Pregnancy Weight Gain Calculator – Your IOM Range, Week by Week
Enter your weight before pregnancy, your height, and what you weigh now. The calculator works out your pre-pregnancy BMI, shows the Institute of Medicine total gain range for your category, and tells you how much of that range you have used and how much is left.
Your numbers
Use your weight from just before pregnancy, not your first appointment weight if that was later. The range shown is a total for the whole pregnancy, so being under it partway through is normal.
Private by design: every number is worked out in your browser. Nothing you type is sent anywhere or stored.
Your result
- The IOM total-gain range for a normal pre-pregnancy BMI is 25 to 35 lb for one baby.
- Ranges are totals for the whole pregnancy, not weekly targets — the guide assumes only about 1.1 to 4.4 lb across the first trimester.
- For twins the IOM ranges are provisional, and no twin range was published for people who started underweight.
- Where your gain sits in the range
- How it works
- Understanding the IOM ranges
- The IOM ranges, all in one place
- How the calculator reads your numbers
- Using the result week by week, without over-reading it
- Worked checks: three starting points, three ranges
- Questions worth taking to your appointment
- One last check before you close the page
- Frequently asked questions
Where your gain sits in the range
The teal band is your recommended total range for the whole pregnancy. The pink marker is what you have gained so far, at week 28 in the default example.
The marker sits at 20.0 lb, just before the range starts. Gaining the remaining 5.0 to 15.0 lb by term would land inside the recommended total.
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How it works
Enter your pre-pregnancy weight and height so the calculator can work out your starting BMI.
Your BMI category picks the Institute of Medicine total gain range, with a separate provisional range if you are carrying twins.
Enter your current weight and week to see your gain so far, your progress, and how much of the range is left.
Understanding the IOM ranges
For one baby, the Institute of Medicine (2009) recommends a total gain of 28 to 40 lb if you started underweight (BMI under 18.5), 25 to 35 lb at a normal weight (BMI 18.5 to 24.9), 15 to 25 lb if overweight (BMI 25 to 29.9), and 11 to 20 lb if obese (BMI 30 or higher). For twins the provisional ranges are 37 to 54 lb at a normal weight, 31 to 50 lb if overweight, and 25 to 42 lb if obese. The IOM did not publish a twin range for people who started underweight, because the data was too limited, so this calculator says so instead of guessing one.
The IOM ranges describe the total gain by the end of pregnancy, not a target for every week. The guidelines assume a small gain of about 1.1 to 4.4 lb across the whole first trimester, then a steadier weekly rate in the second and third trimesters. That is why this calculator compares your gain so far with the total range and shows the weekly rate separately, instead of marking you behind just because it is still early.
The IOM ranges, all in one place
The Institute of Medicine ranges from 2009 are the backbone of this calculator, and they are easier to use when the whole table sits in front of you instead of one row at a time. The singleton ranges below are the ones the calculator shows for one baby, and the twin ranges are provisional: the Institute published them as provisional because the evidence base for twins was thinner. The weekly rates apply to the second and third trimesters only, after a small first-trimester gain, and the twin guidelines give a total only, so no weekly rate is shown for twins. All of these figures are the page’s own published ranges, restated here so you can compare rows without re-entering numbers.
| Pre-pregnancy category | Singleton total gain | Weekly rate, 2nd and 3rd trimester | Twin total gain (provisional) |
|---|---|---|---|
| Underweight (BMI under 18.5) | 28 to 40 lb | About 1 lb a week (1.0 to 1.3) | No published range |
| Normal weight (BMI 18.5 to 24.9) | 25 to 35 lb | About 1 lb a week (0.8 to 1.0) | 37 to 54 lb |
| Overweight (BMI 25.0 to 29.9) | 15 to 25 lb | About 0.6 lb a week (0.5 to 0.7) | 31 to 50 lb |
| Obese (BMI 30 or higher) | 11 to 20 lb | About 0.5 lb a week (0.4 to 0.6) | 25 to 42 lb |
Two rows of that table deserve emphasis because they are the ones people misread. The first is the underweight twin row: the Institute of Medicine did not publish a twin range for people who started underweight, because the data was too limited, so this calculator says so instead of guessing one. If that is your situation, the calculator will flag the gap rather than invent a number, and the right next step is a conversation with your clinician, who can set a target from your individual circumstances. A calculator that invents a range to avoid an awkward blank would be worse than useless here, because a made-up range looks exactly like a real one once it is printed on a results panel.
