Calculator
What the Lean Body Mass Calculator does
Lean Body Mass Calculator: calculates Lean Body Mass and Fat Mass from Weight and Body Fat %. Formula: BMI = weight (kg) / height (m)^2. Example: weight 70 kg and body fat % 20 gives lean body mass 56.0.
With Weight and Body Fat % entered, the Lean Body Mass Calculator works out Lean Body Mass and Fat Mass. Everything recomputes live, which makes it easy to test a few scenarios in a row.
Knowing how a figure is derived makes it much easier to interpret sensibly. The Lean Body Mass Calculator exists to take that particular chore off your hands.
The calculation is local, so no health information ever reaches a server.
Field definitions, the method, and a worked example from the default values are all set out further down the page.
What do the Lean Body Mass Calculator fields mean?
The Lean Body Mass Calculator uses 2 inputs. It opens with a complete example loaded, which is usually the quickest way to understand what a field means.
| Field | What to enter | Default |
|---|---|---|
| Weight | measured in kg; accepts 30 to 200 | 70 |
| Body Fat % | accepts 3 to 60 | 20 |
How does the Lean Body Mass Calculator work?
The result is a ratio, not a body-fat measurement. It does not distinguish muscle from fat.
Inputs are validated before anything is computed, so a bad entry is reported rather than quietly producing a wrong answer.
Worked example
These are the values the Lean Body Mass Calculator loads by default, and the result it produces from them. The numbers below come straight from the default inputs, so you can verify them against the live tool.
| Weight | 70 kg |
|---|---|
| Body Fat % | 20 |
| Lean Body Mass | 56.0 |
|---|---|
| Fat Mass | 14.0 |
| What this means | About 56.0 kg is lean mass (muscle, bone, organs) and 14.0 kg is fat mass. |
Read next: Understanding BMI: What the Number Measures, and What It Misses
How to use it
- Type Weight in kg (30 to 200).
- Enter Body Fat % (3 to 60).
- No submit step is needed; the output updates while you edit.
- Look at Lean Body Mass, Fat Mass and What this means in the output panel.
- To keep the result, either copy the link or print the page. Both preserve what is on screen.
What this tool does not do
- This is a screening estimate, not a diagnosis.
- Body composition, ethnicity and age all shift what a "normal" result means.
- Comparisons across tools can differ where each uses a slightly different definition.
- Garbage in, garbage out: confirm each figure means what you think it does before trusting the result.
Frequently asked questions
Enter Weight in kg. Both the inputs and the result are explicitly labelled, so mixed units cannot slip through unnoticed.
Weight accepts 30 to 200 kg. Anything beyond those bounds is flagged rather than approximated.
With the values loaded when the page opens, lean body mass comes out as 56.0. Try changing one value. The answer refreshes without a page reload.
No. The work runs in JavaScript inside this page, so your values never leave your device. You can confirm it in your browser network tab.
It is free with no conditions: no sign-up, and no limit on how often you use it.
Accurate to the published method. Whether the published method suits your circumstances is a question for someone who knows your history.