12mm in inches
0.4724"
≈ 15/32" (0.4688")
Suspension links and accessory pulleys. Sits almost exactly on 15/32", a size most SAE sets skip entirely.
mm to Inches Conversion: Millimetres to Fractional Inches Chart
Millimetres to Inches Calculator
mm
- Decimal inches
- 0.5512"
- mm ÷ 25.4
- Nearest fraction
- 35/64"
- 0.5469"
- Difference
- +0.0043"
- exact − fraction
14mm equals 0.5512 inches, closest to 35/64" (0.5469"), a difference of +0.0043".
Quick sizes
Decimal inches are exact; the fraction is the nearest 64th, and the difference column shows exactly how much the fraction rounds away.
To convert millimetres to inches, divide by 25.4, one millimetre equals 0.03937 inches. The divisor is exact, not rounded: the inch has been defined as precisely 25.4mm since the 1959 international yard and pound agreement, so every conversion on this page is a definition rather than a measurement. The awkward part is the second step. Decimal inches convert cleanly, but the fractional inches stamped on tools and called out on older drawings come in 64ths, and a millimetre value almost never lands on one. This chart gives both: the exact decimal, and the nearest fraction with the error that fraction introduces. For fastener work specifically, the socket conversion calculator takes the extra step of snapping to sizes that actually exist in a tool set.
Millimetres to Inches: 1mm to 100mm
Every whole millimetre from 1 to 100, with exact decimal inches, the nearest 64th, the rounding error, and a flag on the values that are also real socket sizes.
| Millimetres | Decimal inches | Nearest fraction | Difference | Tool size? |
|---|---|---|---|---|
| 1mm | 0.0394 | 3/64" | −0.0075 | N/A |
| 2mm | 0.0787 | 5/64" | +0.0006 | N/A |
| 3mm | 0.1181 | 1/8" | −0.0069 | N/A |
| 4mm | 0.1575 | 5/32" | +0.0012 | ✓ socket size |
| 5mm | 0.1969 | 13/64" | −0.0063 | ✓ socket size |
| 6mm | 0.2362 | 15/64" | +0.0018 | ✓ socket size |
| 7mm | 0.2756 | 9/32" | −0.0057 | ✓ socket size |
| 8mm | 0.3150 | 5/16" | +0.0025 | ✓ socket size |
| 9mm | 0.3543 | 23/64" | −0.0050 | ✓ socket size |
| 10mm | 0.3937 | 25/64" | +0.0031 | ✓ socket size |
| 11mm | 0.4331 | 7/16" | −0.0044 | ✓ socket size |
| 12mm | 0.4724 | 15/32" | +0.0037 | ✓ socket size |
| 13mm | 0.5118 | 33/64" | −0.0038 | ✓ socket size |
| 14mm | 0.5512 | 35/64" | +0.0043 | ✓ socket size |
| 15mm | 0.5906 | 19/32" | −0.0032 | ✓ socket size |
| 16mm | 0.6299 | 5/8" | +0.0049 | ✓ socket size |
| 17mm | 0.6693 | 43/64" | −0.0026 | ✓ socket size |
| 18mm | 0.7087 | 45/64" | +0.0055 | ✓ socket size |
| 19mm | 0.7480 | 3/4" | −0.0020 | ✓ socket size |
| 20mm | 0.7874 | 25/32" | +0.0062 | ✓ socket size |
| 21mm | 0.8268 | 53/64" | −0.0014 | ✓ socket size |
| 22mm | 0.8661 | 55/64" | +0.0068 | ✓ socket size |
| 23mm | 0.9055 | 29/32" | −0.0007 | ✓ socket size |
| 24mm | 0.9449 | 15/16" | +0.0074 | ✓ socket size |
| 25mm | 0.9843 | 63/64" | −0.0001 | ✓ socket size |
| 26mm | 1.0236 | 1-1/32" | −0.0076 | ✓ socket size |
| 27mm | 1.0630 | 1-1/16" | +0.0005 | ✓ socket size |
| 28mm | 1.1024 | 1-7/64" | −0.0070 | ✓ socket size |
| 29mm | 1.1417 | 1-9/64" | +0.0011 | ✓ socket size |
| 30mm | 1.1811 | 1-3/16" | −0.0064 | ✓ socket size |
| 31mm | 1.2205 | 1-7/32" | +0.0017 | ✓ socket size |
| 32mm | 1.2598 | 1-17/64" | −0.0058 | ✓ socket size |
| 33mm | 1.2992 | 1-19/64" | +0.0023 | ✓ socket size |
