Millimetres to inches (mm to in)

The formula

in = mm × 0.03937007874

Where the factor comes from — each unit by its definition, then its size in SI:

• 1 mm = m / 1000 = 0.001 m
• 1 in = 2.54 × cm = 0.0254 m

Dividing the two SI sizes gives the factor: 0.0393700787402 (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 25.4.

Every formula on this site takes lengths in metres. Heights, distances and stopping distances quoted in feet, inches or miles have to be converted first — getting it wrong by a factor of 3.28 is the most common unit slip in homework.

Worked examples

• A sprint track straight (100 m): 100,000 mm = 3,937.01 in — 100,000 × 0.03937007874 = 3,937.01
• A marathon (42.195 km): 42,195,000 mm = 1,661,220 in — 42,195,000 × 0.03937007874 = 1,661,220
• A tall doorway (6 ft): 1,828.8 mm = 72 in — 1,828.8 × 0.03937007874 = 72
• A school ruler (30 cm): 300 mm = 11.811 in — 300 × 0.03937007874 = 11.811

Frequently asked questions