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Decimal Equivalents Chart: Fractions to Decimal and mm

Guide · Updated October 28, 2026

A decimal equivalent chart turns a fraction on a print or drill into a number you can dial on a mic, DRO or CNC control. Every value below is exact arithmetic: the fraction divided out, then multiplied by 25.4 for millimeters, because one inch is defined as exactly 25.4 mm (international yard and pound agreement, 1959). Fractions in 64ths terminate in six decimal places; the chart rounds to four places in inches, which is finer than a 0.001 in mic reads, and to three places in mm.

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Step by step

Reduce the fraction: Write it in its lowest terms, or in 64ths if you want the chart row.
Divide: Numerator divided by denominator gives decimal inches, for example 7/16 = 0.4375.
Convert to mm: Multiply decimal inches by 25.4; 0.4375 × 25.4 = 11.1125 mm.
Go the other way: Divide millimeters by 25.4 to get inches.
Round sensibly: Round to the resolution of the tool you will use, not further.
Pick the nearest fraction: For a decimal, multiply by 64 and round to get the nearest 64th.

Fractions of an inch: 1/64 to 1 in

1/64 in = 0.0156 in = 0.397 mm
1/32 in = 0.0312 in = 0.794 mm
3/64 in = 0.0469 in = 1.191 mm
1/16 in = 0.0625 in = 1.587 mm
5/64 in = 0.0781 in = 1.984 mm
3/32 in = 0.0938 in = 2.381 mm
7/64 in = 0.1094 in = 2.778 mm
1/8 in = 0.1250 in = 3.175 mm
9/64 in = 0.1406 in = 3.572 mm
5/32 in = 0.1562 in = 3.969 mm
11/64 in = 0.1719 in = 4.366 mm
3/16 in = 0.1875 in = 4.762 mm
13/64 in = 0.2031 in = 5.159 mm
7/32 in = 0.2188 in = 5.556 mm
15/64 in = 0.2344 in = 5.953 mm
1/4 in = 0.2500 in = 6.350 mm
17/64 in = 0.2656 in = 6.747 mm
9/32 in = 0.2812 in = 7.144 mm
19/64 in = 0.2969 in = 7.541 mm
5/16 in = 0.3125 in = 7.938 mm
21/64 in = 0.3281 in = 8.334 mm
11/32 in = 0.3438 in = 8.731 mm
23/64 in = 0.3594 in = 9.128 mm
3/8 in = 0.3750 in = 9.525 mm
25/64 in = 0.3906 in = 9.922 mm
13/32 in = 0.4062 in = 10.319 mm
27/64 in = 0.4219 in = 10.716 mm
7/16 in = 0.4375 in = 11.112 mm
29/64 in = 0.4531 in = 11.509 mm
15/32 in = 0.4688 in = 11.906 mm
31/64 in = 0.4844 in = 12.303 mm
1/2 in = 0.5000 in = 12.700 mm
33/64 in = 0.5156 in = 13.097 mm
17/32 in = 0.5312 in = 13.494 mm
35/64 in = 0.5469 in = 13.891 mm
9/16 in = 0.5625 in = 14.287 mm
37/64 in = 0.5781 in = 14.684 mm
19/32 in = 0.5938 in = 15.081 mm
39/64 in = 0.6094 in = 15.478 mm
5/8 in = 0.6250 in = 15.875 mm
41/64 in = 0.6406 in = 16.272 mm
21/32 in = 0.6562 in = 16.669 mm
43/64 in = 0.6719 in = 17.066 mm
11/16 in = 0.6875 in = 17.462 mm
45/64 in = 0.7031 in = 17.859 mm
23/32 in = 0.7188 in = 18.256 mm
47/64 in = 0.7344 in = 18.653 mm
3/4 in = 0.7500 in = 19.050 mm
49/64 in = 0.7656 in = 19.447 mm
25/32 in = 0.7812 in = 19.844 mm
51/64 in = 0.7969 in = 20.241 mm
13/16 in = 0.8125 in = 20.637 mm
53/64 in = 0.8281 in = 21.034 mm
27/32 in = 0.8438 in = 21.431 mm
55/64 in = 0.8594 in = 21.828 mm
7/8 in = 0.8750 in = 22.225 mm
57/64 in = 0.8906 in = 22.622 mm
29/32 in = 0.9062 in = 23.019 mm
59/64 in = 0.9219 in = 23.416 mm
15/16 in = 0.9375 in = 23.812 mm
61/64 in = 0.9531 in = 24.209 mm
31/32 in = 0.9688 in = 24.606 mm
63/64 in = 0.9844 in = 25.003 mm
1 in = 1.0000 in = 25.400 mm

