Inches to mm Fractions Chart: 1/64 in to 1 in

Quick answer: To turn an inch fraction into millimetres, divide the top number by the bottom number and multiply by 25.4. For example, 1/4 in is 6.35 mm, 3/8 in is 9.525 mm, 1/2 in is 12.7 mm and 3/4 in is 19.05 mm. The chart below gives every 64th of an inch in millimetres.

Fractional inches turn up on drill bits, spanners, American woodworking plans and older tape measures, while almost everything you buy in a UK shop is labelled in millimetres. This chart bridges the two. For decimal conversions and other length units, see the length and distance conversions hub.

One inch enlarged. Each eighth of an inch is exactly 3.175 mm, so the eighths step through 3.175, 6.35, 9.525 and so on up to 25.4 mm.

Table of Contents

How do you convert inch fractions to mm?

Turn the fraction into a decimal, then multiply by 25.4. Because the inch is defined as exactly 25.4 mm, every fraction with a denominator of 2, 4, 8, 16, 32 or 64 converts to a millimetre value that ends cleanly; nothing recurs.

Divide. 5/16 means 5 ÷ 16 = 0.3125 in.
Multiply by 25.4. 0.3125 × 25.4 = 7.9375 mm.
Add any whole inches. For 2 5/16 in, add 2 × 25.4 = 50.8 mm, giving 58.7375 mm, or about 58.7 mm.

A shortcut for sixteenths: each 1/16 in is 1.5875 mm, so multiply the number of sixteenths by 1.5875. For 64ths, use 0.396875 mm per step.

Inches to mm fractions chart (1/64 in to 1 in)

Halves, quarters and eighths are shown in bold; every other row is a 16th, 32nd or 64th. Millimetres are rounded to three decimal places, which is finer than any tape measure or most workshop callipers can read.

Fraction	Decimal inch	Millimetres
1/64 in	0.0156	0.397 mm
1/32 in	0.0313	0.794 mm
3/64 in	0.0469	1.191 mm
1/16 in	0.0625	1.588 mm
5/64 in	0.0781	1.984 mm
3/32 in	0.0938	2.381 mm
7/64 in	0.1094	2.778 mm
1/8 in	0.1250	3.175 mm
9/64 in	0.1406	3.572 mm
5/32 in	0.1563	3.969 mm
11/64 in	0.1719	4.366 mm
3/16 in	0.1875	4.763 mm
13/64 in	0.2031	5.159 mm
7/32 in	0.2188	5.556 mm
15/64 in	0.2344	5.953 mm
1/4 in	0.2500	6.350 mm
17/64 in	0.2656	6.747 mm
9/32 in	0.2813	7.144 mm
19/64 in	0.2969	7.541 mm
5/16 in	0.3125	7.938 mm
21/64 in	0.3281	8.334 mm
11/32 in	0.3438	8.731 mm
23/64 in	0.3594	9.128 mm
3/8 in	0.3750	9.525 mm
25/64 in	0.3906	9.922 mm
13/32 in	0.4063	10.319 mm
27/64 in	0.4219	10.716 mm
7/16 in	0.4375	11.113 mm
29/64 in	0.4531	11.509 mm
15/32 in	0.4688	11.906 mm
31/64 in	0.4844	12.303 mm
1/2 in	0.5000	12.700 mm
33/64 in	0.5156	13.097 mm
17/32 in	0.5313	13.494 mm
35/64 in	0.5469	13.891 mm
9/16 in	0.5625	14.288 mm
37/64 in	0.5781	14.684 mm
19/32 in	0.5938	15.081 mm
39/64 in	0.6094	15.478 mm
5/8 in	0.6250	15.875 mm
41/64 in	0.6406	16.272 mm
21/32 in	0.6563	16.669 mm
43/64 in	0.6719	17.066 mm
11/16 in	0.6875	17.463 mm
45/64 in	0.7031	17.859 mm
23/32 in	0.7188	18.256 mm
47/64 in	0.7344	18.653 mm
3/4 in	0.7500	19.050 mm
49/64 in	0.7656	19.447 mm
25/32 in	0.7813	19.844 mm
51/64 in	0.7969	20.241 mm
13/16 in	0.8125	20.638 mm
53/64 in	0.8281	21.034 mm
27/32 in	0.8438	21.431 mm
55/64 in	0.8594	21.828 mm
7/8 in	0.8750	22.225 mm
57/64 in	0.8906	22.622 mm
29/32 in	0.9063	23.019 mm
59/64 in	0.9219	23.416 mm
15/16 in	0.9375	23.813 mm
61/64 in	0.9531	24.209 mm
31/32 in	0.9688	24.606 mm
63/64 in	0.9844	25.003 mm
1 in	1.0000	25.400 mm