The second is the shape of the table itself: as the starting category rises, the recommended total falls. That is not a judgement about any individual; it is a consequence of the guideline’s aim, which is a healthy outcome for parent and baby across the whole pregnancy, taking into account that the starting point already differs. A person starting at a normal weight is guided toward 25 to 35 lb, while a person starting in the obese category is guided toward 11 to 20 lb, and the weekly rates narrow in the same direction, from about 1 lb a week down to about 0.5 lb a week. The ranges are totals for the whole pregnancy, not targets for any single week, which is why the calculator compares your gain so far with the total range and shows the weekly rate separately, instead of marking you behind just because it is still early.
The default example on the page puts those ideas together. A pre-pregnancy weight of 140 lb at a height of 65 inches gives a pre-pregnancy BMI of 23.3, which is the normal weight category, so the total range is 25.0 to 35.0 lb. At a current weight of 160 lb in week 28, the gain so far is 20.0 lb, which sits below the total range, with 5.0 to 15.0 lb of the range still to go, and the weekly rate shown is about 1 lb a week (0.8 to 1.0). Nothing about that position is alarming on its own: the range is a total for the whole pregnancy, and at week 28 there are still weeks of gain ahead. Switch the same numbers to twins, and the calculator shows the provisional twin range of 37.0 to 54.0 lb instead, with 17.0 to 34.0 lb still to go, and no weekly rate, because the twin guidelines give a total only. The same person, the same weights, and a different range, purely because the pregnancy is a twin pregnancy: that is the table working as intended.
How the calculator reads your numbers
The first thing the calculator does is work out your pre-pregnancy BMI from your pre-pregnancy weight and your height, and that single number decides your category. The category is decided by your exact BMI, not by the rounded figure on the display: the page gives the example of a BMI of 29.9, which is overweight even though it displays rounded to 30.0. That detail matters at the boundaries, because the ranges change abruptly there: a BMI of 24.9 takes the normal range of 25 to 35 lb, while a BMI of 25.0 takes the overweight range of 15 to 25 lb, and a tenth of a BMI point either side of a boundary is well within the error of an ordinary bathroom scale or a remembered height. If you sit close to a boundary, treat the two neighbouring ranges as the honest picture rather than clinging to one row, and raise it with your clinician if the difference worries you.
Use your weight from just before pregnancy, not your first appointment weight if that was later. The distinction is real: weight can already have moved by the first appointment, and since the whole calculation keys off the pre-pregnancy figure, a later weight quietly shifts both the BMI and the category. If you do not know your pre-pregnancy weight exactly, the calculator still runs, but the range it shows inherits that uncertainty, and the page is honest about it in the same way it is honest about the twin underweight gap: an approximate input gives an approximate placement, and the range, which is ten pounds wide in most rows, absorbs small errors gracefully. What it cannot absorb is a systematically wrong input, such as a current weight entered as the pre-pregnancy weight, so the few seconds spent checking which weight goes where are the most valuable seconds in the whole calculation.
The timeline matters as much as the total. The guidelines assume a small gain of about 1.1 to 4.4 lb across the whole first trimester, then a steadier weekly rate in the second and third trimesters, which is why the weekly rates in the table above are labelled for the second and third trimesters only. Early in pregnancy, gaining little or even losing a little is consistent with the guideline shape, and the calculator’s progress reading reflects that: it compares your gain so far with the total range rather than demanding a fixed fraction of the range by a given week. A single weigh-in outside the range is not a diagnosis, either. Weight moves with fluid, with the time of day, and with what you ate and drank, and the baby is only part of the total: the placenta, the amniotic fluid, increased blood volume, and changes in breast and body tissue all contribute. If your gain is well above or below the range, or changes suddenly, the page’s advice is to talk it through with your midwife, doctor, or obstetrician, who can see the full picture.
The unit switches deserve one paragraph because they are a quiet source of error. The calculator accepts weight in pounds or kilograms and height in inches or centimetres, and it converts before computing the BMI, so mixing units across the two weight fields, or entering a height in centimetres while the height unit still says inches, will produce a BMI that looks precise and is wrong. The results panel shows the BMI it computed, so a quick sanity check is always available: a BMI in the teens, twenties, or thirties is plausible, while a BMI in single figures or in three figures means a unit was mixed up. The same discipline applies to the current week: the calculator uses it to frame the progress reading, so an approximate week gives an approximate framing, and the total range itself does not depend on the week at all.
