By using your BMI (Body Mass Index), age and gender you can find out your body fat percentage. Formula to find out Child’s Body Fat percentage, (1.51 x BMI) – (0.70 x Age) – (3.6 x gender) + 1.4. Formula to find out Adult’s Body Fat percentage (1.20 x BMI) + (0.23 x Age) – (10.8 x gender) – 5.4. *Note : Use gender (male = 1, female = 0).