Mm to Inches Converter – With Fractions a Ruler Can Read
A millimetre measurement is only half an answer until you can find it on a ruler. Type any length in millimetres and get it three ways at once: the exact decimal in inches, the nearest 1/16-inch fraction your tape measure actually shows, and feet and inches for the long pieces — or run it backwards, from inches (fractions welcome) to millimetres.
The length
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The same length, three ways
Every way of saying it
How to read this
Cut and measure by the fraction — that is the mark your ruler has. Trust the decimal when the fit is fine, because the fraction is a rounding: here its mark sits 0.01 mm past the true spot. The biggest error a sixteenth can ever hide is half a sixteenth, 0.79 mm.
- 100 mm is 3.937 in, shown as 3 15/16 in — the page gives the decimal and the nearest ruler fraction together.
- The factor 25.4 comes from the 1959 definition the guide cites: 1 inch is exactly 25.4 mm.
- Thirty-seconds are the workshop unit: the guide works through fractions, thirty-seconds, and when decimals serve better.
- See it on the ruler
- Pairs well with this conversion
- Millimetres to inches in three steps
- How the mm to inches converter works
- Going deeper
- Reference table: millimetres, decimals and ruler fractions
- Why 25.4, exactly: the 1959 definition
- Fractions, thirty-seconds, and when to use decimals
- Common sizes people look up
- Reading a tape measure: feet, inches and fractions together
- Feet and inches in one line
- Frequently asked questions
See it on the ruler
The ruler below is the answer above, drawn. The shaded slot is the sixteenth of an inch your length falls in, the red tick is the mark you would read off, and the red triangle is the exact spot — watch how close the two sit.
The shaded slot is the sixteenth your length falls in. The red tick is the mark you read — 3 15/16 in — and the triangle is the exact spot, 0.01 mm short of that mark.
The one honest catch with fractions
A ruler cannot show a decimal. It chops every inch into sixteen equal slots, so any length that falls between two marks gets read as the nearer one — that reading can be out by up to half a slot, which is 1/32 of an inch or 0.79 mm. For hanging a picture that is nothing. For a press fit in a machine shop it is the whole job, which is why machinists work in decimals and thousandths and carpenters work in sixteenths. This page shows you both numbers and the size of the gap between them, every time.
| Millimetres | Decimal inches | On a ruler | You meet it as |
|---|---|---|---|
| 1 mm | 0.0394 in | 1/16 in | A credit card is about this thick |
| 2 mm | 0.0787 in | 1/16 in | A fine pencil lead, a coin’s thickness |
| 3 mm | 0.1181 in | 1/8 in | A standard drill bit size |
| 4 mm | 0.1575 in | 3/16 in | An M4 bolt’s shaft |
| 5 mm | 0.1969 in | 3/16 in | A pencil’s eraser end |
| 6 mm | 0.2362 in | 1/4 in | The classic quarter-inch, near enough |
| 8 mm | 0.3150 in | 5/16 in | An M8 bolt, a very common size |
| 10 mm | 0.3937 in | 3/8 in | The 10 mm socket everyone loses |
| 12 mm | 0.4724 in | 1/2 in | Half-inch plywood is often really 12 mm |
| 15 mm | 0.5906 in | 9/16 in | Copper pipe in much of the world |
| 20 mm | 0.7874 in | 13/16 in | A US quarter is 24.3 mm across, for scale |
| 25 mm | 0.9843 in | 1 in | Reads as a full inch — see the note below |
| 30 mm | 1.1811 in | 1 3/16 in | A watch case, a bottle cap and a bit |
| 40 mm | 1.5748 in | 1 9/16 in | A golf ball is about 43 mm |
| 50 mm | 1.9685 in | 1 15/16 in | Two inches, one sixteenth short |
| 75 mm | 2.9528 in | 2 15/16 in | A baseball’s diameter is about 74 mm |
| 100 mm | 3.9370 in | 3 15/16 in | The width of an adult palm, roughly |
One row deserves a second look: 25 mm is 0.9843 in, not a full inch — but its nearest sixteenth is 16/16, so a ruler reads it as 1 in. The decimal column always tells the exact truth; the fraction column tells you the mark. Where a real 1 inch matters, remember it is 25.4 mm, not 25.
Pairs well with this conversion
The measuring shelf — fractions worked out properly, lumber sizes, and floor areas.
Millimetres to inches in three steps
Divide by 25.4
One inch is exactly 25.4 millimetres, so millimetres divided by 25.4 gives inches — the exact decimal, with nothing rounded away yet.
