A square inch is the area of a one-inch square.
Square inches to Square Millimeters Converter — in2 to mm2
Convert Square inch (in2) to Square Millimeter (mm2) using the exact conversion factor (1 square inch = 645.16 square millimeter). See the formula, worked examples, and conversion table.
Square inches to Square Millimeters converter
Converter
Area Converter
Convert land, construction, mapping, survey, and scientific area units.
Context values
Some unit families need one extra value for a physically accurate result.
Square metric area unit.
Result = input x 0.00064516 / 1e-6.
Area uses the square meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Square inches to Square Millimeters
Comparing Square inch and Square Millimeter is common in agriculture and real estate, where US land is typically measured in acres and most of the rest of the world uses hectares. Specifically: A square inch is the area of a one-inch square, and square metric area unit.
Formula
square millimeters = square inches × 645.16
This factor comes from each unit's defined relationship to the category's base unit: 1 square inch equals 0.00064516 base units, and 1 square millimeter equals 0.000001 base units, so dividing one by the other gives the direct square inch-to-square millimeter factor of 645.16.
Simple example
1 in2 × 645.16 = 645.16 mm2
1 square inch = 645.16 square millimeters.
Real-world example
10 in2 × 645.16 = 6,451.6 mm2
10 square inches = 6,451.6 square millimeters.
Conversion table
| Square inch (in2) | Square Millimeter (mm2) |
|---|---|
| 1 in2 | 645.16 mm2 |
| 5 in2 | 3,225.8 mm2 |
| 10 in2 | 6,451.6 mm2 |
| 25 in2 | 16,129 mm2 |
| 50 in2 | 32,258 mm2 |
| 100 in2 | 64,516 mm2 |
Reverse conversion: Square Millimeter to Square inch
645.16 mm2 × 0.0015500031 = 1 in2
645.16 square millimeters = 1 square inch.
square inches = square millimeters × 0.00155000310001
For a page dedicated to this direction, see Square Millimeter to Square inch.
Understanding the Square inch (in2)
A square inch is the area of a one-inch square.
Understanding the Square Millimeter (mm2)
Square metric area unit.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many square millimeters are in 1 square inch?
1 square inch equals 645.16 square millimeters, using the exact defined conversion factor rather than an estimate.
How do I convert square inch to square millimeter?
Multiply the square inch value by 645.16. The converter above does this instantly to whatever precision you set.
How do I convert square millimeter back to square inch?
Use the reverse factor: 1 square millimeter equals 0.0015500031 square inches. You can also use the swap control in the converter above to flip the direction instantly.
What is a square inch?
Square inch (in2) is a unit of area.
What is a square millimeter?
Square Millimeter (mm2) is a unit of area.
Is the square inch to square millimeter conversion exact?
Yes. Both square inch and square millimeter are defined by fixed standards rather than physical artifacts, so the factor of 645.16 used above is exact to as many digits as you choose to display.