Metric area units are defined in terms of the square of a length unit, for example the square metre is an area of a square with sides 1 metre in length. There is, however, one additional metric unit of area that is not defined as the square of a length unit; that is the hectare, defined as 10,000 square metres.
The hectare is no longer part of the SI metric system but, because of the history of the metric system, it is 'accepted for use with' SI units being derived from the are (100 square metres) that was part of the original French metric system of 1795.
Table of Metric Area Units
| Unit | As sq m | Useful Information |
| sq cm | 0.0001 | |
| sq m | 1 | |
| hectare | 10,000 | Commonly found in measurement of land area - abbreviated to ha |
| sq km | 1,000,000 | Used for large tracts of land |
Similarly to metric area units, imperial area units are for the most part based on units of length for instance square inch, square yard and square mile. The other commonly found imperial area unit is the acre, that is usually used for measurements of areas of land, especially farmland.
Table of Imperial Area Units
| Unit | As sq m | Useful Information |
| sq in | 0.00064516 | |
| sq ft | 0.09290304 | |
| sq yd | 0.83612736 | |
| acre | 4,046.86 | 4,840 square yards or 0.40468564224 hectares |
| sq mi | 2,589,988 | Used for measuring large tracts of land |