Baby Percentile Calculator – Your Baby on the WHO Growth Standards, 0 to 24 Months
A weight or a length on its own is just a number — the percentile says where it sits among babies of the same age and sex.
Your baby’s details
Weight
Length
Head circumference (optional)
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Your baby’s percentiles — WHO growth standards
Right at the WHO median — heavier than about 50 of 100 boys this age.
Right at the WHO median — longer than about 50 of 100 boys this age.
Right at the WHO median — a larger head measurement than about 50 of 100 boys this age.
The comparison, in full
Right in the middle of the chart on all three
The example above is the WHO median baby — a 6-month-old boy sitting exactly on the 50th percentile for weight, length and head circumference. It is a position, not a grade: by definition half of the healthy babies in the WHO study sat below every one of these lines. What pediatricians watch is the pattern across checkups — a baby who tracks steadily along one line, at almost any level, is usually growing exactly as expected. Enter your own baby’s numbers above and this text reads your result instead.
A snapshot, not a verdict: one weighing describes one day. Babies gain in spurts, lose a little with every cold, and are weighed under different conditions at every visit. The notes further down explain what movement between lines does and does not mean in the first two years.
Your baby on the growth curve
The shaded band is the typical range between the 3rd and 97th percentile on the WHO standard. The middle line is the median, and the marker is your baby — pick a measurement to view, and change any detail above to watch it move.
| Percentile band | Weight at 6 months 0 days (boys) | What it usually means |
|---|---|---|
| Below the 3rd | Under 6.44 kg | Below the band that holds about 94 of 100 babies. Common in smaller families; worth raising at the next checkup, especially if the line keeps drifting down. |
| 3rd to 10th | 6.44 to 6.89 kg | The light end of the typical range. Many healthy babies simply live on this line. |
| 10th to 25th | 6.89 to 7.37 kg | Below the middle of the chart, well inside the typical range. |
| 25th to 50th | 7.37 to 7.93 kg | Just under the median — an entirely ordinary place to be. |
| 50th to 75th | 7.93 to 8.54 kg | Just over the median — an entirely ordinary place to be. |
| 75th to 90th | 8.54 to 9.12 kg | Above the middle of the chart, well inside the typical range. |
| 90th to 97th | 9.12 to 9.72 kg | The heavy end of the typical range. Many healthy babies simply live on this line. |
| Above the 97th | Over 9.72 kg | Above the band that holds about 94 of 100 babies. Common in bigger families; worth mentioning at the next checkup if the line keeps climbing. |
A percentile is a position, not a target
The 50th percentile is simply the middle of the WHO reference group — by definition, half of all healthy babies sit below it. There is no prize for a higher line and no failure in a lower one. A baby tracking the 9th percentile steadily, feeding well and developing on time, is usually showing you their blueprint working exactly as intended.
The pattern matters more than the point
One weighing is a snapshot. What pediatricians read is the film: measurements plotted over several visits, forming a channel. A steady channel at almost any level is reassuring. A channel that bends — crossing two or more major lines over time — is the signal doctors act on, and it can only be seen across visits, which is why the checkup measurements count.
WHO median weight and length at a glance
The 50th-percentile weight and length for each age, from the same WHO tables this calculator uses. Half of babies measure above these numbers and half below — they are midpoints, not goals.
