RMR Calculator
Estimate resting metabolic rate from lean body mass using Katch-McArdle and Cunningham — and see how wide the error bar really is.
RMR Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Entirely dependent on the accuracy of the body-fat estimate supplied.
- Population regressions with roughly ±10% error against indirect calorimetry.
- Cannot account for thyroid disease, medication or illness.
In short: At 59 kg of lean mass, Katch-McArdle returns 1,644 kcal a day and Cunningham 1,798. The honest reading is not the number but the interval: a routine ±10% prediction error spans 1,481 to 1,810 kcal, a 329 kcal window wider than the gap between the formulas.
Formula
Both are driven entirely by lean mass, so both inherit the error in your body-fat estimate.
Worked Example
- Split weight into lean and fat. 82 kg at 28% body fat = 59.0 kg lean, 23.0 kg fat.
- Apply Katch-McArdle. 370 + 21.6 × 59.0 = 1,644 kcal/day.
- Apply Cunningham. 500 + 22 × 59.0 = 1,798 kcal/day.
- Compare with Mifflin-St Jeor. 10(82) + 6.25(178) − 5(41) + 5 = 1,733 kcal/day.
- Put an error bar on it. ±10% on Katch-McArdle spans 1,481–1,810 — a 329 kcal window.
The three formulas land within about 150 kcal of each other, and that agreement is less meaningful than it looks. A predictive equation is typically accurate to roughly ±10% against indirect calorimetry, which on this person spans 1,481 to 1,810 kcal — a 329 kcal window that comfortably swallows the disagreement between the formulas. So choosing between Katch-McArdle and Mifflin-St Jeor is not the decision it is often presented as. The genuinely consequential input is the body-fat percentage: Katch-McArdle knows nothing except lean mass, so a five-point error in that estimate moves lean mass by about 4 kg and the answer by around 89 kcal. If your body-fat figure came from a bathroom scale, use Mifflin-St Jeor instead — a guessed input makes the more sophisticated equation worse, not better.
Strengths & Limits Of This Model
Where this engine is strong
- Uses lean mass, which drives resting metabolism, rather than total weight.
- Quantifies the error bar and shows it exceeds the gap between formulas.
- Distinguishes RMR from BMR, which most calculators conflate.
Where it stops
- Requires a body-fat estimate that most people do not have accurately.
- Cannot measure adaptive changes in your own metabolism.
Practical Use Cases
Estimating resting burn when you know your body fat
Katch-McArdle uses lean mass directly, which Mifflin-St Jeor cannot.
Comparing formulas honestly
The ±10% error bar is wider than the gap between any two of them.
Understanding a stalled diet
Resting rate falls during restriction by more than tissue loss alone explains.
Scaling up to a daily target
Apply an activity factor with the TDEE Calculator.
Methodology & Editorial Standards
Resting metabolic rate is estimated here from lean body mass rather than total weight, using the Katch-McArdle equation as the headline and the Cunningham equation alongside it, on the reasoning that skeletal muscle and adipose tissue differ substantially in metabolic activity and that an equation aware of body composition should outperform one that is not. That advantage is conditional: both equations are driven entirely by the lean mass term, so they inherit the full error of the body-fat estimate supplied, and the engine quantifies that sensitivity directly rather than leaving it implicit. Mifflin-St Jeor is computed for comparison because it requires no body-composition input and is the appropriate choice when a reliable body-fat figure is unavailable. The engine reports the interval implied by the approximately ten percent error typical of predictive equations against indirect calorimetry, and states plainly that this interval exceeds the disagreement between the formulas, so that the choice of equation is a smaller decision than the quality of its inputs. The engine also distinguishes resting from basal metabolic rate, the latter requiring laboratory conditions and running roughly ten percent lower, and describes the adaptive decline in resting rate observed during sustained energy restriction. This calculator is an educational estimate and is not medical advice, diagnosis or treatment. It cannot assess your clinical condition, and no result here should be used to start, stop or change any medication or treatment. Discuss results with a qualified clinician who has access to your full history. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
RMR Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is RMR?
Resting metabolic rate — the energy your body uses at rest, typically 60 to 70% of daily burn.
What is the difference between RMR and BMR?
BMR requires strict laboratory conditions. RMR is measured more loosely and runs about 10% higher.
What is the Katch-McArdle formula?
RMR = 370 + 21.6 × lean body mass in kg. It uses lean mass rather than total weight.
How is Cunningham different?
500 + 22 × lean mass. Derived in a leaner cohort, it runs higher and suits trained people.
Which formula is most accurate?
Whichever has the best inputs. The ±10% error bar is wider than the gap between them.
Should I use Katch-McArdle or Mifflin-St Jeor?
Katch-McArdle if your body-fat figure is reliable; Mifflin-St Jeor if you are guessing it.
How accurate are these equations?
Roughly ±10% against indirect calorimetry, and worse at very high or very low body fat.
Why does body fat percentage matter so much?
Lean mass is the only variable in these equations, so an error in body fat passes straight through.
How much does a body-fat error cost?
About 21.6 kcal for every kilogram of lean mass misestimated.
Does RMR fall when I diet?
Yes, and by more than lost tissue alone explains. That is adaptive thermogenesis.
Does adaptive thermogenesis persist?
It can persist well beyond the diet, which contributes to how common regain is.
Can I raise my RMR?
Adding lean mass raises it, but slowly — roughly 21.6 kcal per kilogram gained.
Does muscle burn far more than fat at rest?
More, but less dramatically than often claimed. The difference is real and modest.
How is RMR actually measured?
Indirect calorimetry, measuring oxygen consumption and carbon dioxide production.
Does thyroid disease affect RMR?
Substantially, in both directions, and no equation can detect it. That needs a clinician.
Should I eat at my RMR?
No. RMR is resting burn only, before any activity. Eating at that level is a large deficit.
Is this rmr calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.