Millimeter to Inch
Convert millimeter (mm) to inch (in) instantly. Switch between metric and imperial in a tap.
Millimeter → Inch
0.03937
in
1 mm = 0.03937 in
Frequently asked questions
0.0393701 inches, since an inch is exactly 25.4 mm. Divide millimetres by 25.4 to get inches.
Several are close enough to confuse and far enough apart to matter. A 6 mm bit is 0.236 in against a 1/4 in bit at 6.35 mm — a difference of 0.35 mm. A 12 mm bit is 0.472 in against 1/2 in at 12.7 mm, differing by 0.7 mm. For a clearance hole either may do; for a press fit or a thread the difference is significant.
A designation like 205/55 R16 mixes both systems: 205 is the tyre width in millimetres, 55 is the aspect ratio as a percentage of that width, and 16 is the rim diameter in inches. The convention is historical and universal, which is why rim and tyre dimensions are quoted in different units on the same product.
Divide by 25.4
A millimetre is 0.0393701 inches, since an inch is exactly 25.4 mm.
Inches = millimetres ÷ 25.4
The millimetre is the working unit of precision in engineering and manufacturing, which is why this conversion appears wherever an imperial specification meets a metric workshop.
Where millimetres dominate
Engineering drawings. Mechanical and manufacturing drawings in India and most of the world are dimensioned in millimetres by default, without units marked, because the convention is assumed.
Construction detail. Steel bar diameters, plywood thicknesses, tile dimensions and glass thicknesses are all specified in millimetres.
Rainfall, which is measured and reported in millimetres — a genuinely metric domain worldwide.
Camera lenses and sensor dimensions, photographic film formats.
Paper thickness, and the dimensions of small components generally.
Where inches persist and the conversion is needed
Drill bits and fasteners. Indian hardware carries both metric and imperial sets, and a drawing specifying a 6 mm hole against an available 1/4-inch bit (6.35 mm) is a routine judgement call.
Pipe threading — BSP and NPT standards are imperial-based worldwide.
Wheel and tyre sizes, where the rim diameter is in inches while the tyre width is in millimetres. A "205/55 R16" tyre is 205 mm wide on a 16-inch rim, mixing both systems in one designation.
Bicycle components, split awkwardly between the two.
Common values
- 1 mm = 0.039 in
- 3 mm = 0.118 in (close to 1/8 in)
- 6 mm = 0.236 in (close to 1/4 in)
- 10 mm = 0.394 in
- 12 mm = 0.472 in (close to 1/2 in)
- 25.4 mm = 1 in exactly
- 50 mm = 1.969 in
- 100 mm = 3.937 in
Fractional inch equivalents
Hardware sizing is usually fractional, so the practical table runs the other way:
- 1/16 in = 1.5875 mm
- 1/8 in = 3.175 mm
- 3/16 in = 4.7625 mm
- 1/4 in = 6.35 mm
- 3/8 in = 9.525 mm
- 1/2 in = 12.7 mm
- 3/4 in = 19.05 mm
- 1 in = 25.4 mm
Note how close several metric sizes are to fractional ones — 6 mm against 1/4 inch differs by 0.35 mm, and 12 mm against 1/2 inch by 0.7 mm. Close enough to be confused, and far enough apart to matter for a press fit or a thread.
Precision and significant figures
When converting a measured value, the result should not imply more precision than the input carried. A dimension given as "50 mm" to the nearest millimetre becomes 1.97 inches, not 1.9685039 inches — the extra digits are arithmetic, not information.