| Age | Boys — median weight | Boys — median length | Girls — median weight | Girls — median length |
|---|---|---|---|---|
| Birth | 3.35 kg (7.4 lb) | 49.9 cm (19.6 in) | 3.23 kg (7.1 lb) | 49.1 cm (19.3 in) |
| 3 months | 6.38 kg (14.1 lb) | 61.4 cm (24.2 in) | 5.85 kg (12.9 lb) | 59.8 cm (23.5 in) |
| 6 months | 7.93 kg (17.5 lb) | 67.6 cm (26.6 in) | 7.30 kg (16.1 lb) | 65.7 cm (25.9 in) |
| 9 months | 8.90 kg (19.6 lb) | 72.0 cm (28.3 in) | 8.23 kg (18.1 lb) | 70.1 cm (27.6 in) |
| 12 months | 9.65 kg (21.3 lb) | 75.7 cm (29.8 in) | 8.95 kg (19.7 lb) | 74.0 cm (29.1 in) |
| 18 months | 10.94 kg (24.1 lb) | 82.3 cm (32.4 in) | 10.23 kg (22.6 lb) | 80.7 cm (31.8 in) |
| 24 months | 12.15 kg (26.8 lb) | 87.8 cm (34.6 in) | 11.48 kg (25.3 lb) | 86.4 cm (34.0 in) |
Keep exploring
A weight or a length on its own is just a number. What your pediatrician reads at every checkup is the percentile — where that number sits among babies of the same age and sex on the World Health Organization growth standards. Enter your baby’s weight, length and head circumference below and this calculator places all three on the WHO charts for birth to 24 months, using the same LMS method the WHO publishes, and shows the exact percentile and z-score for each.
Key takeaways
- The shaded band is the typical range between the 3rd and 97th percentile on the WHO standard.
- Half of babies measure above these numbers and half below — they are midpoints, not goals.
- Weigh before a feed, at roughly the same time of day, when you compare visits.
Contents
How to get percentiles you can trust, in three steps
Weigh like a clinic
Naked, or in a dry diaper only, on a flat baby scale — and note what the baby was wearing if you cannot. A full feed can add 100 g or more, and a wet diaper as much again, so weigh before a feed, at roughly the same time of day, when you compare visits.
Measure length lying down, head around the widest part
Length takes two adults: one holds the head gently against a fixed board, the other straightens the legs and brings the foot board to the heels. For head circumference, run a non-stretch tape just above the eyebrows and ears and over the bump at the back — the biggest circle you can find.
Give the exact age, then track the line
Growth charts run on days and months, not round ages — a 5-month-old and a nearly-7-month-old are compared with different groups. Note today’s percentiles, then plot the next set at the next checkup. Staying in the same channel is the expected, healthy pattern.
The method, and every assumption in the open
The default example, worked end to end
Take a boy who is exactly 6 months old. The WHO table gives, for boys at 6 months, a median weight (M) of 7.934 kg with L = 0.1257 and S = 0.1096. Enter his weight as exactly the median, 7.934 kg: the ratio weight / M is 1, so z = ((1)^0.1257 − 1) / (0.1257 × 0.1096) = 0.00, and the normal curve turns a z-score of 0 into the 50.0th percentile. His length and head circumference medians at the same age — 67.62 cm and 43.33 cm — land on exactly the same spot, which is why all three tiles above read 50.0. Now give the same boy a weight of 8.6 kg: the ratio is 8.6 / 7.934 = 1.0839, z comes out at 0.74, and the percentile is the 77th — heavier than about 77 of 100 boys his age. One more anchor: at 8.85 kg he would sit on the z = +1 line itself, the 84th percentile, exactly where the WHO’s printed chart draws it.
Why a formula at all — why not just rank babies?
Baby measurements at any age spread out in a slightly lopsided bell curve, and the lopsidedness changes fast in the first two years — birth weights bunch differently from toddler weights. The LMS method captures each age with three numbers: M, the median; S, the spread around it; and L, a power that straightens the skew so the ordinary normal curve can be used. With those three, any measurement converts to a z-score and any z-score to a percentile. It is the method the WHO itself publishes for its standards, so this page’s numbers are the chart’s numbers, not an approximation of them. For length and head circumference the WHO’s L is exactly 1 at every month, which makes those two curves pure normal spreads around the median; weight carries a small skew that L corrects.
Every assumption, printed
- The dataset: WHO Child Growth Standards LMS values for weight-for-age, length-for-age and head-circumference-for-age, 0 to 24 months, taken from the WHO’s own published monthly tables (the printed percentile-table files, which carry the L, M and S columns) and cross-checked against the WHO’s expanded daily tables. The LMS maths was then verified against the WHO’s printed 3rd, 50th and 97th percentile values at 126 age, sex and measurement points — every computed value landed within the printed tables’ own rounding.