Using the result week by week, without over-reading it
The most useful way to use this calculator is as a slow tracker rather than a daily judge. Weigh yourself at the same time of day, on the same scale, once a week rather than once a day, and compare the trend with the weekly rate for your category instead of reacting to any single reading. A normal-weight gain of about 1 lb a week means a single week’s reading can easily show no change or two pounds, depending on fluid and timing, while the average over a month tells the real story. If the trend over several weeks sits well above or below your category’s weekly rate, that is the point at which the page’s advice applies: raise it with your clinician, who may set a different target based on your health and your pregnancy, rather than adjusting food or activity on the strength of one number.
It is equally important to know what the calculator cannot see. It does not know your medical history, it cannot tell how much of your gain is fluid, and it cannot replace your own clinician, who may set a different target for you for reasons that have nothing to do with the table. The page states this in its disclaimer, and the new sections here repeat it deliberately: this calculator presents the published Institute of Medicine guideline ranges for general information only, for adults, and it is not medical advice. Conditions that change the picture, such as diabetes, thyroid disorders, or a multiple pregnancy beyond twins, are outside the table’s scope, and the calculator does not attempt to adjust for them. The ranges are a starting point for a conversation, not a verdict on any individual pregnancy, and a target your clinician sets for you personally always outranks the table.
Worked checks: three starting points, three ranges
The default case is the normal-weight case: 140 lb before pregnancy, 65 inches tall, 160 lb now, week 28, one baby. The BMI is 23.3, the category is normal weight, the total range is 25.0 to 35.0 lb, the gain so far is 20.0 lb, and 5.0 to 15.0 lb of the range remains, at a weekly rate of about 1 lb a week (0.8 to 1.0). The check is simple: 160 minus 140 is 20, and 20 sits below the bottom of the range, so the remaining gain is 25 minus 20, which is 5, up to 35 minus 20, which is 15. The calculator’s progress reading says exactly that, and the reason it does not call the position a problem is that the range is a total for the whole pregnancy, with weeks still to go.
Change one input, the pre-pregnancy weight, to 110 lb, keeping the height at 65 inches and the current weight at 160 lb. The BMI becomes 18.3, which is the underweight category, so the total range becomes 28.0 to 40.0 lb, and the gain so far is 50.0 lb, which is 10.0 lb above the top of the range. The calculator flags that position as worth raising with your clinician. The example is deliberately stark: the same current weight that sat comfortably inside the normal-weight story now sits above the underweight range, because the starting point moved. That is the whole logic of the table in one comparison, and it is why the pre-pregnancy weight, not the current weight, is the input that decides the category.
Change the pregnancy type instead, back at the default weights: one baby becomes twins. The BMI is still 23.3 and the category is still normal weight, but the range becomes the provisional twin range of 37.0 to 54.0 lb, the gain so far is still 20.0 lb, and the remaining gain becomes 17.0 to 34.0 lb, with the weekly rate shown as not published for twins. The twin ranges are provisional, which means they carry more uncertainty than the singleton ranges, and the absence of a weekly rate is not a gap in the calculator but a faithful reflection of the guideline: the twin guidelines give a total only. For an overweight starting point the twin range is 31 to 50 lb, and for an obese starting point it is 25 to 42 lb, while for an underweight starting point there is no published twin range at all, and the calculator flags that instead of inventing one.
A final check on the arithmetic behind the BMI itself helps when the displayed category surprises you. BMI is weight divided by height squared, with the units converted so the result is comparable across pounds and inches, kilograms and centimetres. At 140 lb and 65 inches the result is 23.3; at 110 lb and the same height it is 18.3. Small changes near a boundary move the category, large changes move it a lot, and the displayed BMI is rounded while the category uses the exact figure, so a displayed 30.0 can still be an overweight 29.9 underneath. If your category ever looks wrong, check the pre-pregnancy weight first, the height and its unit second, and the exact-versus-rounded boundary third. Those three checks resolve nearly every surprise this calculator produces, and anything left over is a question for your clinician, not for a different calculator.
Questions worth taking to your appointment
The calculator gives you a range and a position within it; your clinician can give you the context that turns those numbers into a plan. The most useful questions are the ones the table cannot answer. If your gain sits well above or below your category’s range, ask whether your clinician would set a different target for you personally, and what trend over the coming weeks would reassure or concern them. If you sit close to a category boundary, ask which neighbouring range they would have you watch, since a BMI of 24.9 and a BMI of 25.0 take different rows but describe nearly identical bodies. If you are carrying twins, ask how the provisional nature of the twin ranges should affect the way you read your position, and whether your clinician tracks twin gain against the total alone or against interim milestones of their own. And if the calculator flags that there is no published twin range for an underweight starting point, that flag itself is the question to take in: ask directly what target your clinician would set, rather than borrowing a neighbouring row.
