A water tank is 3m long, 2m broad and 1m deep. How many litre of water can hold?

Volume = 3 x 2 x 1 = 6 cubic meters which we write 6 m^3.

A cubic meter contains 1,000 liters, which is the same as saying a liter is a cube 10 cm on a side.

liters = 6 m^3 x 10^3 L/m^3 = 6 x 10^3 L = 6,000 liters

Normally units are indicated by lower case letters, but the lower case l of liter (or litre) is easily mistaken for upper case I or the digit 1 so we use the upper case letter.