- Interpolation: the WHO publishes one LMS row per month. For an age between rows, L, M and S are interpolated linearly to the exact age in months — the standard approach for growth-chart software.
- Age conversion: age in months = days since birth ÷ 30.4375, the WHO’s own conversion (1 month = 30.4375 days, so 24 months = 730.5 days). Months-and-days entry is converted the same way.
- Extreme z-scores: beyond a z-score of +3 or −3, the WHO’s restricted adjustment is applied — the linear extension the WHO Anthro software uses, because the raw LMS curve is not trusted that far from the median. The percentile you see at the extremes is the WHO figure.
- Length means lying down: the WHO under-2 standard is recumbent length. A standing height reads about 0.7 cm less on the same child and belongs to the 2-to-5-year charts, so measure lying down for this calculator.
- Units: 1 lb = 0.45359237 kg and 1 inch = 2.54 cm exactly. All maths runs in kilograms and centimetres; the imperial figure is the converted result.
- Rounding: percentiles are shown to one decimal place and z-scores to two. Results beyond the chart’s readable edge are shown as below 0.1 or above 99.9 rather than as false-precision decimals.
- The 24-month edge: the WHO infant standards run to 24 months. A baby a few days past 24 months is read on the 24-month row, with a note on the result; beyond that, the CDC stature charts take over and this page points you to the Height Percentile Calculator.
- The reference group: healthy, breastfed babies raised in good conditions in six countries (the WHO Multicentre Growth Reference Study). A percentile compares your baby with that group — a standard for how babies grow under favourable conditions, not a pass mark.
Four ways a home percentile can mislead
Clothes first: at birth, 150 g of diaper and clothing is nearly 5% of body weight — enough to move a newborn several percentile points, so the percentile may be reporting the outfit, not the baby. Length second: a wriggly baby measured by one person, without a board, easily loses a centimetre to bent knees, and a centimetre at 6 months is worth several points. The newborn dip third: most babies lose up to about 7 to 10 percent of birth weight in the first days and regain it by roughly two weeks, so a day-4 percentile almost always reads low and means almost nothing on its own. And chart-shopping fourth: WHO and CDC lines are drawn from different babies, so the same child plots a few points apart on each — pick one chart for the age (WHO under 2, CDC from 2 in US practice) and compare like with like across visits. All four are reasons pediatricians read growth as a series measured their way, and why this page keeps saying: plot the pattern, not just the point.
Going deeper
Most growth charts are references: they describe how some group of children actually grew. The WHO charts are something rarer — a standard, describing how babies grow when conditions are good. Between 1997 and 2003, the WHO’s Multicentre Growth Reference Study followed more than 8,000 children in six countries — Brazil, Ghana, India, Norway, Oman and the United States — selecting families whose babies were breastfed, whose mothers did not smoke, and who had access to good nutrition and healthcare. The striking finding was how alike those babies grew: a well-nourished infant in Accra and one in Oslo traced nearly the same curves, which is why a single international standard is defensible at all. The curves published in 2006 from that study are the ones your pediatrician’s software draws for the first two years, and the ones behind every number on this page. Two consequences are worth holding onto. Because the cohort was breastfed, the standard’s early weight curve rises a little more gently than charts built on mostly formula-fed babies of an earlier era — a breastfed baby who looks small against an old chart often sits dead centre against this one. And because it is a standard rather than a census, sitting away from its middle is a description, not a deviation: the study’s own healthy babies filled every band of it.
Of the three measurements on this page, head circumference is the one parents most often meet without explanation. It earns its place because it is a proxy for brain growth: in the first two years the brain does the fastest growing of a human life, roughly tripling in volume, and the skull expands to fit it. A head that tracks its channel is quiet reassurance that the process is proceeding; a head whose line bends sharply up or down across visits — crossing channels quickly, in either direction — is one of the few single measurements that reliably earns a closer look, because it can flag fluid, growth or development questions early enough to matter. That is also why the measurement has to be taken the same way every time: tape above the eyebrows and ears, over the fullest part of the back of the head, snug but not tight. A centimetre of tape placement error at 4 months is worth several percentile points, so a surprising head percentile is, first of all, a reason to re-measure carefully — and then, if it holds, a reason to bring it to the next checkup rather than to sit with the worry alone.
