RMR from fat-free mass calculator
With 60 kg of lean mass and 15 kg of fat mass, the Nelson equation puts your resting metabolic rate at 1608.6 kcal a day. It works from measured body composition, not from scale weight alone. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Ffm
- 60
- Fm
- 15
- Ffm kg
- 60 kcal/day
- Fm kg
- 15
- Total mass kg
- 75
- Rmr kcal day
- 1,608.6
When you already know fat-free mass and fat mass from DEXA, BodPod, or a quality bioimpedance reading, the Nelson equation turns those kilograms into a resting metabolic rate you can use for meal planning and physique tracking. Unlike weight-and-age formulas that ignore composition, Nelson multiplies fat-free mass by twenty-five point eight and fat mass by four point zero four, then sums the terms into kilocalories per day.
What Nelson RMR measures and which body-composition inputs you need
Resting metabolic rate is the energy your body spends at rest over about twenty-four hours, before exercise multipliers and the thermic effect of food. Classic equations such as Harris-Benedict or Mifflin-St Jeor start from total weight, height, age, and sex, which works when you lack composition data. Nelson et al. (1992) instead use fat-free mass and fat mass because metabolically active tissue dominates resting expenditure while adipose tissue still contributes a smaller coefficient.
Total mass is reported as the sum of the two masses for transparency in body-composition logs. Negative fat mass and non-numeric strings fail the same way. Source FFM and FM from a measurement you already trust; this calculator does not reverse-engineer composition from circumference photos or clothing size.
How to read the result and avoid common mistakes
The unit field is always kcal/day so user interfaces can label the number without hard-coding unit strings in every client. The formula field restates RMR equals twenty-five point eight times FFM plus four point zero four times FM, and method is always nelson so downstream code can branch without sniffing shapes. Do not confuse Nelson with Katch-McArdle: Katch-McArdle estimates basal rate from lean mass alone with a different offset and slope, while Nelson uses both fat-free and fat mass with the published body-composition coefficients.
Extremely large masses above the published isolate safety ceiling are rejected. Remember that fat mass of zero is allowed for theoretical lean-only scenarios, but fat-free mass must stay strictly positive.
What you can do with it
Body-composition coaching logs
Trainers enter measured fat-free and fat mass after DEXA or BodPod sessions to get a Nelson RMR that updates as composition changes, not only as scale weight changes.
FAQ
What is the Nelson RMR formula?
Resting metabolic rate in kcal/day equals 25.8 × fat-free mass (kg) + 4.04 × fat mass (kg). This calculator applies that Nelson et al. (1992) equation exactly.
What happens if fat-free mass is zero or negative?
Fat-free mass must be a positive mass in kilograms; zero and negative values are rejected and not charged.
Which units should I use for FFM and FM?
Send both masses in kilograms. If your UI collects pounds, convert to kg before calling so the published Nelson coefficients apply correctly.
Is the RMR from fat-free mass calculator free?
The browser tool runs free on your device.
How is the Nelson equation different from Katch-McArdle?
Katch-McArdle estimates a basal rate from lean mass alone, with its own offset and slope. Nelson uses both lean mass and fat mass, giving 25.8 kcal for each kilogram of lean tissue and 4.04 for each kilogram of fat.
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/rmr-from-ffm \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"ffm":60,"fm":15}'const res = await fetch("https://api.kit.forhosting.com/health/rmr-from-ffm", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"ffm": 60,
"fm": 15
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/rmr-from-ffm",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"ffm": 60,
"fm": 15
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/rmr-from-ffm", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"ffm":60,"fm":15}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"ffm":60,"fm":15}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/rmr-from-ffm", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"ffm": 60,
"fm": 15
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.rmr_from_ffm",
"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
This capability runs offline as pure arithmetic with no network call and no random seed, so coaching dashboards, sports-science notebooks, and prepaid API clients receive identical JSON for identical inputs every time. Missing fields, non-finite numbers, non-positive fat-free mass, and negative fat mass raise a clear invalid_input error and are not charged.
What Nelson RMR measures and which body-composition inputs you need
Send ffm as a positive finite number in kilograms; aliases fat_free_mass, lean_mass, lbm, and ffm_kg are accepted. Send fm as a non-negative finite number in kilograms; aliases fat_mass, fat_kg, and fm_kg are accepted. The engine multiplies fat-free mass by twenty-five point eight, multiplies fat mass by four point zero four, and adds the products to produce rmr_kcal_day. If fat-free mass is missing, zero, negative, non-finite, or above the isolate safety cap, the request fails with invalid_input. Identical inputs always yield identical JSON because the algorithm never touches the network, a language model, a clock, or a random generator.
Reading the response fields and avoiding common pitfalls
On success the response echoes ffm_kg and fm_kg, then total_mass_kg and rmr_kcal_day in kilocalories per day. Use ffm_kg and fm_kg when a coach wants composition columns beside the calorie line; use rmr_kcal_day when a meal planner only needs the resting baseline before applying an activity multiplier outside this capability. This endpoint does not return total daily energy expenditure: activity factors, training load, and dietary thermogenesis remain out of scope and should be applied by your product after you receive rmr_kcal_day.
API usage, pricing, coaching workflows, and practical limits
Call the free in-browser resting metabolic rate from fat-free mass calculator for quick checks, lecture demos, and client education without installing a nutrition spreadsheet. Prefer kilogram inputs that match the published constants; if your UI collects pounds, convert before calling. Keep expectations clinical and educational: this is a planning estimate for body-composition workflows, not a diagnostic medical device, and users with atypical metabolism or medical conditions should follow professional advice rather than any single predictive equation alone. Send a JSON body with required ffm and fm (or their aliases). Use the prepaid API when a fitness SaaS, telehealth intake form, sports-science lab notebook, or body-composition pipeline needs a stable Nelson 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 DEXA, BodPod, or clinical BIA export on the client side, then store FFM, FM, and RMR in your own database for longitudinal charts as composition changes. Authors shipping SDKs can map rmr_kcal_day directly into dashboards and export CSV columns labeled fat-free mass, fat mass, and resting calories—the same numbers coaches write by hand when they apply Nelson on paper.
Integration use cases
Fitness and nutrition apps
Apps that already store FFM and FM call a stable prepaid endpoint for Nelson resting metabolic rate instead of reimplementing the coefficients in every client language.
Sports science and classroom demos
Labs and courses compare Nelson against weight-only or lean-mass-only equations and pin expected JSON in automated tests with deterministic outputs.
Technical questions
Does this return TDEE or only RMR?
Only RMR (resting kcal/day). Activity multipliers and total daily energy expenditure are out of scope; apply them in your product after you receive rmr_kcal_day.