Consider a square mile (a section of land) as a checker board*. Each of the 64 squares is 1 furlong on a side. If a square is subdivided into 10 strips, each 1 chain wide, each strip will be 1 acre in area. A Canadian Football League field is exactly half a furlong in length, from goal line to goal line.
Using the same checker board, a square kilometre would occupy 5 squares by 5 squares. Each square would be 200 metres on a side. If a square is subdivided into 4 smaller squares, each 100 metres on a side, each smaller square will be 1 hectare in area.
If you can visualize this board, you'll easily be able to remember how to convert between units. Metric/Imperial conversions done this way are accurate to about 99.4%.
There are 10 chains in a furlong. There are 10 acres in a square furlong, and 64 square furlongs (8 × 8 squares) in the section, so there are 640 acres in a square mile.
A kilometre is 1000 metres long, or 5 × 200 metres. So a kilometre is 5 furlongs long, or 5/8 of a mile. Similarly a mile is 8/5 of a kilometre, or 1.6 km.
Each square contains 4 hectares or 10 acres. So 1 hectare is 2.5 acres, and 1 acre is 0.4 hectares.



If you find Metric confusing, that's only because you (like most Americans, Canadians, and British) don't even really understand the old Imperial system. Or did you already know that there are 4 rods in a chain? Or that a rod is 16 ½ feet or 5 ½ yards long? And how many barleycorns are there in an inch?
* And yes, I do know that the checker board is incorrectly positioned. But, I wanted to use the upper left corner for the details, and the lighter coloured squares contrast better with the text.