mm to inches converter

How to convert millimeters to inches

An inch is exactly 25.4 millimetres, so millimetres divided by 25.4 are inches. One millimetre is 0.03937 inches. Ten millimetres, which is one centimetre, is 0.3937 inches, and that matches the centimetre page because 10 mm is 1 cm and 1/2.54 is 0.3937. Fastener sheets, PCB layouts, and feeler gauges live in this pair. A 1/4-20 screw's nominal diameter is not 'about 6 mm'. It is 6.35 mm, which is 0.25 inch.

Mechanical drawings that mix a metric caliper with an inch callout, screen glass specified in millimetres, and drill bits sold in both. 25.4 mm is 1 inch exactly, which is the row to trust when you are checking the converter rather than a measurement.

Exact formula

Formulainches = millimeters ÷ 25.4

inches = millimeters ÷ 25.4 One mm is 0.0393700787402 in. Ten mm is 0.393700787402 in. The worked example below uses that same conversion.

Conversion table

The millimeters column runs from 1 mm (0.0394 in) through 1000 mm (39.37 in).

Worked example: Millimeters to Inches

The definition row is 25.4 mm, which is 1 in. One millimetre is 0.0394 in, and a caliper set to the wrong unit will still look precise while you divide. Ten millimetres is one centimetre and must equal 0.3937 in, the same result as 1 cm on the centimetre page. A mil is a thousandth of an inch, about 0.0254 mm, not a millimetre. An M6 thread is a metric designation, not 'whatever 1/4 inch converts to', even though 1/4 inch is 6.35 mm.

Pitfalls for millimeters to inches

• 0.04 inch per millimetre is a rough 1/25. The exact value is 1/25.4.
• Thou (mil) means 0.001 inch, not a millimetre. 1 mm is 39.37 thou.
• Metric screw threads (M6) are not the inch conversion of a nearby imperial thread.
• Calipers set to the wrong unit look precise. Check the unit icon before you divide.