Inch Fraction to Decimal & mm Chart — 1/64″ to 1″
All 64ths of an inch from 1/64″ to 1″: reduced fraction, decimal inches to 4 places, and exact millimetre equivalents computed at 25.4 mm per inch.
Updated August 18, 2026
This chart lists every 64th of an inch from 1/64″ up to 1″, with the fraction reduced to lowest terms, the decimal-inch equivalent to four places, and the millimetre equivalent to three. Unlike gauge tables or thread charts, nothing here is looked up — every value is pure arithmetic. Since the 1959 international yard and pound agreement, one inch is exactly 25.4 mm by definition, so the mm column is computed as 25.4 × the decimal value, and both columns in this file are generated by the same formula a test re-derives cell by cell.
The 64ths ladder is the working resolution of most inch-based shop measurement: tape measures and rules mark 16ths and 32nds, machinist scales and drill charts go to 64ths, and one 64th is 0.0156 in (0.397 mm) — about the practical limit of reading a scale by eye. Below that, inch work switches to decimals ("thou": 0.001 in = 0.0254 mm). A useful anchor when converting mentally: 1/8″ is almost exactly 3.2 mm, 1/4″ is 6.35 mm, and 1 mm falls between 3/64″ and 1/16″.
Fraction — Decimal — Millimetre, 1/64″ to 1/2″
| Fraction (in) | Decimal (in) | mm |
|---|---|---|
| 1/64″ | 0.0156 | 0.397 |
| 1/32″ | 0.0313 | 0.794 |
| 3/64″ | 0.0469 | 1.191 |
| 1/16″ | 0.0625 | 1.588 |
| 5/64″ | 0.0781 | 1.984 |
| 3/32″ | 0.0938 | 2.381 |
| 7/64″ | 0.1094 | 2.778 |
| 1/8″ | 0.1250 | 3.175 |
| 9/64″ | 0.1406 | 3.572 |
| 5/32″ | 0.1563 | 3.969 |
| 11/64″ | 0.1719 | 4.366 |
| 3/16″ | 0.1875 | 4.763 |
| 13/64″ | 0.2031 | 5.159 |
| 7/32″ | 0.2188 | 5.556 |
| 15/64″ | 0.2344 | 5.953 |
| 1/4″ | 0.2500 | 6.350 |
| 17/64″ | 0.2656 | 6.747 |
| 9/32″ | 0.2813 | 7.144 |
| 19/64″ | 0.2969 | 7.541 |
| 5/16″ | 0.3125 | 7.938 |
| 21/64″ | 0.3281 | 8.334 |
| 11/32″ | 0.3438 | 8.731 |
| 23/64″ | 0.3594 | 9.128 |
| 3/8″ | 0.3750 | 9.525 |
| 25/64″ | 0.3906 | 9.922 |
| 13/32″ | 0.4063 | 10.319 |
| 27/64″ | 0.4219 | 10.716 |
| 7/16″ | 0.4375 | 11.113 |
| 29/64″ | 0.4531 | 11.509 |
| 15/32″ | 0.4688 | 11.906 |
| 31/64″ | 0.4844 | 12.303 |
| 1/2″ | 0.5000 | 12.700 |
Exact by definition: 1 in = 25.4 mm (1959 international inch). Decimal shown to 4 places, mm to 3 places.
Fraction — Decimal — Millimetre, 33/64″ to 1″
| Fraction (in) | Decimal (in) | mm |
|---|---|---|
| 33/64″ | 0.5156 | 13.097 |
| 17/32″ | 0.5313 | 13.494 |
| 35/64″ | 0.5469 | 13.891 |
| 9/16″ | 0.5625 | 14.288 |
| 37/64″ | 0.5781 | 14.684 |
| 19/32″ | 0.5938 | 15.081 |
| 39/64″ | 0.6094 | 15.478 |
| 5/8″ | 0.6250 | 15.875 |
| 41/64″ | 0.6406 | 16.272 |
| 21/32″ | 0.6563 | 16.669 |
| 43/64″ | 0.6719 | 17.066 |
| 11/16″ | 0.6875 | 17.463 |
| 45/64″ | 0.7031 | 17.859 |
| 23/32″ | 0.7188 | 18.256 |
| 47/64″ | 0.7344 | 18.653 |
| 3/4″ | 0.7500 | 19.050 |
| 49/64″ | 0.7656 | 19.447 |
| 25/32″ | 0.7813 | 19.844 |
| 51/64″ | 0.7969 | 20.241 |
| 13/16″ | 0.8125 | 20.638 |
| 53/64″ | 0.8281 | 21.034 |
| 27/32″ | 0.8438 | 21.431 |
| 55/64″ | 0.8594 | 21.828 |
| 7/8″ | 0.8750 | 22.225 |
| 57/64″ | 0.8906 | 22.622 |
| 29/32″ | 0.9063 | 23.019 |
| 59/64″ | 0.9219 | 23.416 |
| 15/16″ | 0.9375 | 23.813 |
| 61/64″ | 0.9531 | 24.209 |
| 31/32″ | 0.9688 | 24.606 |
| 63/64″ | 0.9844 | 25.003 |
| 1″ | 1.0000 | 25.400 |
Computed as n/64 in and 25.4 × n/64 mm — no cell is transcribed from another chart.
Sources & Further Reading
- International yard and pound agreement (1959): 1 in = 25.4 mm exactly, as codified in NIST SP 811 and the Federal Register notice of July 1, 1959 — every cell is computed from this definition, none transcribed
Frequently Asked Questions
Are these conversions exact or rounded?
The underlying values are exact: 1 in = 25.4 mm by definition since 1959, and n/64 is exact binary-fraction arithmetic. Only the display is rounded — decimals to 4 places and millimetres to 3. So 1/64″ is exactly 0.396875 mm, shown as 0.397.
Why do rulers use 64ths instead of decimals?
Halving is the natural way to subdivide a physical scale — each finer graduation splits the previous one in two, giving 2, 4, 8, 16, 32, 64 divisions per inch. 1/64″ (0.0156 in) is about the finest spacing a printed rule can carry and the eye can read; past that, inch work moves to decimal thousandths measured with calipers or micrometers.
What is the quickest way to convert a fraction to millimetres in my head?
Remember two anchors: 1/16″ ≈ 1.6 mm and 1/4″ = 6.35 mm. Scale from there — 3/8″ is 1.5 × 6.35 ≈ 9.5 mm, 3/4″ ≈ 19 mm. For the reverse direction, 1 mm sits just above 3/64″ (1.19 mm is 3/64″ + half a 64th), and 10 mm is a hair under 25/64″.
Which fractions line up closely with common metric drill sizes?
A few near-coincidences are handy: 1/8″ = 3.175 mm (close to a 3.2 mm drill), 5/16″ = 7.938 mm (≈ 8 mm), 1/2″ = 12.7 mm (≈ 12.5 mm), and 25/64″ = 9.922 mm (≈ 10 mm). They are near-misses, not equals — for tapping or press fits use the exact decimal column, not the metric look-alike.
Try the Calculators
Inch Fraction to Decimal & mm Chart — 1/64″ to 1″ — reuven.tools/reference/fraction-decimal-mm — verified against: International yard and pound agreement (1959): 1 in = 25.4 mm exactly, as codified in NIST SP 811 and the Federal Register notice of July 1, 1959 — every cell is computed from this definition, none transcribed