Most of the world outside the United States uses the metric system, so knowing how to convert between metric and English measurements is important. To convert between the two systems, it's helpful to know these conversion factors:

Conversion Factors 
Length 
1 inch = 2.54 centimeters 

1 foot = 0.30 meters 

1 mile = 1.61 kilometers 
Weight and Mass 
1 ounce = 28.35 grams 

1 pound = 0.45 kilograms 
Capacity 
1 fluid ounce = 29.57 milliliters 

1 gallon = 3.78 liters 
The good news is that with these conversion factors and a little multiplication and division, you should be able to tackle conversions. Let's try one: If an object weighs 72 kilograms, what is its weight in pounds?
First, set up the given measurement as a fraction over 1.
72 kg. 1
Now write the conversion factor for the measurements as a fraction. You can write the fraction two different ways:
0.45 kg. OR 1 lb. 1 lb. 0.45 kg.
The correct way depends on what your given measurement is. Since you are given a measurement in kilograms, you should use the conversion factor with kilograms in the denominator. In other words, the denominator of the conversion factor should have the same measurement units as the numerator of the given fraction. The next step will show you why.
Multiply the given fraction by the conversion fraction:
72 kg. X 1 lb. 1 0.45 kg.
The kilograms in the numerator of the given fraction and the denominator of the conversion factor cancel each other out, and you're left with pounds (and your answer):
72 X 1 lb. = 72 lb. = 158.4 lbs. 1 0.45 0.45
So the next time you have to run a 5 K in gym class, you'll be able to calculate exactly what you're in for!