Values are exact to the places shown. Every 1/64 step is 0.015625 in, or 0.396875 mm. Memorizing the eighths (0.125, 0.250, 0.375, 0.500, 0.625, 0.750, 0.875) and the 1/16 offset (0.0625) gets you to any 16th in your head.

Millimeters to inches: 1 to 25 mm

1 mm = 0.0394 in
2 mm = 0.0787 in
3 mm = 0.1181 in
4 mm = 0.1575 in
5 mm = 0.1969 in
6 mm = 0.2362 in
7 mm = 0.2756 in
8 mm = 0.3150 in
9 mm = 0.3543 in
10 mm = 0.3937 in
11 mm = 0.4331 in
12 mm = 0.4724 in
13 mm = 0.5118 in
14 mm = 0.5512 in
15 mm = 0.5906 in
16 mm = 0.6299 in
17 mm = 0.6693 in
18 mm = 0.7087 in
19 mm = 0.7480 in
20 mm = 0.7874 in
21 mm = 0.8268 in
22 mm = 0.8661 in
23 mm = 0.9055 in
24 mm = 0.9449 in
25 mm = 0.9843 in

Because 25.4 mm is exactly 1 in, 1 mm is 0.03937 in (to five places). A quick shop check: 0.1 mm is about 0.004 in, and 0.01 mm is about 0.0004 in, a bit less than half a thousandth.

How to find the nearest fraction

Multiply the decimal by 64 and round. For 0.330 in, 0.330 × 64 = 21.12, so 21/64 (0.3281) is nearest, 0.0019 in under. If you need the nearest 32nd, multiply by 32 instead: 0.330 × 32 = 10.56, so 11/32 (0.3438) is nearest at 0.0138 over versus 5/16 (0.3125) 0.0175 under. For drill choices, also check the number and letter drill chart, which fills the gaps between 64ths.

Worked example: print in fractions, DRO in mm

A print calls for a 1-3/8 in shoulder and your lathe DRO reads in mm. 3/8 = 0.375, so the size is 1.375 in. 1.375 × 25.4 = 34.925 mm. With a ±1/64 fractional tolerance, that is ±0.396875 mm, so anywhere from 34.528 to 35.322 mm passes.

Worked example: metric drill as a fractional substitute

You need an 8.5 mm hole and only have fractional drills. 8.5 ÷ 25.4 = 0.3346 in. In 64ths, 0.3346 × 64 = 21.42, so 21/64 = 0.3281 in (8.334 mm) is 0.166 mm small and 11/32 = 0.3438 in (8.731 mm) is 0.231 mm big. Letter R is 0.339 in (8.611 mm) and Q is 0.332 in (8.433 mm), which are closer. The chart tells you the size; whether the substitute is acceptable depends on what the hole is for, such as a tap drill or a clearance hole.

Common problems

Rounding mm too early in a chain of math; mixing up 5/32 (0.1562) and 5/16 (0.3125); reading a fraction on a print as a decimal tolerance; and assuming a fractional tolerance applies when the print gives decimal places. Always use the title block tolerance for the number of decimal places shown. Before measuring to these numbers, check the mic against a gage block.

FAQ

What is 3/8 as a decimal?

0.375 in, which is 9.525 mm.

What is 1/64 in mm?

0.396875 mm, about 0.397 mm.

How many mm in an inch?

Exactly 25.4 mm by definition.

What fraction is closest to 0.200 in?

13/64 (0.2031) is closest; 0.2 × 64 = 12.8 rounds to 13.

What is 10 mm in inches?

0.3937 in, a hair over 25/64 (0.3906).

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