Inch Fraction to Decimal and Millimeter Chart

Every 1/64 inch from 1/64 to 1 inch in decimal inches and millimeters.

Drawings, drill bits, tape measures and lumber use fractions of an inch; machines, calipers and calculations use decimals. This chart converts every sixty-fourth of an inch to a decimal inch and to millimeters. One inch is exactly 25.4 mm, so every millimeter value is exact to the digits shown.

The table

64 rows
Fraction of an inchDecimal inchMillimeters
1/640.0156250.3969
1/320.031250.7938
3/640.0468751.1906
1/160.06251.5875
5/640.0781251.9844
3/320.093752.3813
7/640.1093752.7781
1/80.1253.175
9/640.1406253.5719
5/320.156253.9688
11/640.1718754.3656
3/160.18754.7625
13/640.2031255.1594
7/320.218755.5563
15/640.2343755.9531
1/40.256.35
17/640.2656256.7469
9/320.281257.1438
19/640.2968757.5406
5/160.31257.9375
21/640.3281258.3344
11/320.343758.7313
23/640.3593759.1281
3/80.3759.525
25/640.3906259.9219
13/320.4062510.3188
27/640.42187510.7156
7/160.437511.1125
29/640.45312511.5094
15/320.4687511.9063
31/640.48437512.3031
1/20.512.7
33/640.51562513.0969
17/320.5312513.4938
35/640.54687513.8906
9/160.562514.2875
37/640.57812514.6844
19/320.5937515.0813
39/640.60937515.4781
5/80.62515.875
41/640.64062516.2719
21/320.6562516.6688
43/640.67187517.0656
11/160.687517.4625
45/640.70312517.8594
23/320.7187518.2563
47/640.73437518.6531
3/40.7519.05
49/640.76562519.4469
25/320.7812519.8438
51/640.79687520.2406
13/160.812520.6375
53/640.82812521.0344
27/320.8437521.4313
55/640.85937521.8281
7/80.87522.225
57/640.89062522.6219
29/320.9062523.0188
59/640.92187523.4156
15/160.937523.8125
61/640.95312524.2094
31/320.9687524.6063
63/640.98437525.0031
1/1125.4

Source and notes: 1 inch = 25.4 mm exactly.

How to use this table

  1. Find the fraction (it is reduced, so 8/64 appears as 1/8).
  2. Read the decimal inch for calculations and the millimeters for metric parts.
  3. To go the other way, filter on the decimal: the nearest rows are the closest fractions.

Worked example

  • 5/16 inch = 0.3125 inch = 7.938 mm.
  • 11/32 inch = 0.34375 inch = 8.731 mm.
  • To convert any decimal to a fraction in 64ths, multiply by 64 and round: 0.40 × 64 = 25.6, so about 26/64 = 13/32 inch.

Notes and cautions

  • Sizes finer than 1/64 inch are normally written as decimals (thousandths of an inch).
  • For drill sizes, including numbered and letter drills, use the tap drill and drill finder tools.

FAQ

How do I convert a fraction of an inch to a decimal?

Divide the numerator by the denominator. For 5/16, 5 ÷ 16 = 0.3125 inch.

How many millimeters is an inch?

Exactly 25.4 mm. Multiply inches by 25.4 to get millimeters.

Sources

  • NIST Handbook 44, Appendix C: the international inch is 25.4 mm exactly.
  • 1 inch = 25.4 mm exactly.