Inches to Mm Converter — Inch Fractions to Millimeters

How to convert inches to millimeters

Multiply the inches by 25.4:

millimeters = inches × 25.4

3/4 in = 0.75 in × 25.4 = 19.05 mm. Because the inch is defined as exactly 25.4 mm, every fraction of an inch has an exact, short value in millimeters. To go the other way — and to find the nearest fraction for a metric size — use the mm to inches converter. For centimeters, see inches to centimeters.

Inch fractions to millimeters

Fraction	Decimal inches	Millimeters
1/64	0.015625 in	0.396875 mm
1/32	0.03125 in	0.79375 mm
1/16	0.0625 in	1.5875 mm
3/32	0.09375 in	2.38125 mm
1/8	0.125 in	3.175 mm
5/32	0.15625 in	3.96875 mm
3/16	0.1875 in	4.7625 mm
1/4	0.25 in	6.35 mm
5/16	0.3125 in	7.9375 mm
3/8	0.375 in	9.525 mm
7/16	0.4375 in	11.1125 mm
1/2	0.5 in	12.7 mm
9/16	0.5625 in	14.2875 mm
5/8	0.625 in	15.875 mm
11/16	0.6875 in	17.4625 mm
3/4	0.75 in	19.05 mm
13/16	0.8125 in	20.6375 mm
7/8	0.875 in	22.225 mm
15/16	0.9375 in	23.8125 mm
1	1 in	25.4 mm

Metric drill and socket sets step in whole and half millimeters, so few fractions have an exact match: a 1/4 in bit (6.35 mm) sits between 6 mm and 6.5 mm, and a 1/2 in socket (12.7 mm) between 12 mm and 13 mm. Pick the closer size for a loose fit and the matching unit for a tight one.

To convert a whole cut list, open Convert a list of values and paste the inches one per line — fractions like 3/8 work there too — then download the millimeters as CSV.

Lumber sizes in millimeters

U.S. softwood lumber is sold by nominal size; the actual dry, planed size is smaller (American Softwood Lumber Standard, PS 20):

Nominal	Actual inches	Actual millimeters
1×4	3/4 × 3 1/2 in	19.05 × 88.9 mm
2×4	1 1/2 × 3 1/2 in	38.1 × 88.9 mm
2×6	1 1/2 × 5 1/2 in	38.1 × 139.7 mm
2×8	1 1/2 × 7 1/4 in	38.1 × 184.15 mm
2×10	1 1/2 × 9 1/4 in	38.1 × 234.95 mm
2×12	1 1/2 × 11 1/4 in	38.1 × 285.75 mm
4×4	3 1/2 × 3 1/2 in	88.9 × 88.9 mm

Board feet are counted from the nominal size, not the actual one — the board foot calculator does that math.

Inches to millimeters chart

Inches (in)	Millimeters (mm)
1 in	25.4 mm
1.5 in	38.1 mm
2 in	50.8 mm
2.5 in	63.5 mm
3 in	76.2 mm
4 in	101.6 mm
5 in	127 mm
6 in	152.4 mm
8 in	203.2 mm
10 in	254 mm
12 in	304.8 mm
18 in	457.2 mm
24 in	609.6 mm
36 in	914.4 mm

Multiplying by 25 is a handy estimate (1.6% low): 4 in ≈ 100 mm, exact 101.6 mm.

Frequently asked questions

How many mm is an inch?

Exactly 25.4 millimeters. The inch was defined as 25.4 mm by international agreement in 1959.

How many mm is 1/4 inch?

6.35 mm. 1/8 inch is 3.175 mm and 1/2 inch is 12.7 mm.

What is 3/8 inch in mm?

9.525 mm. The nearest common metric size is 10 mm, which is 0.475 mm bigger.

How big is a 2×4 in mm?

A dry, surfaced 2×4 is 1 1/2 × 3 1/2 inches, which is 38.1 × 88.9 mm. The 2 and 4 are nominal sizes from before the board was dried and planed.

Can I type fractions?

Yes, on a keyboard: 5/16 or 1 3/8 are read as 0.3125 and 1.375 inches. Phone number pads have no slash, so type decimals there, or use the chart below. The list converter also accepts fractions, one per line.

What is the easiest way to convert inches to mm in my head?

Multiply by 25, then add 1.6%: 4 in × 25 = 100, plus 1.6 is 101.6 mm (exact). Multiplying by 25 alone is 1.6% low.