How to Calculate Body Fat Percentage(BFP)
If the sizes of Waist, Neck and Hip are available, the U.S. Navy method will normally generate a better result. The U.S. Navy method is based on this formula:
 For Men BFP =
\[ \frac{495}{1.0324 0.19077log(W  N) + 0.15456log(He)}  450 \]
 For Women BFP =
\[ \frac{495}{1.29579 0.35004log(W +Hip  N) + 0.221log(He)}  450 \]
Where W is Waist, N is Neck, and He is Height. This formula is based on the metric units that is in cm. For US units, we will convert it into metric firstly and then apply the formula.
Otherwise, the BMI based body fat percentage formula is used:
 Body fat percentage (BFP) formula for adult males:
\[ BFP = 1.20 \times BMI + 0.23 \times Age  16.2 \]

Body fat percentage (BFP) formula for adult males:
\[ BFP = 1.20 \times BMI + 0.23 \times Age  16.2 \]

Body fat percentage (BFP) formula for adult females:
\[ BFP = 1.20 \times BMI + 0.23 \times Age  5.4 \]

Body fat percentage (BFP) formula for boys:
\[ BFP = 1.51 \times BMI  0.70 \times Age  2.2 \]

Body fat percentage (BFP) formula for girls:
\[ BFP = 1.51 \times BMI  0.70 \times Age + 1.4 \]