Frequently asked questions
What does a baby percentile mean?
A baby percentile says how your baby’s measurement compares with the babies in the WHO growth standards — same age, same sex. A baby at the 60th percentile for weight is heavier than about 60 out of every 100 babies in that group, and lighter than about 40. The 50th percentile is the median: exactly in the middle. A percentile is a position, not a score — it does not say your baby is growing well or badly, only where this one measurement falls in the spread of healthy babies.
How is the baby percentile calculated?
With the WHO’s LMS method. For every month of age and each sex, the WHO publishes three numbers — L (the skew of the distribution), M (the median) and S (the spread) — for weight, length and head circumference. Your baby’s measurement becomes a z-score with z = ((measurement / M)^L − 1) / (L × S), with the WHO’s adjustment for extreme values beyond z = ±3, and the standard normal curve converts that z-score into a percentile. This page uses the WHO’s official tables and interpolates L, M and S to your baby’s exact age in months.
Should I use the WHO charts or the CDC charts for my baby?
For babies under 2, the WHO charts are the usual choice — in the United States, the CDC itself recommends the WHO standards for ages 0 to 24 months, because they are built from breastfed babies growing in good conditions around the world. From age 2, US practice switches to the CDC stature-for-age charts, which is why this calculator stops at 24 months and hands over to the Height Percentile Calculator. The same baby can plot a few points differently on the two charts, since they describe different groups — what matters is following one chart consistently at each age.
What if my baby is below the 3rd percentile or above the 97th?
Not automatically a problem. By construction, about 3 of every 100 healthy babies fall below the 3rd percentile and 3 of every 100 above the 97th — the lines have to cut the group somewhere, and smaller or larger parents tend to have smaller or larger babies. What deserves a conversation with your pediatrician is a baby who keeps crossing percentile lines over successive visits, stops gaining as expected, or seems unwell or is not meeting milestones. A single measurement outside the band is a prompt to keep watching and re-measure carefully, not a diagnosis.
How often should I weigh and measure my baby?
Your pediatrician plots weight, length and head circumference at every well-baby visit — frequently in the first months, then at wider intervals up to age 2. At home, weighing every few weeks is plenty. Babies gain in spurts rather than smoothly, and day-to-day numbers mostly reflect feeds, diapers and scale differences, so daily weighing usually creates worry without information. Use the same scale, the same time of day and a dry diaper for readings you plan to compare.
Why did my baby’s percentile change since last time?
Percentiles move for ordinary reasons in the first two years. Newborns typically lose weight in the first days and regain it by about two weeks, so early readings sit low. Growth comes in spurts, illness pauses it briefly, and many babies settle from their birth line to their genetic line over the first 6 to 24 months — a gentle drift toward the channel that matches the family’s size is expected and is called catching down or catching up. Measurement noise matters too: a centimetre of length or 150 g of clothing can shift a small baby’s percentile by several points. Doctors look at the pattern across several visits, and the flag they watch for is a sustained crossing of two or more major percentile lines, not a single wobble.
Related calculators
This calculator places measurements on the WHO Child Growth Standards (birth to 24 months) for education and information. It is not medical advice and cannot diagnose poor growth, a growth disorder or any other condition. A percentile describes one measurement against a reference group; growth is judged on the pattern across several measurements, together with feeding, development, birth history and overall health — all things a calculator cannot see. Babies born early are usually plotted at their corrected age in the first two years; your pediatrician can advise on your baby’s charts. If your baby’s growth seems to slow, their lines keep crossing, or you are worried for any reason, talk to your pediatrician, who can measure accurately and look at the full picture.
One small measurement, placed fairly — the exact WHO percentile, with the method in the open.
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