What are Square Millimeters?
A square millimeter is the area of a square one millimeter on each side, so there are 100 in a square centimeter and 1,000,000 in a square meter. Electrical cable is sized by cross-section in them – 1.5mm² and 2.5mm² are common.
What are Acres?
An acre is 43,560 square feet, which is about 4,047 square meters. It was originally the amount of land a yoke of oxen could plough in one day.
The United States and United Kingdom still measure land in acres, but most of the world uses hectares, which fit neatly into the metric system. One hectare is about 2.47 acres.