The formula
mm² = in² × 645.16
Where the factor comes from — each unit by its definition, then its size in SI:
1 in² = in² = 0.00064516 m²1 mm² = mm² = 1 × 10⁻⁶ m²
Dividing the two SI sizes gives the factor: 645.16, exact — every step above is a definition, not a measurement. The reverse factor is 0.00155000310001.
Area in SI is the square metre. Squaring a length conversion squares its factor: 1 ft = 0.3048 m, so 1 ft² is 0.3048² m².
Worked examples
- A shoe sole (0.02 m²): 31.0001 in² = 20,000 mm² — 31.0001 × 645.16 = 20,000
- A gauge face: 1 in² = 645.16 mm² — 1 × 645.16 = 645.16
- A small room (10 m²): 15,500 in² = 10,000,000 mm² — 15,500 × 645.16 = 10,000,000
Frequently asked questions
How many square millimetres is 1 in²?
1 in² = 645.16 mm², exactly, from the unit definitions.
How do I convert square millimetres back to square inches?
Divide by 645.16 — or multiply by 0.00155000310001: 1 mm² = 0.00155000310001 in².