Body Mass Index (BMI) is a person's weight in kilograms divided by the square of height in meters. A high BMI can be an indicator of high body fatness. BMI can be used to screen for weight categories that may lead to health problems but it is not diagnostic of the body fatness or health of an individual.
BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat. Furthermore, BMI appears to be as strongly correlated with various metabolic and disease outcome as are these more direct measures of body fatness.
Because calculation requires only height and weight, BMI is an inexpensive and easy tool. To see the formula based on either kilograms and meters or pounds and inches, visit https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html#Interpreted (How is BMI calculated?).