Which inch fractions are worth memorising?

Learn the eighths and you can estimate everything else. One eighth of an inch is 3.175 mm, close enough to 3 mm for rough work, and each further eighth adds the same amount:

1/8 in = 3.175 mm (about 3 mm)
1/4 in = 6.35 mm (about 6.4 mm)
1/2 in = 12.7 mm (about 13 mm)
3/4 in = 19.05 mm (about 19 mm)

Going the other way, from a metric measurement to the nearest fraction, is covered in the mm to inches chart.

Whole inches to millimetres

Each whole inch adds 25.4 mm, so 12 inches (one foot) is 304.8 mm. Add the whole-inch value from this table to the fraction from the chart above for mixed measurements such as 6 3/8 in (152.4 + 9.525 = 161.925 mm).

Inches	Millimetres	Centimetres
1 in	25.4 mm	2.54 cm
2 in	50.8 mm	5.08 cm
3 in	76.2 mm	7.62 cm
4 in	101.6 mm	10.16 cm
5 in	127.0 mm	12.70 cm
6 in	152.4 mm	15.24 cm
7 in	177.8 mm	17.78 cm
8 in	203.2 mm	20.32 cm
9 in	228.6 mm	22.86 cm
10 in	254.0 mm	25.40 cm
11 in	279.4 mm	27.94 cm
12 in	304.8 mm	30.48 cm
18 in	457.2 mm	45.72 cm
24 in	609.6 mm	60.96 cm
36 in	914.4 mm	91.44 cm

Where are fractional inches still used in the UK?

Mostly in tools, threads and anything designed in the United States. British trades went metric decades ago, but inch fractions linger in a few places:

Spanners and sockets. Imperial “AF” (across flats) sizes such as 1/2 in and 9/16 in are still sold for classic cars, bikes and American machinery. A 1/2 in AF nut is 12.7 mm across, so a 13 mm spanner fits loosely. The guide to measuring nut size explains how to check.
Drill bits and router cutters. Imperial sets in 1/64 in steps are common in woodworking, and many router collets are 1/4 in or 1/2 in.
Pipe threads. British Standard Pipe (BSP) threads keep inch names such as 1/2 in and 3/4 in, but these are nominal: a 1/2 in BSP thread measures roughly 21 mm across, not 12.7 mm.
Tape measures. Dual-scale tapes mark inches in sixteenths. If you are unsure which line is which, the guide to reading a tape measure walks through the markings.

Common mistakes when converting inch fractions

Rounding before multiplying. Rounding 5/16 to 0.31 first gives 7.874 mm instead of 7.9375 mm. Keep the full decimal until the end.
Swapping a fraction for the nearest metric size. 1/4 in (6.35 mm) is not 6 mm. For holes and fixings that must fit, buy the right system rather than the closest number.
Treating nominal sizes as measurements. Pipe threads and some timber sizes keep inch names that no longer describe the actual dimension.

Frequently asked questions

What is 5/8 of an inch in mm?

5/8 in is 0.625 in, and 0.625 × 25.4 = 15.875 mm, or about 15.9 mm.

How many mm is 3/16 of an inch?

3/16 in is 0.1875 in, which is 4.7625 mm, or about 4.8 mm.

Is 1/4 inch the same as 6 mm?

No. A quarter of an inch is exactly 6.35 mm, so it is 0.35 mm larger than 6 mm. The difference matters for bolts, drill bits and router cutters.

What is 1/16 of an inch in mm?

1/16 in is exactly 1.5875 mm. It is the smallest mark on most imperial tape measures.