Turn the part into sixteenths
Take the part of the answer after the whole inches and multiply it by 16. That is how many sixteenth-marks it spans. The part is always a whole number plus a fraction of a mark.
Round to the nearest mark, ties up
Round that count to the nearest whole sixteenth — an exact half-count rounds up, never down. Simplify (6/16 is written 3/8), and if the count reaches 16 it carries into the next whole inch.
How the mm to inches converter works
inches = millimetres / 25.4 · ruler fraction = nearest 1/16 in
The example on this page. 100 mm. Divide: 100 / 25.4 = 3.937 in. The whole inches are 3, and the remaining 0.937 in spans 0.937 x 16 = 14.99 sixteenths — a hair under 15, which rounds to 15. So the ruler reads 3 15/16 in, and the mark sits 0.01 mm past the true position.
The size everyone asks about. 10 mm. Divide: 10 / 25.4 = 0.3937 in. Sixteenths: 0.3937 x 16 = 6.30, which rounds to 6, and 6/16 simplifies to 3/8. So 10 mm reads as 3/8 in on a ruler — not 7/16. That guess is common and wrong: 7/16 in is 11.1 mm, more than a millimetre too big, while 3/8 in is 9.525 mm, under half a millimetre under.
When the fraction carries. 25 mm is 0.9843 in. The sixteenth count is 0.9843 x 16 = 15.75, which rounds to 16 — and 16/16 is a whole inch, so the ruler reads exactly 1 in. The decimal keeps the real story (25 mm is 0.4 mm short of an inch); the fraction simply has no mark closer.
Going the other way. Inches to millimetres is the same fact read backwards: multiply by 25.4. In inches mode this page also reads ruler fractions as you would say them — type 3 15/16 and it works out 3.9375 in, which is 100.01 mm. Type 5/8 and you get 15.875 mm.
Going deeper
Most unit conversions are measurements, good to so many decimal places and no further. This one is a definition. In 1959 the English-speaking nations signed the international yard and pound agreement and fixed the yard at exactly 0.9144 metres. An inch is a thirty-sixth of a yard, so one inch became exactly 25.4 millimetres — not 25.3998, not 25.4001, exactly 25.4, everywhere, forever, by law in the countries that measure in inches.
That is why this page never has to hedge its decimals. The only rounding anywhere in the conversion is the one you choose at the end: how many decimal places to read, and whether to snap the answer to the nearest mark on a ruler. The conversion itself has no error in it at all — which could not be said before 1959, when the British and American inches genuinely differed by a couple of millionths and precision engineers had to ask which inch a drawing meant.
Ruler fractions are binary: an inch halves to 1/2, quarters, eighths, sixteenths. Each step doubles the marks and halves the possible error. The nearest sixteenth can be out by up to 1/32 in (0.79 mm); the nearest thirty-second by up to 1/64 in (0.40 mm); the nearest sixty-fourth by up to 1/128 in (0.20 mm). Finer rulers exist because some work needs them — drill bits are sold in sixty-fourths, and a 25/64 in bit (9.92 mm) is the closest inch-series bit to a 10 mm hole.
Sixteenths are the default on this page because that is what the common tape measure and workshop ruler are marked in, and because the conversions people usually need — a bolt, a board, a screen size — are carpentry-shaped problems. When the decimal and the fraction disagree by more than you can accept, the honest fixes are a finer ruler, a metric one, or working in millimetres from the start: metric measurements never need the fraction step at all, which is quietly why workshops keep drifting metric-wards.
Reference table: millimetres, decimals and ruler fractions
The converter above gives three answers for any length, the decimal inches, the ruler fraction to the nearest sixteenth, and the feet-and-inches form, but a reference table answers the question most people actually ask, which is what a familiar size reads as on a ruler without typing anything in. The table below gives the decimal and the fraction for common millimetre lengths, computed from the single defined fact on this page: one inch is exactly 25.4 millimetres. Fractions are to the nearest sixteenth, with exact halves rounding up, and simplified where the fraction allows it, exactly as the converter does.
