You have to know how the value of one unit corresponds to the value of the other unit. For example you have to know that there are 100 cm in 1 m or 1000 m in 1 km. It's easy in metric measurement system because it's all about multiplying/dividing by 10 or its multiplications. But in American/English system you just have to learn the values.
For example:
1 km = 1000 m = 10000 dm = 100000 cm = 1000000 mm
But:
1 mile = 1760 yards = 5280 feet = 63360 inches
