Why is BMI used to measure overweight and obesity?
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

BMI calculation:
BMI Calculator is a free app that allows you to monitor BMI and the percentage of fat in your body.
Ideal weight - app calculates the ideal weight you should gain.
To calculate it app uses the D. R. Miller formula.
All measurements use information about your body: gender, age, height, and weight.
The app is designed for people of different ages and supports both metric and imperial.
Track your BMI and stay healthy!

BMI as an indicator of body fatness:
The correlation between BMI and body fatness is fairly strong1,2,3,7, but even if two people have the same BMI, their level of body fatness may differ12.
In general,
At the same BMI, women tend to have more body fat than men.
At the same BMI, the amount of body fat may be higher or lower depending on the racial/ethnic group13-15.
At the same BMI, older people, on average, tend to have more body fat than younger adults.
At the same BMI, athletes have less body fat than do non-athletes.
The accuracy of BMI as an indicator of body fatness also appears to be higher in persons with higher levels of BMI and body fatness16. While, a person with a very high BMI (e.g., 35 kg/m2) is very likely to have high body fat, a relatively high BMI can be the result of either high body fat or high lean body mass (muscle and bone). A trained healthcare provider should perform appropriate health assessments to evaluate an individual’s health status and risks