Body Fat Percentage Calculator (BMI Method) Formula

Understand the math behind the body fat percentage calculator (bmi method). Each variable explained with a worked example.

Use the Body Fat Percentage Calculator (BMI Method)

Formulas Used

Estimated Body Fat %

body_fat_pct = 1.20 * bmi + 0.23 * age - 16.2

BMI

bmi_display = bmi

Lean Mass %

lean_mass_pct = 100 - (1.20 * bmi + 0.23 * age - 16.2)

Variables

Variable	Description	Default
`weight_lbs`	Weight(lbs)	180
`height_feet`	Height (feet)(ft)	5
`height_inches`	Height (inches)(in)	10
`age`	Age(years)	30
`total_inches`	Derived value`= height_feet * 12 + height_inches`	calculated
`bmi`	Derived value`= (weight_lbs * 703) / (total_inches * total_inches)`	calculated

How It Works

BMI-Based Body Fat Estimation (Male)

Body Fat % = 1.20 x BMI + 0.23 x Age - 16.2

This formula was developed by Deurenberg et al. and provides an estimate of body fat percentage using BMI and age.

Note: This is the male formula. The female formula uses different constants (1.20 x BMI + 0.23 x Age - 5.4).

Worked Example

A 30-year-old male, 180 lbs, 5'10".

weight_lbs = 180height_feet = 5height_inches = 10age = 30

01BMI = (180 x 703) / (70 x 70) = 25.8
02Body Fat = 1.20 x 25.8 + 0.23 x 30 - 16.2
03= 31.0 + 6.9 - 16.2 = 21.7%

Frequently Asked Questions

How accurate is the BMI body fat formula?

This method provides a rough estimate. It tends to overestimate body fat in muscular individuals and underestimate it in older adults. Direct methods like DEXA scans are more accurate.

Who developed this formula?

This formula was developed by Deurenberg, Weststrate, and Seidell (1991) and is widely cited in body composition research.

Learn More

Guide

Body Fat Percentage Guide: How to Measure and Interpret Your Results

Learn how to measure body fat percentage, what healthy ranges look like for men and women, and how different measurement methods compare in accuracy and practicality.

Ready to run the numbers?

Open Body Fat Percentage Calculator (BMI Method)