Unit & Measurement Tables
Fraction to Decimal Inch Table
The inch is divided in halves, not in tenths, and that single decision still shapes how hardware is made. A tape measure reads 1/16, a drill index is labelled in 1/64, and a wrench set runs in 1/16 steps up to an inch. Metric drawings give decimals, so anyone working between the two systems is constantly converting a fraction to a decimal to a millimetre and back.
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The arithmetic is trivial but the notation is not. 32/64 has to be written 1/2, because a machinist reading 32/64 on a drawing will assume something is wrong. Every value in the first table is reduced to lowest terms, which is why the fractions do not march in a straight sequence — 1/2 appears where 32/64 would be.
The conversion to millimetres is exact at the anchor points and messy everywhere else. One inch is 25.4 mm by definition, so 1/64 is 0.396875 mm and 1/16 is 1.5875 mm exactly. But 1 mm is 0.03937 inches, which is not a clean fraction of anything, so converting a metric dimension into inches always produces a rounding decision.
The third table makes that decision explicit. For each common millimetre value it gives the nearest 1/64 and the error that rounding introduces. At 5 mm the error is under 0.02 mm and irrelevant; at 50 mm it is over a third of a millimetre, which is enough to leave a visible gap in a joint. Knowing where the rounding stops being free is more useful than the conversion itself.
Every 1/64 of an inch, in lowest terms
| Inch | Decimal | Millimetres |
|---|---|---|
| 0 in | 0.000000 | 0.000 |
| 1/64 in | 0.015625 | 0.397 |
| 1/32 in | 0.031250 | 0.794 |
| 3/64 in | 0.046875 | 1.191 |
| 1/16 in | 0.062500 | 1.588 |
| 5/64 in | 0.078125 | 1.984 |
| 3/32 in | 0.093750 | 2.381 |
| 7/64 in | 0.109375 | 2.778 |
| 1/8 in | 0.125000 | 3.175 |
| 9/64 in | 0.140625 | 3.572 |
| 5/32 in | 0.156250 | 3.969 |
| 11/64 in | 0.171875 | 4.366 |
| 3/16 in | 0.187500 | 4.762 |
| 13/64 in | 0.203125 | 5.159 |
| 7/32 in | 0.218750 | 5.556 |
| 15/64 in | 0.234375 | 5.953 |
| 1/4 in | 0.250000 | 6.350 |
| 17/64 in | 0.265625 | 6.747 |
| 9/32 in | 0.281250 | 7.144 |
| 19/64 in | 0.296875 | 7.541 |
| 5/16 in | 0.312500 | 7.938 |
| 21/64 in | 0.328125 | 8.334 |
| 11/32 in | 0.343750 | 8.731 |
| 23/64 in | 0.359375 | 9.128 |
| 3/8 in | 0.375000 | 9.525 |
| 25/64 in | 0.390625 | 9.922 |
| 13/32 in | 0.406250 | 10.319 |
| 27/64 in | 0.421875 | 10.716 |
| 7/16 in | 0.437500 | 11.112 |
| 29/64 in | 0.453125 | 11.509 |
| 15/32 in | 0.468750 | 11.906 |
| 31/64 in | 0.484375 | 12.303 |
| 1/2 in | 0.500000 | 12.700 |
| 33/64 in | 0.515625 | 13.097 |
| 17/32 in | 0.531250 | 13.494 |
| 35/64 in | 0.546875 | 13.891 |
| 9/16 in | 0.562500 | 14.288 |
| 37/64 in | 0.578125 | 14.684 |
| 19/32 in | 0.593750 | 15.081 |
| 39/64 in | 0.609375 | 15.478 |
| 5/8 in | 0.625000 | 15.875 |
| 41/64 in | 0.640625 | 16.272 |
| 21/32 in | 0.656250 | 16.669 |
| 43/64 in | 0.671875 | 17.066 |
| 11/16 in | 0.687500 | 17.463 |
| 45/64 in | 0.703125 | 17.859 |
| 23/32 in | 0.718750 | 18.256 |
| 47/64 in | 0.734375 | 18.653 |
| 3/4 in | 0.750000 | 19.050 |
| 49/64 in | 0.765625 | 19.447 |
| 25/32 in | 0.781250 | 19.844 |
| 51/64 in | 0.796875 | 20.241 |
| 13/16 in | 0.812500 | 20.638 |
| 53/64 in | 0.828125 | 21.034 |
| 27/32 in | 0.843750 | 21.431 |
| 55/64 in | 0.859375 | 21.828 |
| 7/8 in | 0.875000 | 22.225 |
| 57/64 in | 0.890625 | 22.622 |
| 29/32 in | 0.906250 | 23.019 |
| 59/64 in | 0.921875 | 23.416 |
| 15/16 in | 0.937500 | 23.813 |
| 61/64 in | 0.953125 | 24.209 |
| 31/32 in | 0.968750 | 24.606 |
| 63/64 in | 0.984375 | 25.003 |
| 1 in | 1.000000 | 25.400 |
1/64 inch is 0.3969 mm. Most tape measures stop at 1/16 and most vernier scales read 1/128, so 1/64 is the practical floor for hand work.