| 34mm | 1.3386 | 1-11/32" | −0.0052 | ✓ socket size |
| 35mm | 1.3780 | 1-3/8" | +0.0030 | ✓ socket size |
| 36mm | 1.4173 | 1-27/64" | −0.0046 | ✓ socket size |
| 37mm | 1.4567 | 1-29/64" | +0.0036 | N/A |
| 38mm | 1.4961 | 1-1/2" | −0.0039 | ✓ socket size |
| 39mm | 1.5354 | 1-17/32" | +0.0042 | ✓ socket size |
| 40mm | 1.5748 | 1-37/64" | −0.0033 | ✓ socket size |
| 41mm | 1.6142 | 1-39/64" | +0.0048 | ✓ socket size |
| 42mm | 1.6535 | 1-21/32" | −0.0027 | N/A |
| 43mm | 1.6929 | 1-11/16" | +0.0054 | N/A |
| 44mm | 1.7323 | 1-47/64" | −0.0021 | N/A |
| 45mm | 1.7717 | 1-49/64" | +0.0060 | N/A |
| 46mm | 1.8110 | 1-13/16" | −0.0015 | ✓ socket size |
| 47mm | 1.8504 | 1-27/32" | +0.0066 | N/A |
| 48mm | 1.8898 | 1-57/64" | −0.0009 | N/A |
| 49mm | 1.9291 | 1-59/64" | +0.0073 | N/A |
| 50mm | 1.9685 | 1-31/32" | −0.0002 | ✓ socket size |
| 51mm | 2.0079 | 2" | +0.0079 | ✓ socket size |
| 52mm | 2.0472 | 2" | +0.0472 | N/A |
| 53mm | 2.0866 | 2" | +0.0866 | N/A |
| 54mm | 2.1260 | 2" | +0.1260 | ✓ socket size |
| 55mm | 2.1654 | 2" | +0.1654 | ✓ socket size |
| 56mm | 2.2047 | 2" | +0.2047 | N/A |
| 57mm | 2.2441 | 2" | +0.2441 | ✓ socket size |
| 58mm | 2.2835 | 2" | +0.2835 | ✓ socket size |
| 59mm | 2.3228 | 2" | +0.3228 | N/A |
| 60mm | 2.3622 | 2" | +0.3622 | ✓ socket size |
| 61mm | 2.4016 | 2" | +0.4016 | N/A |
| 62mm | 2.4409 | 2" | +0.4409 | N/A |
| 63mm | 2.4803 | 2" | +0.4803 | ✓ socket size |
| 64mm | 2.5197 | 2" | +0.5197 | N/A |
| 65mm | 2.5591 | 2" | +0.5591 | N/A |
| 66mm | 2.5984 | 2" | +0.5984 | N/A |
| 67mm | 2.6378 | 2" | +0.6378 | N/A |
| 68mm | 2.6772 | 2" | +0.6772 | N/A |
| 69mm | 2.7165 | 2" | +0.7165 | N/A |
| 70mm | 2.7559 | 2" | +0.7559 | N/A |
| 71mm | 2.7953 | 2" | +0.7953 | N/A |
| 72mm | 2.8346 | 2" | +0.8346 | N/A |
| 73mm | 2.8740 | 2" | +0.8740 | N/A |
| 74mm | 2.9134 | 2" | +0.9134 | N/A |
| 75mm | 2.9528 | 2" | +0.9528 | N/A |
| 76mm | 2.9921 | 2" | +0.9921 | N/A |
| 77mm | 3.0315 | 2" | +1.0315 | N/A |
| 78mm | 3.0709 | 2" | +1.0709 | N/A |
| 79mm | 3.1102 | 2" | +1.1102 | N/A |
| 80mm | 3.1496 | 2" | +1.1496 | N/A |
| 81mm | 3.1890 | 2" | +1.1890 | N/A |
| 82mm | 3.2283 | 2" | +1.2283 | N/A |
| 83mm | 3.2677 | 2" | +1.2677 | N/A |
| 84mm | 3.3071 | 2" | +1.3071 | N/A |
| 85mm | 3.3465 | 2" | +1.3465 | N/A |
| 86mm | 3.3858 | 2" | +1.3858 | N/A |
| 87mm | 3.4252 | 2" | +1.4252 | N/A |
| 88mm | 3.4646 | 2" | +1.4646 | N/A |
| 89mm | 3.5039 | 2" | +1.5039 | N/A |
| 90mm | 3.5433 | 2" | +1.5433 | N/A |
| 91mm | 3.5827 | 2" | +1.5827 | N/A |
| 92mm | 3.6220 | 2" | +1.6220 | N/A |
| 93mm | 3.6614 | 2" | +1.6614 | N/A |
| 94mm | 3.7008 | 2" | +1.7008 | N/A |
| 95mm | 3.7402 | 2" | +1.7402 | N/A |
| 96mm | 3.7795 | 2" | +1.7795 | N/A |
| 97mm | 3.8189 | 2" | +1.8189 | N/A |
| 98mm | 3.8583 | 2" | +1.8583 | N/A |
| 99mm | 3.8976 | 2" | +1.8976 | N/A |
| 100mm | 3.9370 | 2" | +1.9370 | N/A |
Reading down the Difference column shows something useful: the fractional error has no pattern. 25mm lands within 0.0001" of 63/64", while 26mm misses its nearest fraction by nearly 0.008". That is the practical consequence of two systems built on different bases, closeness is coincidence, never structure. The final column marks the 47 millimetre values that exist as sockets; everything else on the chart is a measurement rather than a tool.
