Fractions of an inch to millimeters

Tools, fasteners and drill bits in inches are sized with fractions: 1/4, 3/8, 5/8… To convert a fraction to millimeters, divide the top number by the bottom number and multiply by 25.4. The chart lists every fraction down to 1/64.

Published October 3, 2026 · Updated October 23, 2026 · Calculated and checked by Pulgadas a CM · How we check our numbers

How to convert a fraction of an inch to mm

Example: 3/8 inch = 3 ÷ 8 = 0.375 inch; 0.375 × 25.4 = 9.525 mm. For a mixed number such as 1 3/4, add first: 1 + 0.75 = 1.75 inches = 44.45 mm. You can also type the fraction straight into the converter.

Inch and metric wrenches

Some inch sizes are almost identical to a metric size, others are not. A metric wrench that is smaller than the nut will not go on; one that is larger can round off the corners.

Wrench	Millimeters	Nearest metric	Fit
1/4″	6.35	6 mm	6 mm is 0.35 mm smaller: will not fit
5/16″	7.94	8 mm	Practically the same
3/8″	9.53	10 mm	10 mm is 0.48 mm larger: may round the nut
7/16″	11.11	11 mm	Almost the same, may be very tight
1/2″	12.7	13 mm	13 mm is 0.3 mm larger: may round the nut
9/16″	14.29	14 mm	14 mm is 0.29 mm smaller: will not fit
5/8″	15.88	16 mm	Almost the same
11/16″	17.46	17 mm	17 mm is 0.46 mm smaller: will not fit
3/4″	19.05	19 mm	Practically the same
13/16″	20.64	21 mm	21 mm is 0.36 mm larger: may round the nut
7/8″	22.23	22 mm	22 mm is 0.22 mm smaller: will not fit
15/16″	23.81	24 mm	24 mm is 0.19 mm larger
1″	25.4	25 mm	25 mm is 0.4 mm smaller: will not fit

Fractions of an inch chart (1/64–1″)

Fraction	Inches	Millimeters	Centimeters
1/64″	0.015625	0.397	0.0397
1/32″	0.03125	0.794	0.0794
3/64″	0.046875	1.191	0.1191
1/16″	0.0625	1.588	0.1588
5/64″	0.078125	1.984	0.1984
3/32″	0.09375	2.381	0.2381
7/64″	0.109375	2.778	0.2778
1/8″	0.125	3.175	0.3175
9/64″	0.140625	3.572	0.3572
5/32″	0.15625	3.969	0.3969
11/64″	0.171875	4.366	0.4366
3/16″	0.1875	4.763	0.4763
13/64″	0.203125	5.159	0.5159
7/32″	0.21875	5.556	0.5556
15/64″	0.234375	5.953	0.5953
1/4″	0.25	6.35	0.635
17/64″	0.265625	6.747	0.6747
9/32″	0.28125	7.144	0.7144
19/64″	0.296875	7.541	0.7541
5/16″	0.3125	7.938	0.7938
21/64″	0.328125	8.334	0.8334
11/32″	0.34375	8.731	0.8731
23/64″	0.359375	9.128	0.9128
3/8″	0.375	9.525	0.9525
25/64″	0.390625	9.922	0.9922
13/32″	0.40625	10.319	1.0319
27/64″	0.421875	10.716	1.0716
7/16″	0.4375	11.113	1.1113
29/64″	0.453125	11.509	1.1509
15/32″	0.46875	11.906	1.1906
31/64″	0.484375	12.303	1.2303
1/2″	0.5	12.7	1.27
33/64″	0.515625	13.097	1.3097
17/32″	0.53125	13.494	1.3494
35/64″	0.546875	13.891	1.3891
9/16″	0.5625	14.288	1.4288
37/64″	0.578125	14.684	1.4684
19/32″	0.59375	15.081	1.5081
39/64″	0.609375	15.478	1.5478
5/8″	0.625	15.875	1.5875
41/64″	0.640625	16.272	1.6272
21/32″	0.65625	16.669	1.6669
43/64″	0.671875	17.066	1.7066
11/16″	0.6875	17.463	1.7463
45/64″	0.703125	17.859	1.7859
23/32″	0.71875	18.256	1.8256
47/64″	0.734375	18.653	1.8653
3/4″	0.75	19.05	1.905
49/64″	0.765625	19.447	1.9447
25/32″	0.78125	19.844	1.9844
51/64″	0.796875	20.241	2.0241
13/16″	0.8125	20.638	2.0638
53/64″	0.828125	21.034	2.1034
27/32″	0.84375	21.431	2.1431
55/64″	0.859375	21.828	2.1828
7/8″	0.875	22.225	2.2225
57/64″	0.890625	22.622	2.2622
29/32″	0.90625	23.019	2.3019
59/64″	0.921875	23.416	2.3416
15/16″	0.9375	23.813	2.3813
61/64″	0.953125	24.209	2.4209
31/32″	0.96875	24.606	2.4606
63/64″	0.984375	25.003	2.5003
1″	1	25.4	2.54

Frequently asked questions

How many mm is 3/8 inch?

9.525 mm.

How many mm is 1/4 inch?

6.35 mm.

Will a 13 mm wrench fit a 1/2-inch nut?

1/2″ = 12.7 mm, so a 13 mm wrench is 0.3 mm larger: on a tight nut it may round the corners.

More size guides

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