| 43560 | square feet |
| 0.001563 | square miles |
| 4047 | square meters |
| 0.4047 | hectares |
An acre is “the amount of land that could be ploughed in one day with a yoke of oxen,” in the Middle Ages.1 Which is totally alright and a little bit neat, but I don't really feel like sitting around to watch oxen mark out space.
More on acres at Wolfram Alpha. Source on Github.