In Japan, the floor area of rooms are often measured in terms of Tatami mats (-Jo). You will see this on floor plans when looking at apartments, as well as when buying some items like lights or air purifiers that are advertised as being for certain sized rooms.

It's actually a pretty convenient unit, as long as you're familiar with how large tatami mats are.

Note that this unit isn't used for measuring an entire apartment's floor area though, that's done in meters.

See: https://en.wikipedia.org/wiki/Tatami#Size