Convert Square Millimeters to Square Inches
1 square millimeter (mm²) equals 0.0015500031 square inches (in²).
1 mm² = 0.0015500031 in²
Type in either box. Scientific notation works: 1.5e-3.
Converting a list? Use the bulk square millimeter to square inch converter to paste a column from Excel.
How to Convert Square Millimeter to Square Inch
- 1 mm² = 0.0015500031 in²
- 1 in² = 645.16 mm²
- Example: convert 15 mm² to in²: 15 mm² = 15 × 0.0015500031 in² = 0.0232500465 in²
Formula
in² = mm² × 0.001550003100006
or in² = mm² ÷ 645.16
Derivation from SI units:
- 1 mm² = (0.001 m)² = 10⁻⁶ m²
- 1 in² = (0.0254 m)² = 6.4516 × 10⁻⁴ m² = 6.4516 cm²
- 1 mm² = 0.000001 m² ÷ 0.00064516 m² per in² = 0.001550003100006 in²
Both units are defined exactly, so the factor is exact. Square Millimeter definition: square millimeter; Square Inch definition: square inch.
Worked examples
- 645.16 mm² is exactly 1 in².
645.16 mm² × 0.0015500031 = 1 in² - A 2.5 mm² cable cross-section is 0.00387500775 in².
2.5 mm² × 0.0015500031 = 0.00387500775 in²
Square Millimeter to Square Inch conversion table
| Square Millimeters (mm²) | Square Inches (in²) |
|---|---|
| 0.5 mm² | 0.00077500155 in² |
| 0.75 mm² | 0.001162502325 in² |
| 1 mm² | 0.0015500031 in² |
| 1.5 mm² | 0.00232500465 in² |
| 2.5 mm² | 0.00387500775 in² |
| 4 mm² | 0.0062000124 in² |
| 6 mm² | 0.0093000186 in² |
| 10 mm² | 0.015500031 in² |
| 16 mm² | 0.0248000496 in² |
| 25 mm² | 0.0387500775 in² |
| 35 mm² | 0.0542501085 in² |
| 50 mm² | 0.077500155 in² |
| 70 mm² | 0.108500217 in² |
| 95 mm² | 0.1472502945 in² |
| 100 mm² | 0.15500031 in² |
| 500 mm² | 0.77500155 in² |
| 645.16 mm² | 1 in² |
| 1000 mm² | 1.5500031 in² |
Where it's used
Cable cross-sections and stressed areas in mm² are converted to in² for North American codes and stress calculations in psi.
Square Millimeter to other area units
| Conversion | 1 mm² equals |
|---|---|
| square millimeter to square meter | 0.000001 m² |
| square millimeter to square centimeter | 0.01 cm² |
| square millimeter to square foot | 0.00001076391042 ft² |
| square millimeter to square inch | 0.0015500031 in² |
Frequently asked questions
How many mm² are in a square inch?
Exactly 645.16 mm².
How many square inches are in 1 square millimeter?
1 mm² = 0.0015500031 in².
How do I convert square millimeters to square inches?
Multiply the value in mm² by 0.0015500031, or divide it by 645.16. For example, 15 mm² × 0.0015500031 = 0.0232500465 in².
Is the mm² to in² conversion exact?
Yes. Both units are defined exactly, so the ratio is exact; its decimal form does not terminate and is shown here to 10 significant figures (more with the precision selector).
Sources
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures , Table 2 with SI prefix milli.
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "square inch (in²)".
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Convert Square Millimeters to Square Inches. Retrieved October 17, 2026, from https://scientificconverters.com/area/square-millimeters-to-square-inches/
IEEE
Scientific Converters, "Convert Square Millimeters to Square Inches," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/area/square-millimeters-to-square-inches/