Worked example
A drawing calls for a 1 3/8 in bore. As a decimal, 3/8 = 0.375, so the bore is 1.375 in. Multiply by 25.4: 1.375 × 25.4 = 34.925 mm. The other direction: a 10 mm pin is 10 / 25.4 = 0.393701 in, which is just over 25/64 in (0.390625 in = 9.92 mm) and just under 13/32 in (0.40625 in = 10.32 mm) – no common fraction matches it, which is why metric and fractional-inch parts rarely swap.
Fractions of an inch in decimal inches and millimetres
Every fraction in steps of 1/64 inch up to one inch. The millimetre column is exact, not rounded.
| Fraction (in) | Decimal (in) | Millimetres |
|---|---|---|
| 1/64 | 0.015625 | 0.396875 |
| 1/32 | 0.03125 | 0.79375 |
| 3/64 | 0.046875 | 1.190625 |
| 1/16 | 0.0625 | 1.5875 |
| 5/64 | 0.078125 | 1.984375 |
| 3/32 | 0.09375 | 2.38125 |
| 7/64 | 0.109375 | 2.778125 |
| 1/8 | 0.125 | 3.175 |
| 9/64 | 0.140625 | 3.571875 |
| 5/32 | 0.15625 | 3.96875 |
| 11/64 | 0.171875 | 4.365625 |
| 3/16 | 0.1875 | 4.7625 |
| 13/64 | 0.203125 | 5.159375 |
| 7/32 | 0.21875 | 5.55625 |
| 15/64 | 0.234375 | 5.953125 |
| 1/4 | 0.25 | 6.35 |
| 17/64 | 0.265625 | 6.746875 |
| 9/32 | 0.28125 | 7.14375 |
| 19/64 | 0.296875 | 7.540625 |
| 5/16 | 0.3125 | 7.9375 |
| 21/64 | 0.328125 | 8.334375 |
| 11/32 | 0.34375 | 8.73125 |
| 23/64 | 0.359375 | 9.128125 |
| 3/8 | 0.375 | 9.525 |
| 25/64 | 0.390625 | 9.921875 |
| 13/32 | 0.40625 | 10.31875 |
| 27/64 | 0.421875 | 10.715625 |
| 7/16 | 0.4375 | 11.1125 |
| 29/64 | 0.453125 | 11.509375 |
| 15/32 | 0.46875 | 11.90625 |
| 31/64 | 0.484375 | 12.303125 |
| 1/2 | 0.5 | 12.7 |
| 33/64 | 0.515625 | 13.096875 |
| 17/32 | 0.53125 | 13.49375 |
| 35/64 | 0.546875 | 13.890625 |
| 9/16 | 0.5625 | 14.2875 |
| 37/64 | 0.578125 | 14.684375 |
| 19/32 | 0.59375 | 15.08125 |
| 39/64 | 0.609375 | 15.478125 |
| 5/8 | 0.625 | 15.875 |
| 41/64 | 0.640625 | 16.271875 |
| 21/32 | 0.65625 | 16.66875 |
| 43/64 | 0.671875 | 17.065625 |
| 11/16 | 0.6875 | 17.4625 |
| 45/64 | 0.703125 | 17.859375 |
| 23/32 | 0.71875 | 18.25625 |
| 47/64 | 0.734375 | 18.653125 |
| 3/4 | 0.75 | 19.05 |
| 49/64 | 0.765625 | 19.446875 |
| 25/32 | 0.78125 | 19.84375 |
| 51/64 | 0.796875 | 20.240625 |
| 13/16 | 0.8125 | 20.6375 |
| 53/64 | 0.828125 | 21.034375 |
| 27/32 | 0.84375 | 21.43125 |
| 55/64 | 0.859375 | 21.828125 |
| 7/8 | 0.875 | 22.225 |
| 57/64 | 0.890625 | 22.621875 |
| 29/32 | 0.90625 | 23.01875 |
| 59/64 | 0.921875 | 23.415625 |
| 15/16 | 0.9375 | 23.8125 |
| 61/64 | 0.953125 | 24.209375 |
| 31/32 | 0.96875 | 24.60625 |
| 63/64 | 0.984375 | 25.003125 |
| 1/1 | 1 | 25.4 |
Whole inches to millimetres
| Inches | Millimetres |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 7 | 177.8 |
| 8 | 203.2 |
| 9 | 228.6 |
| 10 | 254 |
| 11 | 279.4 |
| 12 | 304.8 |
| 13 | 330.2 |
| 14 | 355.6 |
| 15 | 381 |
| 16 | 406.4 |
| 17 | 431.8 |
| 18 | 457.2 |
| 19 | 482.6 |
| 20 | 508 |
| 21 | 533.4 |
| 22 | 558.8 |
| 23 | 584.2 |
| 24 | 609.6 |
Common millimetre sizes in inches
| Millimetres | Inches (5 decimals) |
|---|---|
| 1 | 0.03937 |
| 2 | 0.07874 |
| 3 | 0.11811 |
| 4 | 0.15748 |
| 5 | 0.19685 |
| 6 | 0.23622 |
| 8 | 0.31496 |
| 10 | 0.39370 |
| 12 | 0.47244 |
| 15 | 0.59055 |
| 16 | 0.62992 |
| 20 | 0.78740 |
| 25 | 0.98425 |
| 30 | 1.18110 |
| 40 | 1.57480 |
| 50 | 1.96850 |
| 100 | 3.93701 |
Tips for the shop floor
- Convert once, at the end. Round only the final number; converting a rounded value and rounding again can add up to a few microns.
