Mm to Inches Converter
Convert millimeters to inches — hardware, drill bits, screens, and drawings.
Updated
inches
3.94in
100 mm = 3.94 in
In short
How many inches is 1 mm?
One millimeter is 0.03937 inches, so divide millimeters by 25.4. The inch has been exactly 25.4 mm since the 1959 International Yard and Pound Agreement, which makes this conversion exact rather than approximate. 100 mm is 3.94 inches, and 25.4 mm is one inch on the nose.
Below about 50 mm the answer is far more useful as a fraction than a decimal, because that is how drill bits, wrenches and stock sizes are labelled.
millimeters to inches — quick reference
| millimeters (mm) | inches (in) |
|---|---|
| 1 mm | 0.04 in |
| 5 mm | 0.20 in |
| 10 mm | 0.39 in |
| 25 mm | 0.98 in |
| 50 mm | 1.97 in |
| 100 mm | 3.94 in |
| 200 mm | 7.87 in |
| 300 mm | 11.81 in |
The formula, worked line by line
The inch is defined in metric terms, and it has been for longer than most people assume. The chain runs from the yard: fix the yard at 0.9144 meters, divide by 36 to get the inch at 0.0254 meters, and multiply by a thousand to state it in millimeters. That gives 25.4, exactly, with nothing after it.
“The avoirdupois pound is exactly 0.45359237 kilograms; the yard is exactly 0.9144 meters.”
Because 25.4 is exact, the inch is a metric unit wearing imperial clothes. Any conversion between the two systems in either direction is a single division or multiplication with no approximation involved, which is precisely why international manufacturing tolerated the two systems side by side for as long as it did.
inches = millimeters ÷ 25.4
= millimeters × 0.0393700787…
nearest 64th = round(inches × 64) ÷ 64
thousandths = millimeters ÷ 0.0254
100 mm → 100 ÷ 25.4 = 3.937007… in- Millimeters entered
- 100
- Exact divisor
- 25.4 mm per in
- Division
- 100 ÷ 25.4 = 3.937007…
- Times 64, rounded
- 251.97 → 252, so 252/64
- Inches
- 3.94 in, or 3 15/16
One hundred millimeters is a useful anchor because it sits so close to four inches without reaching it. The gap is 1.6 mm, about a sixteenth of an inch, which is enough to leave a visible step where a metric panel meets an imperial opening and enough to stop a 100 mm part dropping into a 4 inch hole cut to size.
The mental version
Divide by 25 instead of 25.4 and you have the answer within 1.6 percent, always on the high side. One hundred millimeters becomes 4 inches against a true 3.937, and 50 mm becomes 2 against 1.969. That is close enough to picture the size and never close enough to cut to.
- At 25 mm: estimate 1 in
- true 0.9843 in
- At 50 mm: estimate 2 in
- true 1.9685 in
- At 100 mm: estimate 4 in
- true 3.9370 in
The shortcut always over-reads by 1.6 percent, which is a sixteenth of an inch across 100 mm.
Reading the fraction correctly
To reach a fraction, multiply the decimal inches by 64, round to a whole number, and put it over 64. Ten millimeters gives 0.3937 times 64, which is 25.2, so 25/64. That does not reduce, which is the tell: a metric size that lands on an unreduceable sixty-fourth has no natural fractional equivalent at all.
Sizes that reduce are easier to remember, but only one is a genuine crossover. Three millimeters rounds to 8/64, which reduces to 1/8, though the true gap is 0.175 mm. Nineteen rounds to 48/64, which reduces to 3/4, and there the gap is only 0.05 mm. That second pair is the one you can actually substitute.
How to use the mm to inches converter
Millimeters are the working unit of nearly all manufacturing. Engineering drawings, fastener specifications, bearing sizes, sheet thicknesses and component footprints are dimensioned in millimeters even in countries that still sell lumber by the foot. The conversion into inches is what lets a metric spec meet an imperial toolbox.
Type the millimeters in and the inches appear as you type. The tool divides by 25.4, an exact international definition rather than a measured ratio, so there is no error to accumulate. Press Swap when the number you are holding is already in inches and the drawing in front of you is metric.
3.94 in
100 mm
100 ÷ 25.4, exact factor
25.4 mm
One inch, by definition
0.03937 in
One millimeter
The reason this conversion feels harder than it is comes down to notation. Metric sizes are whole numbers and imperial sizes are fractions, so the answer to a millimeter question is almost never a tidy figure. Ten millimeters is 0.3937 inches, which is close to three eighths and not equal to it.
- 3 mm
- 0.1181 in, near 1/8
- 6 mm
- 0.2362 in, near 15/64
- 10 mm
- 0.3937 in, near 25/64
- 13 mm
- 0.5118 in, near 33/64
- 19 mm
- 0.7480 in, near 3/4
- 25 mm
- 0.9843 in, near 63/64
Near means the closest sixty-fourth, not an equivalent. Only 25.4 mm is exactly an inch.
