mm to Inches Chart UK: Printable Conversion Table

Quick answer: To convert millimetres to inches, divide by 25.4, because one inch is exactly 25.4 mm. So 10 mm is 0.394 in, 25 mm is 0.984 in (just under an inch) and 100 mm is 3.937 in. The chart below lists 1 to 100 mm with decimal inches and the nearest fraction on an imperial tape.

Millimetres are the working unit for almost everything built or sold in the UK, from copper pipe to plasterboard, yet plenty of tape measures, older plans and American instructions still use inches. This printable mm to inches chart is part of the length and distance conversions collection.

A to-scale ruler: millimetres on the top edge, inches in sixteenths on the bottom. One inch lands at 25.4 mm, and 100 mm falls just short of 4 inches.

Table of Contents

How many millimetres are in an inch?

There are exactly 25.4 millimetres in one inch. The figure is a definition, not a rounded measurement: since 1959 the inch has been fixed at 25.4 mm, and the Weights and Measures Act 1985 defines it as 1/36 of a yard of 0.9144 m, which comes to the same figure. That means any conversion you do with 25.4 is exact before you round the answer.

mm to inches: inches = mm ÷ 25.4 (or mm × 0.03937)
Inches to mm: mm = inches × 25.4

Worked example: a kitchen worktop is commonly 38 mm thick. 38 ÷ 25.4 = 1.496 in, which is almost exactly 1½ inches.

mm to inches converter

mm=1.496 in

mm to inches conversion chart (1 to 1,000 mm)

Every millimetre is about 0.039 inches, so 1 mm is a little under 1/16 in. Decimal inches are rounded to three places up to 100 mm and two places above that. The fraction column gives the nearest 1/32 in up to 100 mm and the nearest 1/16 in for the larger sizes, which is as fine as most tape measures are marked.

Millimetres	Inches (decimal)	Nearest fraction
1 mm	0.039 in	1/32 in
2 mm	0.079 in	3/32 in
3 mm	0.118 in	1/8 in
4 mm	0.157 in	5/32 in
5 mm	0.197 in	3/16 in
6 mm	0.236 in	1/4 in
7 mm	0.276 in	9/32 in
8 mm	0.315 in	5/16 in
9 mm	0.354 in	11/32 in
10 mm	0.394 in	13/32 in
11 mm	0.433 in	7/16 in
12 mm	0.472 in	15/32 in
13 mm	0.512 in	1/2 in
14 mm	0.551 in	9/16 in
15 mm	0.591 in	19/32 in
16 mm	0.630 in	5/8 in
17 mm	0.669 in	21/32 in
18 mm	0.709 in	23/32 in
19 mm	0.748 in	3/4 in
20 mm	0.787 in	25/32 in
21 mm	0.827 in	13/16 in
22 mm	0.866 in	7/8 in
23 mm	0.906 in	29/32 in
24 mm	0.945 in	15/16 in
25 mm	0.984 in	31/32 in
26 mm	1.024 in	1 1/32 in
27 mm	1.063 in	1 1/16 in
28 mm	1.102 in	1 3/32 in
29 mm	1.142 in	1 5/32 in
30 mm	1.181 in	1 3/16 in
35 mm	1.378 in	1 3/8 in
40 mm	1.575 in	1 9/16 in
45 mm	1.772 in	1 25/32 in
50 mm	1.969 in	1 31/32 in
55 mm	2.165 in	2 5/32 in
60 mm	2.362 in	2 3/8 in
65 mm	2.559 in	2 9/16 in
70 mm	2.756 in	2 3/4 in
75 mm	2.953 in	2 15/16 in
80 mm	3.150 in	3 5/32 in
85 mm	3.346 in	3 11/32 in
90 mm	3.543 in	3 17/32 in
95 mm	3.740 in	3 3/4 in
100 mm	3.937 in	3 15/16 in
150 mm	5.91 in	5 7/8 in
200 mm	7.87 in	7 7/8 in
250 mm	9.84 in	9 13/16 in
300 mm	11.81 in	11 13/16 in
500 mm	19.69 in	19 11/16 in
1,000 mm	39.37 in	39 3/8 in

Fractions are approximate. For example, 13 mm is listed as 1/2 in, but half an inch is really 12.7 mm, so a 13 mm spanner will be slightly loose on a 1/2 in nut. When parts must fit, convert from the original unit rather than from the rounded fraction.

Where are millimetres and inches both used in the UK?

Building, plumbing and engineering in the UK work in millimetres, but inch names survive in several trades. Some common examples:

Item	Usual UK size	In inches
Copper water pipe (outside diameter)	15 mm and 22 mm	0.591 in and 0.866 in
Standard plasterboard	12.5 mm	0.492 in
A4 paper	210 × 297 mm	8.27 × 11.69 in
Legal minimum car tyre tread	1.6 mm	0.063 in (about 1/16 in)
Common kitchen worktop thickness	38 mm	1.496 in

Tyre tread depth is checked in millimetres with a gauge; the guide to measuring tyre tread in the UK explains the method. Screws and bolts are another area where metric and imperial overlap, as covered in how to measure a screw size.

How do you convert mm to inches without a calculator?

Divide by 25 instead, then take off about 2 per cent. For 50 mm: 50 ÷ 25 = 2, and 2 minus 2 per cent is 1.96, close to the true 1.969 in. Two other handy anchors are 100 mm ≈ 4 in (actually 3.94) and 300 mm ≈ 1 ft (actually 11.81 in, so a 300 mm ruler is about 5 mm short of a foot).

Common mistakes to avoid

Mixing up mm and cm. 10 mm is 1 cm. If you divide centimetres by 25.4 your answer will be ten times too small; use 2.54 for cm.
Trusting nominal sizes. Timber, pipe threads and some fittings carry names that do not match their measured size, so measure the actual item.
Reading the wrong edge of the tape. Many UK tapes show mm on one edge and inches on the other. The guide to reading a tape measure shows how to read both.

For centimetre measurements, the cm to inches converter does the same job with a factor of 2.54.

Frequently asked questions

How many inches is 10 mm?

10 mm is 0.394 inches, which is a little over 3/8 in (9.525 mm). It is the same as 1 cm.

Is 25 mm the same as 1 inch?

Not quite. One inch is exactly 25.4 mm, so 25 mm is 0.984 in, about 0.4 mm short of an inch.

What is 6 mm in inches as a fraction?

6 mm is 0.236 in, which is closest to 1/4 in. A true quarter inch is 6.35 mm.

How do I convert mm to inches quickly?

Divide the millimetres by 25.4. For a mental estimate, divide by 25 and knock off about 2 per cent.

How many inches is 100 mm?

100 mm is 3.937 inches, just under 4 inches and very close to 3 15/16 in.

Sources

Weights and Measures Act 1985, Schedule 1 (definitions of UK units of measurement)
National Physical Laboratory (NPL)
The Highway Code, Annex 6: vehicle maintenance (1.6 mm minimum tread depth)