The formula
How to calculate lean body mass
Lean body mass is everything that is not fat: muscle, bone, organs, skin and body water. It is the figure drug dosing is often based on, and the one that should be preserved during weight loss.
The Boer formula estimates lean mass from height and weight alone. It is population-derived, so it describes a typical body at those measurements rather than measuring yours — a bodybuilder and a sedentary person of identical height and weight get the same answer.
What to enter:
- Weight (kg)
- Height (cm)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why lean body mass matters
This kind of calculation gets used both out of general interest and ahead of a specific conversation — with a trainer, a midwife or a GP — where having an actual number ready, rather than a description, makes that conversation more concrete and specific.
This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.
It is also worth being honest about what a formula like this is actually built from. Most of the equations behind calculators like this one were derived from population studies — averages taken across large groups of people — and applied here as a general-purpose estimate rather than a measurement tailored to any one individual. That is a reasonable trade-off for a quick, free estimate, and it is also exactly why a result that looks unusual is worth discussing with a professional rather than acted on alone.
In practice, this kind of figure is often calculated more than once with slightly different inputs to see how sensitive it is — trying a measurement taken this morning against one taken yesterday evening, for instance, to see how much normal daily variation actually moves the result. That habit is a good one: it builds a feel for how much a single reading can be trusted on its own before drawing any conclusion from it.
Worked example
Here is the calculation with the starting values:
- Weight: 75 kg
- Height: 175 cm
That gives:
- Lean body mass, male formula: 58.05 kg
- Lean body mass, female formula: 53.37 kg
- Implied fat mass, male formula: 16.95 kg
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
Use the trend, not the number. If weight falls while the estimate holds steady, the loss is coming from fat, which is the outcome you want.
Where this goes wrong. At the extremes the formula breaks down, and it can produce implausible results for very short, very tall or very heavy people. Where lean mass matters clinically, a measured method such as DEXA or bioimpedance should be used instead.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
Boer is the most widely used and generally the most accurate for people in the normal to overweight range. James tends to underestimate at higher weights, and Hume was derived from a small sample.
No. Lean mass includes bone, organs and body water as well as muscle. Skeletal muscle is usually around half of it.
The headline figure is lean body mass, male formula. With 75 kg weight and 175 cm height, that comes to 58.05 kg. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.