Square Millimeters to Square Inches
Convert square millimeters to square inches instantly — one square millimeter is 0.00155 square inches.
| Square Millimeters | Square Inches |
|---|---|
| 1 mm² | 0.00155 in² |
| 2 mm² | 0.0031 in² |
| 3 mm² | 0.00465 in² |
| 5 mm² | 0.00775 in² |
| 10 mm² | 0.0155 in² |
| 25 mm² | 0.03875 in² |
| 50 mm² | 0.0775 in² |
| 100 mm² | 0.155 in² |
How to use
- Enter a value in the Square Millimeters field.
- The Square Inches figure appears immediately — no button to press.
- Typing in the Square Inches field converts back to Square Millimeters the same way.
- Use the reference table for the values that come up most often.
Square Millimeters to Square Inches formula
Converting square millimeters to square inches is a single multiplication by 0.0015500031.
in² = mm² × 0.0015500031
Why this factor: the square millimeter and the square inch are both defined against the square meter, so dividing one definition by the other gives 0.0015500031 directly. Both definitions are set out in full below.
To go the other way, divide by 0.0015500031 — or use the Square Inches to Square Millimeters converter, which is the same calculation reversed.
Worked example
Take 50 mm² and multiply by the factor 0.0015500031: the result is 0.0775 in².
50 mm² × 0.0015500031 = 0.0775 in²
The converter above does this for any value you type, in either direction.
What is a square millimeter?
A square millimeter is the area of a one-millimeter square.
Definition: Exactly 1×10⁻⁶ square meter.
Where it is used: Cable cross-sections, engineering tolerances, and material stress calculations.
What is a square inch?
A square inch is the area of a one-inch square, used for small areas and pressure.
Definition: Exactly 0.00064516 square meter, following from the exact 25.4 mm inch.
Where it is used: Print and label areas, gasket and seal faces, and the denominator in psi.
Square Millimeters vs Square Inches
The square millimeter is a metric unit while the square inch is an imperial unit. That is why the factor between them is not a round number: the two systems were defined independently and only later tied together by exact agreement.
Frequently asked questions
How many square inches are in a square millimeter?
One square millimeter equals 0.0015500031 square inches.
What is the formula to convert square millimeters to square inches?
Multiply the number of square millimeters by 0.0015500031. To reverse it, divide by 0.0015500031 instead.
How do I convert square inches back to square millimeters?
Divide by 0.0015500031, which is the same as multiplying by 645.16.
How many square inches are in 10 square millimeters?
10 mm² works out to 0.0155 in².
How is the square millimeter defined?
Exactly 1×10⁻⁶ square meter.