Body mass index is a very common measurement used by doctors to try and predict health based on your height and weight. This system is flawed for those who are not children or completely sedentary. Which in light of the average American, it might be more accurate today than it used to be.
The big issue with only taking someone's height and weight into account is that it completely ignores body composition. Body composition is what your body is actually made up of. Fat mass, or Fat-Free Mass (FFM), which makes up muscle, tendons ligaments, water content, and bone. For athletes and those are heavy into resistance training, it often predicts them as overweight because they are at a higher weight due to the muscle mass being denser. If you looked at them you wouldn't think they were overweight, but if you read those two numbers without seeing the person, you might assume they are the average American.
All this to say, it is still widely used and because of that it still can be a good educational point, especially when you compare it to your client's actual body composition.
Measure your clients height
Measure your clients weight
Plug those numbers into the calculation below for your BMI number.
Calculation of BMI: Use average weight and height measurements.
BMI=______________________kg * m^-2
If you do not want to calculate it on your own, plug the numbers into this calculator. (Feet, inches and pounds will need to be measured or converted to from the metric numbers).
Haff, G. G., & Dumke, C. (2018). Laboratory Manual for Exercise Physiology, 2E: Human Kinetics.