MM to Fraction Calculator
| MM | Exact inch | Nearest 64th | Error |
|---|---|---|---|
| 6 mm | 0.2362 | 15/64 | -0.78% |
| 8 mm | 0.3150 | 5/16 | -0.78% |
| 9 mm | 0.3543 | 23/64 | +1.42% |
| 10 mm | 0.3937 | 25/64 | -0.78% |
| 12 mm | 0.4724 | 15/32 | -0.78% |
| 13 mm | 0.5118 | 33/64 | +0.75% |
| 15 mm | 0.5906 | 19/32 | +0.54% |
| 17 mm | 0.6693 | 43/64 | +0.39% |
| 19 mm | 0.7480 | 3/4 | +0.26% |
| 21 mm | 0.8268 | 53/64 | +0.16% |
| 22 mm | 0.8661 | 55/64 | -0.78% |
| 25 mm | 0.9843 | 63/64 | +0.01% |
Type a millimeter number and the calculator answers the question a workshop actually asks: not 21 mm = 0.8268 inches, but 21 mm is 53/64 to the nearest sixty-fourth - about 0.16 percent under - and 13/16 if you are working to a sixteenth. That is the whole bridge between a metric tool and an imperial workpiece, and the error percentage tells you whether the swap is honest for your tolerance or wishful thinking.
The inch side is exact by treaty, not by luck: the 1959 international yard and pound agreement fixed one inch at exactly 25.4 millimeters in the US, UK, Canada, Australia and New Zealand. So every metric-to-fraction answer comes from one clean division and one rounding decision - no calibration tables, no vintage differences. Anything you read that says 1 inch = 25.40005 mm is pre-1959 survey data, not what your drill bits obey.
Why sixty-fourths and not nicer numbers: imperial length fractions double as they refine - 1/2, 8/16, 32/64 - and machine and wood shops standardize on the 64th as the smallest marked graduation on a rule. Rounding to the nearest 64th caps the worst error at 0.78 percent of whatever you measure (half a 64th, about 0.2 mm at 25 mm), then the fraction reduces itself: 32/64 prints as 1/2, 53/64 stays 53/64 because nothing divides it.
How to use
- Enter the millimeter measurement - sockets, drill shanks, bolt heads, camera sensor widths, anything metric.
- Read the nearest fraction at the scale you work to: 64ths for machining, 32nds for general shop work, 16ths for carpentry.
- Check the error percentage before you commit: under half a percent is invisible in woodworking, over one percent matters for tapped holes and press fits.
Frequently asked questions
Why is 25.4 mm per inch exact?
The 1959 international yard and pound agreement defined the yard as exactly 0.9144 meters, which makes the inch exactly 25.4 millimeters - agreed by the US, UK, Canada, Australia and New Zealand to end a century of slightly different inches. Before that, the US inch and the UK inch differed by about 2 parts per million, enough to matter only in surveying.
Which fraction should I actually use - 64th, 32nd or 16th?
Match the tolerance of the job, not the biggest number. Carpentry and framing live on 16ths (a 64th is thinner than a saw kerf). General shop work and hardware store sizing use 32nds. Machining, drill selection and tapped holes deserve 64ths because a 1/64 error on a 10 mm hole is a third of a millimeter of interference - the calculator shows all three scales plus the exact decimal so you can pick with the error in front of you.
What is 21 mm in inches as a fraction?
21 mm = 0.8268 inches exactly (21 divided by 25.4). Nearest 64th: 53/64 (0.828125, about 0.16 percent over). Nearest 16th: 13/16 (0.8125, about 1.7 percent under) - which is why a 13/16 spark plug socket is the standard stand-in for a 21 mm nut and fits with a little slop.
Why not round to 128ths for even more precision?
Because nothing is marked in 128ths - rules, tapes and drill gauges stop at 64ths. A tighter rounding would give you a fraction you cannot find on a tool. If a job truly needs better than a 64th (about 0.2 mm at 25 mm), the honest move is a digital caliper in decimal millimeters, not a finer fraction.
My drill bit set is fractional and the bolt is metric - how do I pick?
Convert the metric diameter to a fraction, then choose the fractional bit one size under for a close fit or matching for a clearance hole. The classic traps: a 10 mm bolt wants a 25/64 (9.9 mm) close-fit bit, not 3/8 (9.53 mm, half a millimeter loose). The error column is the decision maker - anything over one percent on a drill diameter will be felt.
Is this the same as the decimal inch the calculator shows?
No - and using them interchangeably is a classic shop mistake. 19 mm is 0.748 inches decimal but 3/4 as a fraction; ordering a 0.748 reamer when the drawing says 3/4 means buying a tool 0.5 mm undersize. Decimal and fractional inch systems coexist on purpose: decimal for machining, fractions for wood and hardware. This page shows both and labels the gap.