Convert Square millimetres to Square inches
1 square millimetre is 0.00155 sq in. One square millimetre is exactly 0.00155000310001 square inches.
One square millimetre is 0.0015500031 square inches. To convert square millimetres to square inches, multiply by 0.0015500031; to go the other way, divide by 0.0015500031 or multiply by 645.16. The relation is exact but the figure is not: it is a rounding of a number that does not terminate, and it follows from NIST Office of Weights and Measures. A millionth of a square metre, because the millimetre is a thousandth of a metre and area squares the factor.
Factor checked against the defining standard · How we check
- Square millimetres × 0.0015500031
1 * 0.001550.002 sq in
1 square millimetre in every area unit
| Unit | Value | Exact factor |
|---|---|---|
| square inches | 0.0015500031 sq in | × 0.00155000310001 |
| square feet | 0.00001076391042 sq ft | × 0.0000107639104167 |
| square yards | 0.000001195990046 sq yd | × 0.0000011959900463 |
| acres | 0.0000000002471053815 ac | × 0.000000000247105381467 |
| square meters | 0.000001 m² | × 0.000001 |
| square millimetres (as entered) | 1 mm² | — |
| square centimetres | 0.01 cm² | × 0.01 |
| square kilometres | 0.000000000001 km² | × 0.000000000001 |
| square miles | 0 mi² | × 0.000000000000386102158542 |
| hectares | 0.0000000001 ha | × 0.0000000001 |
Factors are entered by hand from NIST Office of Weights and Measures rather than derived from a units library, so the ones that terminate are exact rather than rounded.
The same converter answers this pair however it is written: sqmm to sqin is how the symbols are typed on a keyboard with no superscript on it.
How do you convert square millimetres to square inches? The converter and the working
- 1.Take the measurement in square millimetres.
- 2.Multiply by 0.0015500031 — that is how many square inches fit in one square millimetre.
- 3.To go back the other way, divide by 0.0015500031, or multiply by 645.16.
Worked example: 25 square millimetres to square inches
- Formula
- mm² × 0.0015500031 = sq in
- Arithmetic
- 25 × 0.0015500031 = 0.04 sq in
- Answer
- 25 mm² = 0.04 sq in
The factor is rounded for display; the figure above is computed from the full value.
Square millimetres to square inches conversion table
| Square millimetres | Square inches |
|---|---|
| 1 mm² | 0.00155 sq in |
| 2 mm² | 0.0031 sq in |
| 3 mm² | 0.00465 sq in |
| 4 mm² | 0.01 sq in |
| 5 mm² | 0.01 sq in |
| 6 mm² | 0.01 sq in |
| 7 mm² | 0.01 sq in |
| 8 mm² | 0.01 sq in |
| 9 mm² | 0.01 sq in |
| 10 mm² | 0.02 sq in |
| 12 mm² | 0.02 sq in |
| 15 mm² | 0.02 sq in |
| 20 mm² | 0.03 sq in |
| 25 mm² | 0.04 sq in |
| 50 mm² | 0.08 sq in |
| 100 mm² | 0.16 sq in |
| 200 mm² | 0.31 sq in |
| 300 mm² | 0.47 sq in |
| 500 mm² | 0.78 sq in |
| 1000 mm² | 1.55 sq in |
| 2000 mm² | 3.1 sq in |
| 5000 mm² | 7.75 sq in |
| 10000 mm² | 15.5 sq in |
What is a square millimetre?
A millionth of a square metre, because the millimetre is a thousandth of a metre and area squares the factor. The working unit of engineering drawings and of cable cross-sections.
What is a square inch?
An inch on a side. 144 of them make a square foot.
How many square inches are in a square millimetre?
One square millimetre is 0.0015500031 square inches. Going the other way, one square inch is 645.16 square millimetres. Both figures come from the same defined relation rather than from a measurement, which is why they are the same today as they were in 1959.
Every area unit here
The whole group — 10 units with their symbols and the size of each one in square meters — is on the conversion index, along with every pair it makes. The table above already gives each of them relative to a square millimetre.
Cite this page
- APA
- Rule Calculator. (2026). Convert Square millimetres to square inches. Rule Calculator. https://rulecalculators.com/convert/square-millimeter-to-square-inch
- MLA
- "Convert Square millimetres to square inches." Rule Calculator, August 16, 2026, https://rulecalculators.com/convert/square-millimeter-to-square-inch.
- Chicago
- Rule Calculator. "Convert Square millimetres to square inches." Rule Calculator. Last reviewed August 16, 2026. https://rulecalculators.com/convert/square-millimeter-to-square-inch.
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Questions people actually ask
- How many square inches are in a square millimetre?
- 0.0015500031. An inch on a side. 144 of them make a square foot.
- How do I convert square inches back to square millimetres?
- Divide by 0.0015500031, or multiply by 645.16 — they are the same operation.
Other area conversions
Square inches to square millimetres — the same conversion the other way.