Identify the base unit. This will be the last or only letter next to the number. Common base units are m, measuring meters, the metric unit of length, L for liters, unit for volume and g for grams, measuring mass.
Identify the prefix. Prefixes indicate how much of the base unit there is in powers of ten. Common prefixes include k, for kilo, the base times 1,000, c for centi, the base times -100 and m for milli, the base times -1,000. While they are the most common, there are many prefixes besides kilo, centi and milli.
Put the prefix and base units together. Kg is a kilogram, or 1,000 grams; mm is millimeter, or -1,000 meters.
Convert between units by multiplying or dividing by tens, or just moving the decimal point. Twenty meters would be 20,000 millimeters or 0.02 kilometers.