Measure · 12 units
Area
Areas are squares of lengths, so every area unit here is exact once its length is. The acre is a chain by a furlong (66 by 660 feet): a strip of ploughland one furlong long.
Convert area
Then and now
Land measures varied from place to place for centuries; the rood and the square perch survive only in old deeds.
All area units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| square centimetre cm² | 1 cm × 1 cm | 0.000 1 m² | exact | BIPM SI Brochure |
| square metre m² | 1 m × 1 m | 1 m² | exact | BIPM SI Brochure |
| square kilometre km² | 1 km × 1 km | 1 000 000 m² | exact | BIPM SI Brochure |
| Metric | ||||
| are a | 100 m² | 100 m² | exact | BIPM SI Brochure |
| Metric (accepted) | ||||
| hectare ha | 100 ares = 10 000 m² | 10 000 m² | exact | BIPM SI Brochure |
| Imperial / US | ||||
| square inch in² | 1 in × 1 in | 0.000 645 16 m² | exact | NIST Handbook 44 |
| square foot ft² | 144 square inches | 0.092 903 04 m² | exact | NIST Handbook 44 |
| square yard yd² | 9 square feet | 0.836 127 36 m² | exact | NIST Handbook 44 |
| acre ac | 43 560 square feet = 160 square rods | 4 046.856 422 4 m² | exact | NIST Handbook 44 |
| square mile mi² | 640 acres | 2 589 988.110 336 m² | exact | NIST Handbook 44 |
| Surveying | ||||
| square rod (perch) | 30¼ square yards | 25.292 852 64 m² | exact historical | NIST Handbook 44 |
| rood | 1/4 acre = 40 square rods | 1 011.714 105 6 m² | exact historical | NIST Handbook 44 |