It also helps to bring the same numbers the calculator used, in the same units, so the conversation starts from agreement rather than reconstruction. Your pre-pregnancy weight and height, your current weight and week, whether the pregnancy is a singleton or a twin pregnancy, and the range the calculator showed are enough for your clinician to see exactly what you saw. If your clinician’s target differs from the table, that is not a contradiction to resolve by re-running the calculator with different inputs until it agrees; it is the expected outcome when individual circumstances override a population guideline. The guideline ranges describe what was associated with good outcomes across large groups of pregnancies in the Institute of Medicine’s 2009 review. Your clinician’s target describes what is right for one pregnancy, yours, with its own history, and the individual target always outranks the table when the two differ.
Finally, a word about what happens after the range ends. The Institute of Medicine ranges describe gain during pregnancy only; they say nothing about how quickly weight comes off afterwards, and this calculator makes no claim about that either. Bodies recover on very different timelines, feeding choices and sleep disruption both affect appetite and activity, and the postnatal period brings its own clinical guidance that is separate from the pregnancy gain table. If you are reading this page after delivery and wondering what the numbers mean now, the honest answer is that the pregnancy ranges no longer apply, and the right next step is the same as at every other point in this guide: take your numbers to your clinician and ask what target, if any, makes sense for the months ahead. The calculator’s job ends at delivery; your clinician’s does not.
One last check before you close the page
Before you trust any result this calculator shows, run through three quick checks that take less than a minute. First, confirm the pre-pregnancy weight is really the weight from just before pregnancy, and that the height and its unit match the height you entered: a height in centimetres entered while the unit still says inches is the single easiest way to get a confident-looking but wrong BMI. Second, confirm the pregnancy type: a twin pregnancy on the singleton setting, or a singleton on the twin setting, changes the range by more than ten pounds at a normal starting weight, from 25 to 35 lb to 37 to 54 lb, so a wrong toggle dwarfs every other error on the page. Third, read the progress line as a position within a total, not as a pass or fail mark: below the range partway through is normal, above it is a prompt for a conversation, and neither is a diagnosis on its own. With those three checks done, the range on the screen is as good as the published guideline can make it, and anything beyond that belongs with your clinician.
The second and third trimester weekly rates
The IOM ranges are totals, but weight is gained week by week, and the weekly rates in the second and third trimesters are what make the totals usable. After a first-trimester gain of roughly 1.1 to 4.4 lb, the expected weekly rate is about 1 lb a week for underweight and normal-weight pregnancies, about 0.6 lb a week for overweight pregnancies, and about 0.5 lb a week for obese pregnancies. Those rates, sustained across the final two trimesters, land inside the total ranges.
IOM 2009 recommendations for singleton pregnancies. Totals assume a first-trimester gain of 1.1 to 4.4 lb.
| Pre-pregnancy BMI | Total gain | Weekly rate, 2nd and 3rd trimester |
|---|---|---|
| Underweight, below 18.5 | 28-40 lb | 1 lb per week |
| Normal, 18.5-24.9 | 25-35 lb | 1 lb per week |
| Overweight, 25.0-29.9 | 15-25 lb | 0.6 lb per week |
| Obese, 30 and above | 11-20 lb | 0.5 lb per week |
The weekly rate is a guide, not a weekly test. Weight fluctuates with fluid, meals, and time of day, so a single week above or below the rate means nothing; the pattern across a month is what matters. A sustained drift away from the rate, in either direction, is the thing worth taking to an appointment, not one week’s reading.
Twins change the range, not the method
Twin pregnancies use wider ranges but the same method: start from pre-pregnancy BMI, read the range, and track the weekly pattern against it. For twins, the IOM totals are 37 to 54 lb for normal-weight pregnancies, 31 to 50 lb for overweight pregnancies, and 25 to 42 lb for obese pregnancies. No range is published for underweight twin pregnancies, which is itself information: those pregnancies need individual guidance rather than a table.