- Match the tolerance to the unit. A tolerance of ±0.005 in is ±0.127 mm – do not write ±0.13 unless the drawing allows the rounding.
- Check the machine's unit mode. Many controls take G20 for inch and G21 for millimetre; a program run in the wrong mode is out by a factor of 25.4.
Frequently asked questions
How many millimetres are in an inch?
Exactly 25.4. Since 1959 the international inch is defined as 25.4 millimetres, so 1 in = 25.4 mm and 1 mm = 0.0393701 in (rounded).
How do I convert a fraction of an inch to mm?
Divide the fraction to get a decimal and multiply by 25.4. For 3/8 in: 3 ÷ 8 = 0.375, and 0.375 × 25.4 = 9.525 mm. The calculator accepts fractions such as 1 3/8 or 3/4 directly.
What is 1/64 inch in mm?
1/64 in is 0.015625 in, which is 0.396875 mm. Every other 64th is a multiple of that: 32/64 (1/2 in) is exactly 12.7 mm.
How many thou is a millimetre?
One millimetre is 39.3701 thou (thousandths of an inch); one thou is exactly 0.0254 mm, or 25.4 micrometres.
Why does the control show the wrong size after changing units?
Most controls switch between inch and millimetre with G20 and G21. A program written for one unit and run in the other is out by a factor of 25.4.
More free machinist calculators
- RPM and feed – Calculate spindle RPM from surface speed and diameter, surface speed from RPM, and feed rate from chip load – metric and imperial, with formulas.
- Taper – Taper ratio, half angle and taper per foot from the large and small diameter and the length, or the small diameter from the angle.
- Bolt circle – X and Y coordinates of evenly spaced holes on a bolt circle, with the hole spacing, start angle and centre offset. Copy the table into your program.
- Sine bar – Gauge block height for a sine bar angle, or the angle from a known stack, for 5 in, 10 in, 100 mm, 200 mm or any other bar length.
- Metal removal rate – Metal removal rate in cm³/min or in³/min from depth, width and feed – milling and turning, metric and imperial, with the formulas.
- Drill point depth – Length of a drill's point cone from diameter and point angle, and the total depth needed for a full-diameter hole to depth.
- Right triangle – Hypotenuse and angles from two legs, or the legs from the hypotenuse and an angle – for chamfers, dovetails, tapers and hole positions.
- Machining time – Cutting time from length, feed and spindle speed – turning with feed per revolution, milling with feed rate and number of passes.
- Decimal to fraction – Turn a decimal inch or a millimetre value into the nearest fraction – 8ths to 128ths – with the error in millimetres.
- Degrees to DMS – Convert decimal degrees to degrees, minutes and seconds and back, with the angle in radians.
- SFM to m/min – Convert surface feet per minute to metres per minute and back, and inches per minute to mm per minute, with the exact factors.
- TPI to pitch – Convert a thread pitch in millimetres to threads per inch (TPI) and back, for matching metric and inch threads.
- Cpk calculator – Paste measurements from Excel, enter USL and LSL and get Cp, Cpk, Pp and Ppk. Says plainly when 25 values are too few for an index.
- Tolerance stack-up – Add up the tolerances of a chain of parts: closing gap, worst case and statistical RSS for up to eight dimensions. Free, in your browser.
- Fit clearance – Enter hole and shaft limits and get minimum and maximum clearance and the kind of fit – clearance, transition or interference.
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Making parts for a customer who wants paperwork?
Automotive and aerospace customers ask for proof that the parts match the drawing. See first article inspection (FAI), PPAP explained and the true position calculator.