The sixteenths, which is what tape measures actually show
| Inch | Decimal | Millimetres | Closest metric |
|---|---|---|---|
| 0 in | 0.0000 | 0.000 | 0 mm |
| 1/16 in | 0.0625 | 1.588 | 1.6 mm |
| 1/8 in | 0.1250 | 3.175 | 3.2 mm |
| 3/16 in | 0.1875 | 4.762 | 4.8 mm |
| 1/4 in | 0.2500 | 6.350 | 6.4 mm |
| 5/16 in | 0.3125 | 7.938 | 7.9 mm |
| 3/8 in | 0.3750 | 9.525 | 9.5 mm |
| 7/16 in | 0.4375 | 11.112 | 11.1 mm |
| 1/2 in | 0.5000 | 12.700 | 12.7 mm |
| 9/16 in | 0.5625 | 14.288 | 14.3 mm |
| 5/8 in | 0.6250 | 15.875 | 15.9 mm |
| 11/16 in | 0.6875 | 17.463 | 17.5 mm |
| 3/4 in | 0.7500 | 19.050 | 19 mm |
| 13/16 in | 0.8125 | 20.638 | 20.6 mm |
| 7/8 in | 0.8750 | 22.225 | 22.2 mm |
| 15/16 in | 0.9375 | 23.813 | 23.8 mm |
| 1 in | 1.0000 | 25.400 | 25.4 mm |
1/16 inch is 1.5875 mm exactly, so the metric column never repeats and the two systems drift apart by about 0.4 mm every step.
Millimetres to the nearest 1/64 inch
| Millimetres | Exact inches | Nearest 1/64 | Error (mm) |
|---|---|---|---|
| 1 | 0.0394 | 3/64 in | 0.191 |
| 2 | 0.0787 | 5/64 in | 0.016 |
| 3 | 0.1181 | 1/8 in | 0.175 |
| 4 | 0.1575 | 5/32 in | 0.031 |
| 5 | 0.1969 | 13/64 in | 0.159 |
| 6 | 0.2362 | 15/64 in | 0.047 |
| 8 | 0.3150 | 5/16 in | 0.062 |
| 10 | 0.3937 | 25/64 in | 0.078 |
| 12 | 0.4724 | 15/32 in | 0.094 |
| 16 | 0.6299 | 5/8 in | 0.125 |
| 20 | 0.7874 | 25/32 in | 0.156 |
| 25 | 0.9843 | 63/64 in | 0.003 |
| 30 | 1.1811 | 19/16 in | 0.162 |
| 40 | 1.5748 | 101/64 in | 0.084 |
| 50 | 1.9685 | 63/32 in | 0.006 |
The error column is the price of rounding to 1/64. At 50 mm it is over a third of a millimetre, which is enough to open a joint in cabinet work.
Fractions worth memorising
| Fraction | Decimal | Millimetres | Where it appears |
|---|---|---|---|
| 1/32 | 0.0313 | 0.794 | Sheet metal gauges, fine joinery |
| 1/16 | 0.0625 | 1.588 | Tape measure smallest mark |
| 1/8 | 0.1250 | 3.175 | Plywood nominal thickness |
| 3/16 | 0.1875 | 4.763 | Bolts, rivets |
| 1/4 | 0.2500 | 6.350 | Standard dowel, socket drives |
| 5/16 | 0.3125 | 7.938 | Common bolt diameter |
| 3/8 | 0.3750 | 9.525 | Hose fittings, rebar |
| 1/2 | 0.5000 | 12.700 | Nominal half inch pipe |
| 5/8 | 0.6250 | 15.875 | Garden hose, sheet goods |
| 3/4 | 0.7500 | 19.050 | Nominal three quarter pipe |
| 7/8 | 0.8750 | 22.225 | Large bolts, sockets |
| 1 | 1.0000 | 25.400 | The definition of the inch |
Nominal pipe sizes are not the actual diameter: a 1/2 inch pipe measures 0.840 inches outside. The fraction names a trade size, not a dimension.
How to Use These Tables
- Reading a drill index: a 1/4 inch bit is 0.2500 in or 6.350 mm. The metric equivalent is 6.35 mm, and a 6.5 mm bit is a different hole.
- Converting a drawing: multiply the decimal inch by 25.4. 0.375 in is 9.525 mm, which is why 3/8 hardware is often called 9.5 mm in metric catalogues.
- Choosing the nearest stock: 8 mm is closest to 5/16 in (7.938 mm), an error of 0.06 mm. 10 mm is closest to 25/64 in (9.922 mm), which no one stocks.
- Reducing a measured value: 0.4375 in is 28/64, which reduces to 7/16. Always divide by the largest common factor before writing it down.
- Working in both systems: keep one reference dimension in each. 1 inch is 25.4 mm and 1 mm is 0.0394 in; everything else follows by multiplication.
Common Questions
What is 3/8 of an inch in millimetres?
9.525 mm exactly. Multiply 0.375 by 25.4. A 9.5 mm drill is 0.025 mm smaller, which is within tolerance for most work but not for a press fit.
Why do tape measures use sixteenths?
Because halving is easy to mark and to read. A sixteenth is the smallest division most people can resolve at arm's length, and the pattern of progressively shorter marks makes the value readable without counting.
Is 5/16 the same as 8 mm?
Close but not the same: 5/16 inch is 7.938 mm, so an 8 mm hole is 0.062 mm larger. For a bolt that is usually fine; for a bearing seat it is not.
How do I convert a decimal inch to a fraction?
Multiply by 64 and round to the nearest whole number, then reduce. 0.4375 × 64 = 28, and 28/64 reduces to 7/16.
What is 1/64 of an inch in millimetres?
0.3969 mm. It is the finest division on a standard rule and the smallest step in most imperial drill indexes.
Why is a 1/2 inch pipe not half an inch across?
Because pipe sizes are nominal trade sizes inherited from the inside diameter of old iron pipe. A 1/2 inch pipe measures 0.840 inches outside, and the wall thickness decides the bore.
More Unit & Measurement Tables
Sources
- NIST Handbook 44 — Appendix C, units of measurement and inch fractions — National Institute of Standards and Technology
- ASME B1.1 — Unified inch screw threads — American Society of Mechanical Engineers
- ISO 286-1 — Limits and fits, ISO system of tolerances — International Organization for Standardization
Filed with Unit Conversion calculations.