| Millimetres | Decimal inches | Ruler fraction |
|---|---|---|
| 1 mm | 0.039 in | 1/16 in |
| 5 mm | 0.197 in | 3/16 in |
| 10 mm | 0.394 in | 3/8 in |
| 12 mm | 0.472 in | 1/2 in |
| 25 mm | 0.984 in | 1 in |
| 50 mm | 1.969 in | 1 15/16 in |
| 100 mm | 3.937 in | 3 15/16 in |
| 150 mm | 5.906 in | 5 7/8 in |
| 300 mm | 11.811 in | 11 13/16 in |
Two rows deserve a second look, because they show the fraction’s limits. The 1 mm row reads as 1/16 in, but a sixteenth of an inch is 1.5875 mm, so the mark sits more than half a millimetre from the true length, an error of nearly sixty percent of the length itself. Fractions are a coarse language for small lengths, and below about 10 mm the decimal column is the only honest answer. The 25 mm row is the opposite lesson: 25 mm is 0.9843 in, not a full inch, but its nearest sixteenth is 16/16, so a ruler reads it as 1 in, and the decimal column is again the place where the exact truth lives. The page’s own guide makes the same point for 100 mm, where the fraction mark sits 0.01 mm past the true spot: fractions tell you the mark, decimals tell you the length, and the two agree closely only when the length is large compared with a sixteenth.
Why 25.4, exactly: the 1959 definition
One inch is exactly 25.4 millimetres, not approximately, and the exactness is a matter of definition rather than measurement. In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa adopted the international yard and pound agreement, which defines the yard as exactly 0.9144 metres. A yard is 36 inches, so an inch is 0.9144 divided by 36 metres, which is 0.0254 metres, or 25.4 millimetres, with no remainder and no measurement uncertainty. Before 1959 the inch differed slightly between countries, by amounts that mattered to engineers and not to anyone else; since 1959 there has been one inch, and every millimetre-to-inch conversion on Earth is a division by 25.4.
The practical consequence is that millimetre-to-inch conversion never needs a lookup table in principle, only in convenience: divide by 25.4 for the decimal, and the decimal is exact up to whatever rounding you display. The converter above shows three decimals, which is enough for woodwork, metalwork and most engineering, and its fraction column is the only part that involves judgment, because a ruler cannot show thousandths of an inch. A machinist who needs finer work uses thousandths of an inch directly, called thou in Britain and mil in the United States, where one thou is 0.001 in, or 0.0254 mm, and the conversion is again a division, this time by 0.0254 rather than 25.4. The system is consistent at every scale because it rests on one definition.
Fractions, thirty-seconds, and when to use decimals
The ruler fraction on this page is to the nearest sixteenth, because sixteenths are the finest division most tape measures and rulers show clearly. The biggest error a sixteenth can ever hide is half a sixteenth, which is 0.79375 mm, shown on this page as 0.79 mm: any length is at most that far from its nearest sixteenth mark, and most lengths are closer. That error is acceptable for cutting timber, framing a wall or marking a shelf, where a fraction of a millimetre disappears into the saw cut, and it is unacceptable for fine joinery, metal fitting or any work where parts must mate, where the decimal column, or a direct millimetre measurement, is the right language.
Finer rulers divide the inch into thirty-seconds, halving the worst error to half a thirty-second, which is 0.396875 mm, shown as 0.40 mm. A thirty-second ruler would read 100 mm as 3 30/32 in, which simplifies to 3 15/16 in, the same mark the sixteenth ruler gives, because 100 mm sits almost exactly on a sixteenth mark anyway; the finer division only helps lengths that fall between sixteenth marks, such as 12 mm, which a thirty-second ruler reads as 15/32 in, or 11.91 mm, against the sixteenth ruler’s 1/2 in, or 12.70 mm. The finer mark sits 0.09 mm from the true length, while the coarser mark sits 0.70 mm past it. The rule for choosing is simple: use fractions when the work is done with a ruler and the tolerance is larger than the fraction’s error, use decimals or millimetres when the tolerance is smaller, and never mix the two in one measurement, because a length recorded as a fraction and then converted back to millimetres has already lost the precision the decimal column preserved.
Common sizes people look up
Three everyday objects account for a large share of millimetre-to-inch lookups, and their conversions are worth stating because they are also the objects most often misquoted. An A4 sheet of paper is 210 by 297 mm, which is 8.27 by 11.69 inches, since 210 divided by 25.4 is 8.2677 and 297 divided by 25.4 is 11.6929. A credit card, under the international standard for identification cards, is 85.60 by 53.98 mm, which is 3.37 by 2.13 inches, since 85.60 divided by 25.4 is 3.3701 and 53.98 divided by 25.4 is 2.1252. A standard photographic print of four by six inches is 101.6 by 152.4 mm, the reverse conversion, multiplying by 25.4 rather than dividing. These are the sizes that appear on packaging, in software dialogs and in ordering forms, and they are the sizes where a rounded conversion in one direction, read back in the other, produces a figure a fraction of a millimetre from the standard.
