Unit converter

Inches to Millimeters (in to mm)

Multiply inches by 25.4 to get millimeters. The factor is exact, so 1/4 in is exactly 6.35 mm and 3/4 in is 19.05 mm. The table lists the common fractional sizes on drill bits, sockets and tape measures.

Convert inches to mm

inches to mm

1 in = 25.4 mm

1 in = 25.4 mm · 1 mm = 0.0393701 in

Show the math

mm = in × 25.4

Uses exact legal definitions; results are rounded to 6 significant figures for display.

How to convert inches to mm

mm = in × 25.4

One inch is exactly 25.4 millimeters, so multiply by 25.4.

Worked example: converting 1 in:

mm = in × 25.4
1 in × 25.4 = 25.4 mm

Inches to mm conversion table

Common values, each computed with the exact formula and rounded to 6 significant figures.

Inches (in) to Millimeters (mm)
Inches (in)	Millimeters (mm)	Fraction
0.0625 in	1.5875 mm	1/16
0.125 in	3.175 mm	1/8
0.1875 in	4.7625 mm	3/16
0.25 in	6.35 mm	1/4
0.3125 in	7.9375 mm	5/16
0.375 in	9.525 mm	3/8
0.4375 in	11.1125 mm	7/16
0.5 in	12.7 mm	1/2
0.5625 in	14.2875 mm	9/16
0.625 in	15.875 mm	5/8
0.6875 in	17.4625 mm	11/16
0.75 in	19.05 mm	3/4
0.8125 in	20.6375 mm	13/16
0.875 in	22.225 mm	7/8
0.9375 in	23.8125 mm	15/16
1 in	25.4 mm	1
1.25 in	31.75 mm	1 1/4
1.5 in	38.1 mm	1 1/2
1.75 in	44.45 mm	1 3/4
2 in	50.8 mm	2
2.5 in	63.5 mm	2 1/2
3 in	76.2 mm	3
3.5 in	88.9 mm	3 1/2
4 in	101.6 mm	4
5 in	127 mm	5
6 in	152.4 mm	6
8 in	203.2 mm	8
10 in	254 mm	10
12 in	304.8 mm	12

Quick reference: everyday inches in millimeters

1/16 in (finest common tape mark)
0.0625 in = 1.5875 mm
1/4 in drill bit
0.25 in = 6.35 mm
1/2 in socket
0.5 in = 12.7 mm
3/4 in plywood (nominal)
0.75 in = 19.05 mm
One inch
1 in = 25.4 mm
Actual width of a 2×4
3.5 in = 88.9 mm

Drill bits, sockets and the nearest metric size

Fractional inch sizes rarely land on whole millimeters, so a metric tool is usually a near match rather than an exact one. 1/4 in (6.35 mm) sits between 6 and 6.5 mm drills, 5/16 in (7.94 mm) is close to 8 mm, and 1/2 in (12.7 mm) falls between 12 and 13 mm sockets.

For a clearance hole, round up to the next metric size. For a hole that a screw has to grip, round down. For sockets and wrenches, use the exact system the fastener was made in, because the 0.2–0.3 mm gap between near sizes is enough to strip a bolt head under load.

Nominal sizes: pipe and lumber

Some inch sizes are names, not measurements. 1/2-inch nominal steel pipe has an outside diameter of 0.840 in (21.3 mm), which is why a fitting labelled '1/2 inch' won't measure 12.7 mm anywhere. Metric plumbing is sized by actual outside diameter, so match pipe by its standard, not by converting the name.

Lumber works the same way. A 2×4 is 1.5 × 3.5 in (38 × 89 mm), and a 1×6 board is 0.75 × 5.5 in (19 × 140 mm). When you mix metric hardware with North American lumber, convert the actual dimensions. The fraction calculator helps when adding fractional cuts before you convert, and the concrete calculator takes slab thickness in inches or centimeters.

Frequently asked questions

How many millimeters are in an inch?

Exactly 25.4 mm. The figure was fixed by international agreement in 1959 and isn't an approximation, so conversions from inches to millimeters can be exact to as many decimals as the inch value has. The other direction (mm to inches) produces repeating decimals, which is why the reverse table rounds to fractions.

Is 3/4-inch plywood really 19 mm thick?

Often not. 3/4 in is 19.05 mm, but sheets sold as 3/4-inch in North America are frequently a little thinner, around 23/32 in (18.3 mm), because of sanding and manufacturing tolerances. Metric plywood sold as 18 mm is close to that. Measure the actual sheet before cutting dadoes or rebates.

What's 1/8 inch in mm?

1/8 in is 3.175 mm. A 3.2 mm bit is only 0.025 mm oversize, close enough for most woodworking, while a 3 mm bit is 0.175 mm undersize. That's why drill-size charts pair 1/8 in with 3.2 mm. The exact figure is shown in the table above.