The Lookups People Search For Most
18mm in inches
0.7087"
≈ 45/64" (0.7031")
Crank pulley and camshaft sprocket bolts. Falls between 11/16" and 23/32" with no good SAE substitute.
30mm in inches
1.1811"
≈ 1-3/16" (1.1875")
Front hub and spindle nuts. Lands near 1-3/16", comfortably inside a 1/2" drive socket set.
Those three cards state the decimal first because that is the exact answer; the fraction underneath is the approximation. It is worth internalising the direction of the error, 12mm is slightly over 15/32", 18mm is slightly over 11/16", and a fastener that is slightly over the tool size is one that a tool will not seat on properly. Whether that overhang actually matters depends on how large it is, which is what the socket interchangeability calculator rates directly for any pairing.
Worked Example: 5/8" Back to Millimetres
Running the conversion the other way keeps the arithmetic honest. Start with 5/8". Divide 5 by 8 to get 0.625 inches. Multiply by 25.4 to get 15.875mm. The closest socket in a metric set is 16mm, which sits 0.125mm oversize, about 0.005 inches, right on the boundary of a free swap. So a 16mm socket will turn a 5/8" fastener, and a 5/8" socket will turn a 16mm fastener, provided it is a 6-point and the joint is not torque-critical. That single example contains the whole method: divide to a decimal, multiply by 25.4, then find the tool that exists. The SAE to metric chart has this worked out for every fractional size, and the fraction to decimal reference covers the first step alone down to a 64th.
Length Reference Beyond Fastener Sizes
The same divisor covers any length, not just tool sizes.
| Imperial | Metric | Notes |
|---|---|---|
| 1 inch | 25.4mm exactly | The defining relationship. Every other row derives from it |
| 1 foot | 304.8mm | 12 × 25.4 |
| 1 yard | 914.4mm | 36 × 25.4 |
| 1/64 inch | 0.397mm | The finest graduation on a typical rule |
| 1 thou (0.001") | 0.0254mm | Feeler gauge and machining territory |
| 1 metre | 39.37 inches | Just over 3 feet 3 inches |
The thou row is the one that matters for fastener work. A thousandth of an inch is 0.0254mm, so the ±0.005" band that separates a safe socket substitution from a risky one is only about 0.13mm, roughly the thickness of a sheet of paper. That is how little slack it takes to move a fastener from gripped to rounded, and it is why the wrench chart applies an even tighter band than the socket chart does.
Frequently Asked Questions
The millimetre-to-inch questions that come up most often in the workshop.
How many inches is 1mm?
1mm is 0.03937 inches, which is just under 1/25th of an inch. The relationship is fixed by definition: one inch equals exactly 25.4mm, so one millimetre equals 1 ÷ 25.4 inches. The nearest common fraction is 3/64" at 0.0469", which overshoots by more than 20 per cent, millimetres below about 6mm rarely have a useful fractional equivalent.
What is 25mm in inches?
25mm is 0.9843 inches, just under one inch. The nearest fraction is 63/64" at 0.9844", which is within 0.0001". One of the closest fraction matches anywhere on the chart. In tooling terms 25mm and 1" are close enough to look interchangeable and far enough apart at 0.4mm to round a fastener, which is why 25mm rates Not Recommended against a 1" socket.
Is 10mm the same as 3/8 inch?
No. 10mm is 0.3937" and 3/8" is 0.375", a gap of 0.0187 inches. That is roughly four times the safe substitution band for a socket and nearly five times the band for a wrench. 10mm is the most common bolt head on a modern vehicle and it has no workable SAE equivalent, which is the single strongest argument for owning both sets.
How do I convert mm to a fraction of an inch?
Divide the millimetre figure by 25.4 to get decimal inches, then multiply the decimal by 64 and round to the nearest whole number to get the count of 64ths. 14mm ÷ 25.4 = 0.5512", and 0.5512 × 64 = 35.3, so 35/64" is the nearest fraction. Reduce it if it divides: 36/64" would reduce to 9/16".
Why do drawings use decimal inches instead of fractions?
Because fractions cannot express arbitrary precision. A drawing calling for 0.6875" is exactly 11/16", but one calling for 0.6900" has no fractional equivalent at all, the nearest 64th is 0.6875" and it is 0.0025" out. Engineering drawings work in decimals and let the machinist decide which tool comes closest, which is the same problem this chart solves in the other direction.
More Conversion Tools
The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.
Bolt Size Chart Bolt head dimensions matched to the socket that fits them Drill Size Chart Number, letter, fractional and metric drill bit sizes Tap Drill Chart Correct drill size before cutting metric or SAE threads Thread Size Chart Metric, UNC, UNF and Whitworth thread standards compared Impact Socket Sizes Impact-rated socket sizing for 1/4 through 3/4 inch drives Deep Socket Sizes Deep well socket lengths for recessed fasteners Drive Size Adapters Stepping between 1/4, 3/8, 1/2 and 3/4 inch drive sizes Allen Wrench Sizes Metric and SAE hex key equivalents