What is fat-free mass?

Fat-free mass (FFM) is the total mass of your body excluding all fat tissue. It is effectively the same as lean body mass (LBM) and includes muscle, bone, organs, and water.

How is fat-free mass calculated?

This tool uses the Boer (1984) formula. For men: FFM = 0.407w + 0.267h − 19.2. For women: FFM = 0.252w + 0.473h − 48.3. Where w is weight in kg and h is height in cm.

Can this calculator track muscle gain?

The Boer formula uses only weight, height, and sex, so it cannot see a change in body composition at a steady weight. Each extra kilogram raises the lean mass estimate by 0.407 kg for men (0.252 kg for women) and adds the rest to fat mass, whatever you actually gained. To follow muscle gain, take regular waist and neck readings with the body fat calculator, which responds to changes in those measurements.

Does the calculator work offline?

Once the page has loaded, yes. The formula runs in the open tab, so it keeps working if your connection drops. Reloading or opening another page needs a connection again.

Fat-free mass calculator

Fat-free mass (FFM) and lean body mass (LBM) are often used interchangeably. This calculator uses the Boer (1984) formula to estimate your fat-free mass from your weight and height. All results are shown in both kg and lbs. All calculations run locally in your browser.

Sex

Units

Weight (kg)

Height (cm)

Results are for informational purposes and do not constitute medical advice.

How it works

The Boer (1984) formula estimates lean body mass from weight and height. For men: LBM = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2. For women: LBM = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3. Imperial inputs are converted to metric before calculation. Fat mass is calculated as total weight minus LBM, and body fat percentage as fat mass divided by total weight multiplied by 100. All arithmetic runs in JavaScript in your browser.

Processing runs in your browser

All calculations happen locally in your browser tab. Our servers are not involved at any point.

Technical specification

The Boer formula was published by P. Boer in the American Journal of Physiology, volume 247 (1984), as a way to estimate lean body mass from weight and height. It uses separate equations for men and women. Because the only inputs are weight, height, and sex, the estimate cannot separate muscle from fat at a given weight.

Men: 0.407 × kg + 0.267 × cm − 19.2
Women: 0.252 × kg + 0.473 × cm − 48.3
Inputs: Weight up to 650 kg, height 50–272 cm
Browser API: Pure JavaScript arithmetic. No library

Related operations

For measuring body fat directly with circumference inputs, try the body fat calculator. To check a general weight category, use the BMI calculator. For a reference weight range tied to height, see the ideal weight calculator.

Frequently asked questions

What is fat-free mass?: Fat-free mass (FFM) is the total mass of your body excluding all fat tissue. It is effectively the same as lean body mass (LBM) and includes muscle, bone, organs, and water.
How is fat-free mass calculated?: This tool uses the Boer (1984) formula. For men: FFM = 0.407w + 0.267h − 19.2. For women: FFM = 0.252w + 0.473h − 48.3. Where w is weight in kg and h is height in cm.
Can this calculator track muscle gain?: The Boer formula uses only weight, height, and sex, so it cannot see a change in body composition at a steady weight. Each extra kilogram raises the lean mass estimate by 0.407 kg for men (0.252 kg for women) and adds the rest to fat mass, whatever you actually gained. To follow muscle gain, take regular waist and neck readings with the body fat calculator, which responds to changes in those measurements.
Does the calculator work offline?: Once the page has loaded, yes. The formula runs in the open tab, so it keeps working if your connection drops. Reloading or opening another page needs a connection again.

Last reviewed September 30, 2026

Other pages for this tool

Related tools

Background reading: Complete guide to health tools