If you are unable to use the BMI calculator, or if you are interested in how BMI is calculated, this page has the mathematical formulas. You can calculate BMI using wither feet, inches, and pounds, or meters, centimeters, and kilograms.
English Formula
Body Mass Index can be calculated using pounds and inches with this equation
BMI = (  Weight in Pounds (Height in inches) x (Height in inches) 
) x 703 
For example, a person who weighs 220 pounds and is 6 feet 3 inches tall has a BMI of 27.5.
(  220 lbs. (75 inches) x (75 inches) 
) x 703 = 27.5 
What Does This All Mean?
Metric Formula
Body Mass Index can also be calculated using kilograms and meters (or centimeters).
BMI =  Weight in Kilograms (Height in Meters) x (Height in Meters) 

or  
BMI = (  Weight in Kilograms (Height in centimeters) x (Height in centimeters) 
) x 10,000 
For example, a person who weighs 99.79 Kilograms and is 1.905 Meters (190.50 centimeters) tall has a BMI of 27.5.
99.79 Kg (1.905 m) x (1.905 m) 
= 27.5 
What Does This All Mean?
BMI Table for Adults
For adults BMI can also be found by using
Body Mass Index for Adults Table (PDF15K)
BMI for children and teens (2 – 20 years) is plotted on age and gender specific growth charts. For more information link to BMI for Children and Teens.