Fastener and tool sizes are the second family of lookups, and they follow a simpler pattern because they are defined in inches first. A quarter inch is 6.35 mm, a half inch is 12.70 mm, and three quarters of an inch is 19.05 mm, all exact, because they are 25.4 multiplied by 0.25, 0.5 and 0.75. Metric fasteners, by contrast, are defined in millimetres first, and their inch equivalents are the repeating decimals the converter shows: an 8 mm bolt is 0.315 in, a 10 mm bolt is 0.394 in, and a 13 mm bolt is 0.512 in. The asymmetry is the point: a size defined in one system converts exactly into the other only when the arithmetic happens to land on a round number, and most of the time it does not. When a fit matters, work in the system the part was defined in, and convert only for communication, not for measurement.
Reading a tape measure: feet, inches and fractions together
Most tape measures show feet, inches and fractions together, and reading one correctly means holding three notations at once, which is where millimetre conversions most often help. A standard interior door, 1,981 mm tall in many markets, is 77.99 inches, which is 6 feet and 5.99 inches, effectively 6 feet 6 inches, the nominal door height in feet and inches. A five-metre tape measure, 5,000 mm long, is 196.85 inches, which is 16 feet and 4.85 inches, because 196.85 divided by twelve is 16.404 feet, so sixteen whole feet account for 192 inches and the leftover is 4.85 inches. A room 3.6 metres wide is 141.73 inches, which is 11 feet and 9.73 inches, because 141.73 divided by twelve is 11.811 feet, so eleven whole feet account for 132 inches and the leftover is 9.73 inches.
The working in each case is the same: divide millimetres by 25.4 for inches, divide inches by twelve for feet, keep the whole feet, and multiply the fractional foot by twelve for the leftover inches. The fraction column on this page then gives the nearest sixteenth for the leftover inches, so 9.73 inches reads as 9 and three-quarter inches on a tape, since 0.73 times sixteen is 11.68, which rounds to twelve sixteenths, or three quarters. The commonest tape-measure error is to read the leftover inches as a decimal of a foot, so that 11 feet 9.73 inches is misread as 11.973 feet or as 11 feet 9.7 inches without converting, and the discipline that prevents it is to convert in stages, millimetres to inches, inches to feet and leftover inches, and to check the final figure by converting it back: 11 feet is 132 inches, plus 9.73 inches is 141.73 inches, times 25.4 is 3,600 mm, which is where the room started.
Feet and inches in one line
When a length is quoted in feet and inches together, the conversion to millimetres runs through inches first, and the only arithmetic is multiplication: feet times twelve gives inches, plus the leftover inches gives total inches, times 25.4 gives millimetres. Six feet exactly is 72 inches, which is 72 times 25.4, giving 1,828.8 mm. Six feet six inches is 78 inches, which is 78 times 25.4, giving 1,981.2 mm, the door height of the previous section to within a fifth of a millimetre of rounding. Five feet nine inches, a common height, is 69 inches, which is 69 times 25.4, giving 1,752.6 mm. The reverse runs the same chain backwards: divide millimetres by 25.4 for inches, divide inches by twelve for feet, and the remainder is the leftover inches, which the fraction column on this page then gives to the nearest sixteenth. A single chain, used in both directions, is the whole of feet-and-inches conversion, and every other method is this chain with steps omitted or reordered.
Why 25.4, exactly: the 1959 definition
Before 1959 the inch differed slightly between countries. The international yard and pound agreement defined the yard as 0.9144 metres exactly, fixing the inch at 25.4 millimetres exactly. That is why this conversion has no uncertainty in the factor itself: any error is rounding in the display.
Fractions, thirty-seconds, and when to use decimals
At sixteenths, each step is 1.5875 mm, so a value can sit 0.8 mm from the nearest mark. For fine work use thirty-seconds or decimals. And remember that 6 mm is not 1/4 inch: a quarter inch is 6.35 mm, which is why metric fasteners fit loosely in imperial holes.
Frequently asked questions
How many millimetres are in an inch?
Exactly 25.4. Since the international yard and pound agreement of 1959, the inch has been defined as 25.4 millimetres — not measured, defined — so the figure is exact with no rounding in it. Half an inch is 12.7 mm, a quarter inch is 6.35 mm, and a sixteenth of an inch is 1.5875 mm.
How do I convert mm to inches?
Divide the millimetres by 25.4. To get a fraction a ruler can show, take the part after the whole inches, multiply it by 16, and round to the nearest whole number — an exact half rounds up. That number is the sixteenths; simplify it (6/16 becomes 3/8), and if it comes out as 16 it carries into the next whole inch. This page does all of that the moment you type.
