Square inches to square millimetres – convert in² to mm²
Convert square inches to square millimetres (in² to mm²) for mechanical drawings, PCB layouts, machining and product design. This page opens the main area converter with in² → mm² pre-selected.
Open the interactive in² → mm² converter:
Use main converter (in² → mm²) or switch to mm² → in²On the homepage you can also convert between cm², m², ft², yards², acres and hectares.
Quick reference: in² to mm²
One square inch is exactly 645.16 mm², so: mm² = in² × 645.16. Examples:
| Area (in²) | Area (mm²) |
|---|---|
| 1 in² | 645.16 mm² |
| 2 in² | 1,290.32 mm² |
| 5 in² | 3,225.8 mm² |
| 10 in² | 6,451.6 mm² |
| 20 in² | 12,903.2 mm² |
Why convert square inches to square millimetres?
Square inches to square millimetres is helpful when you:
- Work with imperial datasheets but design in metric area units.
- Specify component footprints in mm² but receive values in in².
- Compare PCB, gasket or plate areas across mixed unit systems.
- Standardise documentation to SI units while keeping imperial references.
Other area converters on this site
This converter uses the exact inch definition (1 in = 25.4 mm), so 1 in² = 645.16 mm².
Page last updated: 4 December 2025.