Fat-free mass calculator
Splits your scale weight into fat and lean mass: at 80 kg with 15% body fat, lean mass is 68 kg and fat mass 12 kg. It works from the body-fat percentage you already measured and never guesses one. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Weight
- 80
- Body fat
- 15
- Weight (kg)
- 80 kg
- Body fat percent
- 15
- Fat free mass kg
- 68
- Fat mass kg
- 12
Fat-free mass is the portion of body weight that is not fat—muscle, bone, organs, and water—expressed in kilograms. Coaches, athletes, clinicians, and self-trackers need that number whenever a measured body-fat percentage is available but the scale only shows total weight. Body-fat percentages outside zero through one hundred are rejected as invalid input because fat-free mass cannot be negative or exceed total weight.
What fat-free mass means and which inputs you need
Total body weight mixes fat tissue and everything else. Fat-free mass, also called lean body mass in everyday coaching language, isolates the non-fat compartment so progress during a cut or bulk can be judged without treating every kilogram on the scale as muscle. The classic two-compartment formula is simple and universal: fat-free mass in kilograms equals body weight in kilograms times one minus body-fat percentage divided by one hundred.
Equivalently, fat mass equals weight times the fat fraction, and fat-free mass is weight minus fat mass. This capability applies that definition exactly. The engine does not invent a body-fat estimate from circumference, photos, age, or sex—you must already have a percentage from DEXA, hydrostatic weighing, a quality bioimpedance device, skinfolds, or another method you trust.
Reading your results and avoiding common mistakes
The unit field is always kg so UIs can label absolute mass without hard-coding strings. Lean-mass percent is simply one hundred minus the body-fat percentage; it is included for dashboards that prefer a complementary percentage next to the kilogram figures. Do not confuse it with BMI, BMR, or total daily energy expenditure: those answer stature-adjusted adiposity, resting metabolism, or activity-scaled calories.
Extremely large weights above the published safety ceiling are rejected. Source your body-fat percentage carefully—garbage composition input produces a precise but meaningless fat-free mass—and prefer the same measurement method over time when charting lean mass across a season so method drift does not masquerade as muscle gain or loss.
Everyday uses and practical limits
Call the free in-browser fat-free mass calculator for quick checks, lecture demos, and client education without installing a sports-science spreadsheet. Prefer kilogram weight that matches the published formula; if your UI collects pounds, convert to kilograms before calling. Results are safe to pin in automated tests because the algorithm is fully deterministic across runtimes.
Keep expectations clinical: this is an educational and planning estimate, not a diagnostic medical device, and users with atypical body composition or medical conditions should follow professional advice rather than any single calculator alone.
What you can do with it
Coaching cut and bulk tracking
Coaches enter scale weight and a measured body-fat percentage to track fat-free mass in kilograms as athletes cut fat or add lean tissue over a training block.
FAQ
What is the fat-free mass formula?
Fat-free mass in kilograms equals body weight in kg times (1 − body fat percent / 100). Fat mass is the complement: weight minus fat-free mass. This calculator applies that definition exactly.
What happens if body fat is above 100 percent?
Body-fat percentage cannot exceed 100 percent; values above that limit are rejected and not charged.
How is this different from fat-free mass index (FFMI)?
This calculator returns absolute fat-free mass in kilograms from weight and body fat only. FFMI divides lean mass by height squared and requires height; it is a separate capability with a different purpose.
Which units should I use for weight?
Send body weight in kilograms. If your UI collects pounds, convert to kilograms before calling so the published FFM definition applies correctly.
Is the fat-free mass calculator free?
The browser tool runs free on your device.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/health/fat-free-mass-calc \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"weight":80,"body_fat":15}'const res = await fetch("https://api.kit.forhosting.com/health/fat-free-mass-calc", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"weight": 80,
"body_fat": 15
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/fat-free-mass-calc",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"weight": 80,
"body_fat": 15
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/fat-free-mass-calc", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"weight":80,"body_fat":15}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"weight":80,"body_fat":15}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/fat-free-mass-calc", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"weight": 80,
"body_fat": 15
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.fat_free_mass_calc",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
The algorithm is pure arithmetic with no network calls and no random seed, so free browser checks and prepaid API calls always agree.
What fat-free mass means and which inputs you need
Send weight as a positive finite number in kilograms; aliases weight_kg, body_weight, and mass_kg are accepted so fitness apps can map their existing field names without renaming payloads. Send body_fat as a measured percentage from zero through one hundred inclusive; aliases body_fat_percent, bf, fat_percent, and bodyfat are accepted. Missing fields, empty strings, non-finite numbers, non-positive weight, and body fat below zero or above one hundred all fail with invalid_input and are not billed. Identical inputs always produce identical JSON because the path never touches the network, a language model, a clock, or a random generator.
Reading the response fields and avoiding common mistakes
On success the response echoes weight_kg and body_fat_percent, then fat_free_mass_kg, fat_mass_kg, and lean_mass_percent. The formula field restates FFM equals weight times one minus body fat over one hundred, and method is always fat_free_mass so downstream code can branch without sniffing shapes. Do not confuse this endpoint with fat-free mass index (FFMI), which divides lean mass by height squared and lives on a different capability with a different SEO intent.
API usage, pricing, workflows, and practical limits
Send a JSON body with required weight and body_fat (or their aliases). Use the prepaid API when a fitness SaaS, team performance platform, telehealth intake form, nutrition app, or research notebook needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with whatever body-fat measurement workflow you already run on the client, then store fat_free_mass_kg and fat_mass_kg in your own database for longitudinal charts. Authors shipping SDKs can map fat_free_mass_kg and fat_mass_kg directly into dashboards and export CSV columns labeled lean mass and fat mass—the same numbers coaches compute by hand when they apply the two-compartment split on paper.
Integration use cases
Fitness and nutrition apps
Apps that already store composition readings call a stable prepaid endpoint for FFM instead of reimplementing weight times one minus fat fraction in every client language.
Classroom and research demos
Courses and labs illustrate the two-compartment body model and pin expected JSON in automated tests with fully deterministic outputs.