Convert Millimeters to Inches
Translate millimeter precision into fractional inches for engineering drawings, jewelry sizing, watch bands and printer paper.
1 mm = 0.03937 in
One millimetre equals approximately 0.0393701 inches, since an inch is defined as exactly 25.4 mm. To convert millimetres to inches, divide by 25.4 — so 100 mm = 3.94 in and 25.4 mm = exactly 1 in. Millimetres are the standard unit for engineering tolerance work because they express sub-inch precision without fractions. The conversions worth memorising are the common fractions: 1/16 in is 1.59 mm, 1/8 in is 3.18 mm, 1/4 in is 6.35 mm and 1/2 in is 12.7 mm. This matters most with fasteners and tooling, where metric and imperial sizes are close enough to appear interchangeable and are not — a 10 mm socket and a 3/8 in socket differ by 0.48 mm, which is enough to round off a bolt head. Timber is a separate trap, since nominal sizes are not actual dimensions in either system.
Quick reference: millimeters to inches
| Millimeters | Inches |
|---|---|
| 1 | 0.0394 in |
| 5 | 0.1969 in |
| 6 | 0.2362 in |
| 10 | 0.3937 in |
| 12.7 | 0.5 in (exact) |
| 25.4 | 1 in (exact) |
| 50 | 1.9685 in |
| 100 | 3.9370 in |
| 300 | 11.811 in |
How do you convert millimeters to inches?
Formula: inches = millimetres ÷ 25.4 • A 6 mm drill bit → 6 ÷ 25.4 = 0.2362 in, closest to a 15/64 in bit at 0.2344 in rather than the 1/4 in many people reach for. • An M8 bolt with an 8 mm shank → 8 ÷ 25.4 = 0.315 in, which is between 5/16 in (0.3125) and 21/64 in — not interchangeable with either. • A 1.75 mm 3D-printer filament → 1.75 ÷ 25.4 = 0.0689 in, the figure a US-built extruder specification might quote.
When and why people use millimeters to inches
Workshop and fabrication work is the main driver. Metric and imperial hardware coexist in almost every toolbox, and knowing that a 13 mm spanner is 0.512 in — very close to but not the same as 1/2 in at 0.500 in — is the difference between a clean fit and a rounded fastener. 3D printing and CNC are a second. Model files are usually authored in millimetres while some machine documentation, nozzle sizes and stock material thicknesses are given in inches. Converting at the design stage keeps the whole toolchain consistent. The third case is product specification. Phone thickness, watch case diameter, lens filter threads and PCB dimensions are published in millimetres worldwide, so anyone comparing against an inch-based specification — a US case, mount or enclosure — needs the conversion to check clearance before ordering.
Common mistakes to watch for when converting millimeters to inches
The biggest practical error is assuming near-equal metric and imperial fasteners are interchangeable. A 13 mm nut on a 1/2 in bolt will start to turn and then strip, because the 0.012 in difference is enough to round the corners. The second issue is precision: dividing by 25 instead of 25.4 introduces a 1.6% error, which on a 200 mm part is over 3 mm — far beyond any machining tolerance. Third, converting to a decimal inch and then hunting for the nearest fraction loses information; 0.2362 in sits between two standard bits and picking the larger one leaves a loose hole. Finally, some drawings specify millimetres to one decimal place as a tolerance statement, and converting to inches at more decimal places implies a precision the original never claimed.
How to estimate millimeters to inches in your head
Divide by 25 and then subtract 1.5%. For 100 mm: 100 ÷ 25 = 4, minus 0.06 gives 3.94 inches, matching the true 3.937. For workshop use the more valuable trick is fractional recognition rather than decimal conversion. 25 mm is almost exactly 1 inch, 12.5 mm is almost exactly half an inch, 6 mm is close to a quarter inch (6.35), and 3 mm is close to an eighth (3.175). Each of these is a couple of hundredths of a millimetre off, which is fine for identifying a fastener and not fine for machining a fit. The important discipline is knowing when to stop estimating. Anything involving a hole, a thread or a press fit needs the exact 25.4 divisor, because the gap between 6 mm and a quarter inch is 0.35 mm — enough to make a bolt refuse to enter or a shaft spin loose.
Millimeters to Inches FAQ
How many inches is 1 mm?
- 0.0393701 in. The inch is defined as exactly 25.4 mm, so the conversion carries no measurement uncertainty.
Is a 13 mm spanner the same as 1/2 inch?
- No. 13 mm is 0.512 in against 0.500 in for the half-inch. The 0.012 in gap is enough to round off a fastener.
How many mm is half an inch?
- Exactly 12.7 mm.
Why not just divide by 25?
- That is a 1.6% error. On a 200 mm dimension it is more than 3 mm, which no machining tolerance would accept.
What imperial drill bit matches 6 mm?
- 15/64 in at 0.2344 in is closest. A 1/4 in bit at 0.250 in would leave the hole noticeably oversized.
Are M8 and 5/16 inch bolts interchangeable?
- No. M8 is 0.315 in and 5/16 in is 0.3125 in. They will cross-thread.
How precise should my converted figure be?
- Match the source. If a drawing gives one decimal place in millimetres, quoting five decimals in inches implies precision the original never claimed.
Does the tool store what I enter?
- No. The calculation happens in your browser and nothing is transmitted.
Last updated:
Sources: 1959 international yard and pound agreement defining the inch as exactly 25.4 mm; ISO 261 general-purpose metric screw threads; ANSI/ASME B18 fastener standards.