The method stays the same because the logic stays the same: the range is a planning aid built from BMI, not a prescription, and the clinician managing the pregnancy is the one who adapts it to the individual case. This calculator shows where a gain sits inside the range; it does not set a target, and it is not medical advice. Anything the range cannot cover — an underweight twin pregnancy, a medical condition, a gain pattern that worries you — belongs in an appointment, not in a table.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your BMI before pregnancy. The Institute of Medicine recommends a total of 28 to 40 lb if you started underweight, 25 to 35 lb at a normal weight, 15 to 25 lb if overweight, and 11 to 20 lb if obese. Enter your numbers above to see your own range.
How is my recommended range worked out?
Your pre-pregnancy weight and height give your starting BMI. That BMI puts you in one of four categories, and each category has its own IOM total gain range. The category is decided by your exact BMI, so a BMI of 29.9 is overweight even though it displays rounded to 30.0.
What if I am carrying twins?
Choose Twins above. The IOM gives provisional twin ranges of 37 to 54 lb at a normal starting weight, 31 to 50 lb if overweight, and 25 to 42 lb if obese. There is no published IOM twin range for people who started underweight, so the calculator flags that instead of inventing a number.
How much should I gain each week?
After a small first-trimester gain of about 1.1 to 4.4 lb in total, the IOM weekly rates for the second and third trimesters are about 1 lb a week if underweight (1.0 to 1.3), about 1 lb a week at a normal weight (0.8 to 1.0), about 0.6 lb a week if overweight (0.5 to 0.7), and about 0.5 lb a week if obese (0.4 to 0.6). The twin guidelines give a total only, so no weekly rate is shown for twins.
What if I have gained more or less than the range?
A single weigh-in outside the range is not a diagnosis. Weight moves with fluid, timing, and the baby is only part of the total. If your gain is well above or below the range, or changes suddenly, talk it through with your midwife, doctor, or obstetrician, who can see the full picture.
Is this calculator medical advice?
No. It presents the published IOM guideline ranges for general information only. It does not know your medical history, and it cannot replace your own clinician, who may set a different target for you.
Does the calculator use my current BMI or my pre-pregnancy BMI?
Your pre-pregnancy BMI. Your pre-pregnancy weight and height give your starting BMI, and that figure decides your category and your range. The category uses your exact BMI, so a BMI of 29.9 is overweight even though it displays rounded to 30.0. Use your weight from just before pregnancy, not your first appointment weight if that was later.
Why does the page show no weekly rate for twins?
Because the twin guidelines give a total only. The Institute of Medicine published provisional twin totals, such as 37 to 54 lb at a normal starting weight, but no weekly rate to go with them, so this calculator shows the total range and reports the weekly rate as not published for twins rather than inventing one.
I am under the range at week 28. Does that mean I am behind?
Not necessarily. The range is a total for the whole pregnancy, not a target for week 28. In the default example, a gain of 20.0 lb against a range of 25.0 to 35.0 lb leaves 5.0 to 15.0 lb still to go, and the calculator shows the weekly rate separately instead of marking you behind just because it is still early. A single weigh-in outside the range is not a diagnosis; weight moves with fluid and timing, and a sudden change or a gain well outside the range is worth raising with your clinician.
What if there is no IOM twin range for my starting weight?
That happens for an underweight starting point: the Institute of Medicine did not publish a twin range for people who started underweight, because the data was too limited. This calculator flags that gap instead of guessing a number, and the right next step is to ask your midwife, doctor, or obstetrician to set a target from your individual circumstances.
How much weight should I gain per week in pregnancy?
In the second and third trimesters, about 1 lb a week for underweight and normal-weight pregnancies, 0.6 lb a week for overweight pregnancies, and 0.5 lb a week for obese pregnancies, after a first-trimester gain of roughly 1.1 to 4.4 lb. Judge the pattern across a month, not a single week.
What are the IOM weight gain ranges for twins?
For twin pregnancies: 37 to 54 lb for normal weight, 31 to 50 lb for overweight, and 25 to 42 lb for obese. No range is published for underweight twin pregnancies, which need individual clinical guidance.
Is this calculator medical advice?
No. It shows where a weight gain sits inside the published IOM ranges for planning and discussion. Targets for an individual pregnancy, and any worry about a gain pattern, belong with the clinician managing the pregnancy.
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Disclaimer: This calculator is for general information only and is not medical advice. The ranges shown are the Institute of Medicine (2009) guideline ranges; the twin ranges are provisional. Your own clinician may recommend a different target based on your health and your pregnancy. Please discuss your weight gain with your midwife, doctor, or obstetrician.
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