How can you understand the bmi when calculating with muscle?

BMI and muscle: The bmi is a good guide for most people, but it is not an indicator of obesity in some groups. Bmi is just weight/height squared. It does not take into account muscle. Muscular men can have a high bmi and be lean.

BMI & body fat %.: The BMI reflects your weight with respect to your height, but as you suggested does not reflect body composition. People who are very muscular could have a higher BMI that would not be truly reflective of their fitness and health. Combining the BMI with a body fat percentage would greatly increase its accuracy.