Lean body mass calculator

Reading fat free mass index

Fat free mass index is lean mass in kilograms divided by height in meters squared. The ranges quoted come from Schutz and colleagues, who reported reference values from a large Caucasian adult sample, so they are a comparison against that population rather than a personal target.

• Low fat free mass index: Under about 17 in men, under about 14 in women. Below the range reported for healthy adult reference populations. Worth attention, particularly if you are losing weight quickly or are over 60.
• Typical fat free mass index: About 17 to 22 in men, about 14 to 18 in women. The range most untrained and moderately active adults fall into.
• High fat free mass index: Above about 22 in men, above about 18 in women. Consistent with sustained resistance training. Values well above this range are uncommon in people who have never trained heavily.

Why lean mass matters more than weight

Two people at the same weight and height can carry completely different amounts of muscle, and their metabolic risk and physical capability differ accordingly. Lean body mass is everything that is not fat: muscle, organs, bone, connective tissue and the water in all of it.

It matters practically because lean mass is where most of your resting energy expenditure comes from, it is where glucose is disposed of after a meal, and it is what determines whether you can get off the floor unaided in your eighties. Loss of it with age is the thing worth defending.

The formulas used here

When you enter a measured body fat percentage, the math is simple: lean mass equals weight multiplied by one minus your body fat fraction. That is the version to rely on.

Without a measured value we use the Boer formula, which was derived from body composition data and is the equation most often used clinically when lean mass is needed for drug dosing calculations. It uses only height, weight and sex, so it cannot distinguish a lean athlete from an untrained person of the same size.

• Boer, men: lean mass equals 0.407 times weight in kilograms, plus 0.267 times height in centimeters, minus 19.2.
• Boer, women: lean mass equals 0.252 times weight in kilograms, plus 0.473 times height in centimeters, minus 48.3.
• From measured body fat: lean mass equals weight multiplied by one minus body fat percent divided by 100.

How to get a number worth trusting

A DEXA scan is the practical reference standard for most people. It separates fat, lean tissue and bone, reports them by region, and is repeatable enough that a follow up scan on the same machine tells you something real about direction.

Bioimpedance scales are convenient and heavily affected by hydration, so their absolute body fat number should be treated with suspicion while their trend, measured under the same conditions each time, is more useful. Skinfold calipers depend almost entirely on who is holding them.

Whatever method you pick, the rule is to keep using the same one. Comparing a DEXA result against a bathroom scale reading tells you about the machines, not about you.

Where this number is misleading

• The Boer estimate cannot see muscle. In a very muscular or very lean person it can be off by several kilograms.
• Fat free mass index does not distinguish muscle from water, so it rises with fluid retention and falls with dehydration.
• Neither number is a diagnosis. Sarcopenia is defined clinically using muscle mass together with strength and function, not by a calculator.
• In pregnancy, in edema, and in people with amputations or significant fluid shifts, these formulas do not apply.

Common questions

Is lean body mass the same as muscle mass?

No. Lean body mass includes muscle plus organs, bone, connective tissue and body water. Skeletal muscle is roughly half of it in most adults, which is why a change in lean mass is not a one for one change in muscle.

How do I work out muscle versus fat from a DEXA scan?

A DEXA report gives you fat mass, lean tissue mass and bone mineral content directly, usually by region. Enter the body fat percentage from that report here and the lean and fat mass split shown will match your scan rather than a formula.

What is a good fat free mass index?

There is no single target. Most untrained adults sit around 17 to 22 in men and 14 to 18 in women in the Schutz reference data, and sustained resistance training moves people to the top of that range or above it.

Sources and disclosures

• Boer P. Estimated lean body mass as an index for normalization of body fluid volumes. American Journal of Physiology, 1984.
• Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18 to 98 years. International Journal of Obesity, 2002.
• Cruz-Jentoft AJ, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age and Ageing, 2019.
• These calculators are for education. They do not diagnose, they do not replace testing, and they are not a prescription. Baseline is a wellness consultancy, and any prescription is written by an independent licensed provider in our network who has reviewed your history, your goals and any lab work you choose to share.