Small sizes are where the mismatch bites hardest, because the fractional ladder gets coarse fast. Between 1/16 and 1/8 of an inch there is nothing but 3/32, a gap of 1.5875 mm, so a metric size landing in the middle has no fractional neighbour worth using and has to stay a decimal.
Going the other way, inches into millimeters
When the number you have is a fraction of an inch and the supplier, the drawing or the machine wants millimeters, the same exact 25.4 runs in reverse.
Open the inches to mm converter →A ladder of millimeter sizes converted to decimal inches and to the nearest sixty-fourth, which is the finest fraction marked on an ordinary rule and the resolution most fractional tooling is sold in.
| Millimeters | Inches | Nearest 1/64 in |
|---|---|---|
| 1 mm | 0.0394 in | 3/64 |
| 2 mm | 0.0787 in | 5/64 |
| 3 mm | 0.1181 in | 1/8 |
| 4 mm | 0.1575 in | 5/32 |
| 5 mm | 0.1969 in | 13/64 |
| 6 mm | 0.2362 in | 15/64 |
| 8 mm | 0.3150 in | 5/16 |
| 10 mm | 0.3937 in | 25/64 |
| 12 mm | 0.4724 in | 15/32 |
| 13 mm | 0.5118 in | 33/64 |
| 14 mm | 0.5512 in | 35/64 |
| 16 mm | 0.6299 in | 5/8 |
| 17 mm | 0.6693 in | 43/64 |
| 19 mm | 0.7480 in | 3/4 |
| 20 mm | 0.7874 in | 25/32 |
| 22 mm | 0.8661 in | 55/64 |
| 25 mm | 0.9843 in | 63/64 |
| 30 mm | 1.1811 in | 1 3/16 |
| 50 mm | 1.9685 in | 1 31/32 |
| 100 mm | 3.9370 in | 3 15/16 |
Which imperial wrench fits a metric nut
Every mixed toolbox eventually faces the same question: can this imperial socket turn that metric fastener without rounding it off. The answer is arithmetic, not luck. Convert the fraction to millimeters, compare it with the nut, and look at the sign and size of the gap.
A fraction larger than the nut will go on and may be loose enough to slip. A fraction smaller than the nut will not go on at all. The only pairing that is genuinely interchangeable across the common range is 3/4 inch against 19 mm, where the wrench is 19.05 mm and the gap is five hundredths of a millimeter.
- 3/8 in is 9.53 mm, on a 10 mm nut
- 0.48 mm too small
- 7/16 in is 11.11 mm, on an 11 mm nut
- 0.11 mm loose
- 1/2 in is 12.70 mm, on a 13 mm nut
- 0.30 mm too small
- 9/16 in is 14.29 mm, on a 14 mm nut
- 0.29 mm loose
- 3/4 in is 19.05 mm, on a 19 mm nut
- 0.05 mm loose
- 7/8 in is 22.23 mm, on a 22 mm nut
- 0.23 mm loose
Every figure is the fraction multiplied by 25.4 and compared with the nominal metric size.
The practical reading is that a quarter of a millimeter of slop is enough to round a soft fastener under load, so the loose pairings are for a nut you can already turn by hand. Anything seized, anything on an engine, and anything with a torque spec deserves the right socket rather than the near one.
Millimeters, thousandths and drawing tolerances
Machinists working in inches do not use fractions for tolerance. They use the thousandth of an inch, spoken as a thou and written 0.001 in. One thou is exactly 0.0254 mm, or 25.4 micrometers, which means a millimeter is 39.37 thou and a tenth of a millimeter is just under four thou.
That relationship is the one worth carrying, because tolerance blocks on a metric drawing are written in hundredths of a millimeter. A tolerance of plus or minus 0.05 mm is about two thou. A tolerance of 0.01 mm is four tenths of a thou, which is finer than most shop measurement and tells you the feature needs grinding rather than milling.
0.0254 mm
One thousandth of an inch
39.37 thou
One millimeter
2 thou
A 0.05 mm drawing tolerance
Sheet and plate stock shows the same split. Metric sheet comes in whole and half millimeter thicknesses while imperial stock follows gauge numbers and fractions, so a 1.5 mm sheet is 0.0591 inches and sits between the imperial gauges rather than on one. Substituting the nearest gauge changes stiffness, which goes as the cube of thickness.
Every millimeters value, worked out
33 common millimeters figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.
Questions people ask
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The inch is defined as exactly 25.4 millimetres.
NIST Guide to the SI, Appendix B.8 — Factors for units listed alphabetically — NIST
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