What is 10 mm in inches?
10 mm is 0.3937 in. On a ruler the nearest sixteenth is 3/8 in — the count is 6.30 sixteenths, which rounds to 6, and 6/16 simplifies to 3/8. It is often guessed as 7/16, but 7/16 in is 11.1 mm, over a millimetre too large; 3/8 in is 9.525 mm, less than half a millimetre under the true 10 mm.
What is 100 mm in inches?
100 mm is 3.937 in — 3 15/16 in on a ruler, one sixteenth short of 4 inches. The fraction mark sits just 0.01 mm past the exact position, so for any ordinary purpose 100 mm and 3 15/16 in are the same length; only the decimal shows the hair of difference.
Why does the fraction not match the decimal exactly?
Because a ruler only has marks every sixteenth of an inch, and most lengths fall between two of them. The fraction you read is the nearer mark, so it can differ from the exact decimal by up to half a sixteenth — 1/32 in, or 0.79 mm. The converter shows the size and direction of that gap on every answer. When the gap matters, work from the decimal, or use a ruler marked in thirty-seconds or sixty-fourths, which cuts the possible error to 0.40 mm or 0.20 mm.
How do I convert inches back to millimetres?
Multiply the inches by 25.4. Fractions convert by their decimal value: 3/8 in is 0.375 in, which is 9.525 mm, and 3 15/16 in is 3.9375 in, which is 100.01 mm. Switch this page to inches mode and you can type the fraction exactly as you would say it — whole number, space, fraction — and get the millimetres straight back.
What is 2 inches in millimetres?
50.8 mm, exactly. Inches to millimetres is the same definition read backwards: multiply by 25.4, so 2 times 25.4 is 50.8, with no rounding involved. The same multiplication gives 1 inch as 25.4 mm, 6 inches as 152.4 mm, and 12 inches, a foot, as 304.8 mm. The exactness matters when a length is specified in inches and manufactured in millimetres, or the reverse: a part drawn as 2 inches and made as 51 mm is 0.2 mm oversize, which is within tolerance for timber and outside it for a machined fit. Convert first, then round to the tolerance the work allows, never the other way round.
Why does my ruler not have a mark for my exact length?
Because rulers show marks at fixed fractions, usually sixteenths of an inch, and most lengths fall between marks. The converter’s fraction column tells you the nearest mark, and its decimal column tells you the exact length; the distance between them is at most half a sixteenth, or 0.79 mm, and for small lengths it can be a large fraction of the length itself. A length of 1 mm reads as 1/16 in on a ruler, but the mark sits 0.59 mm from the true length. For work finer than the ruler’s divisions, use the decimal inches or measure directly in millimetres, where the scale is continuous rather than stepped.
Should I measure in millimetres or inches for woodwork?
Whichever system your materials, tools and plans already use, and then stay in it end to end. Mixing systems is where errors enter: a board measured in inches, cut from a plan in millimetres, and checked against a ruler in fractions accumulates a rounding at every change of system, and the roundings do not cancel. Millimetres suit fine work because the unit is small and the scale is decimal, so halving and doubling stay exact; inches suit work where the stock, the fasteners and the plans are all imperial, because the fractions then match the materials. The worst choice is not either system but switching between them halfway through a project.
What is a thou?
A thousandth of an inch, called a thou in Britain and a mil in the United States, equal to 0.0254 mm. It is the unit of machinists and engineers, for whom a sixteenth of an inch is far too coarse: a tolerance of five thou is 0.127 mm, and a clearance of two thou is 0.051 mm, figures that a ruler cannot show and a micrometer can. The conversion is the same definition as ever: multiply thou by 0.0254 for millimetres, or divide millimetres by 0.0254 for thou. A sheet of paper about 0.1 mm thick is about four thou, which is a useful sense of the scale: a thou is roughly the thickness of a fine human hair divided by twenty, small enough that it only matters when parts must fit.
Is the mm to inches factor exact?
Yes. One inch is exactly 25.4 mm by the 1959 international agreement, so the only approximations are in rounding the displayed result, not in the conversion itself.
Why does my tape measure disagree with the calculator?
Usually because of fraction rounding: a tape reads to the nearest sixteenth, which is 1.5875 mm per step. A value between marks rounds to the nearest one, and the calculator decimal is the exact figure.
How do I read feet and inches from millimetres?
Divide by 25.4 for inches, divide by 12 for feet, and keep the remainder as inches. For example, 1,750 mm is 68.898 inches, or 5 ft 8.9 in.
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