What is resting metabolic rate and why does it matter?

Resting metabolic rate (RMR) is the number of calories your body burns at complete rest just to keep vital functions running — breathing, circulation, cell repair, temperature regulation, and organ function. It typically accounts for 60–75% of your total daily energy expenditure (TDEE). Knowing it gives you a principled starting point for any calorie target, whether you want to lose fat, maintain weight or build muscle.

Which formula is the most accurate?

Mifflin-St Jeor (1990) consistently shows the lowest mean prediction error (~8%) in large validation studies of non-obese adults — which is why it is the default. Revised Harris-Benedict (Roza and Shizgal 1984) is nearly as accurate and is slightly better calibrated for older adults. Katch-McArdle and Cunningham use lean body mass rather than total body weight, so they outperform weight-based equations for athletes and very lean or very overweight individuals — but only when you supply a reliable body fat percentage.

What is the Katch-McArdle formula?

Katch-McArdle (1996) calculates RMR directly from lean body mass (LBM): RMR = 370 + 21.6 × LBM (kg). Because it ignores height, age and sex once LBM is known, it side-steps the systematic errors that weight-based equations introduce for people with high or low muscle mass. Enter your body fat percentage and the calculator derives LBM automatically.

What is the difference between RMR and BMR?

Basal metabolic rate (BMR) is measured under strict laboratory conditions — 12-hour fast, full night of sleep, thermoneutral room, lying completely still for at least 30 minutes. RMR allows slightly less stringent conditions and is therefore a bit higher (typically 10–20% above true BMR). In practice the two terms are used interchangeably in diet and fitness apps, and all five equations in this calculator estimate the resting, not basal, rate.

How is TDEE calculated from RMR?

TDEE (total daily energy expenditure) multiplies RMR by an activity factor: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active, 1.9 for extremely active, and 2.2 for elite athletes training twice daily. These multipliers (derived from doubly-labelled water studies) account for all movement and the thermic effect of food.

How many calories should I eat to lose 0.5 kg per week?

One kilogram of body fat stores roughly 7,700 kcal. To lose 0.5 kg per week you need a daily deficit of about 550 kcal below your TDEE. The goal table labels this "Steady loss" and shows the exact number for your stats. The absolute minimum for safe weight loss is 1,200 kcal/day for women and 1,500 kcal/day for men — the calculator flags when any target drops below those thresholds.

Resting Metabolic Rate Calculator

Your resting metabolic rate is the engine number under the bonnet of your metabolism — the calories your body burns around the clock even when you are doing nothing at all. This calculator runs five peer-reviewed equations side by side, shows you the full arithmetic, and converts the result into practical calorie targets and gram-level macros for any goal. Everything computes privately in your browser.

How it works

Step 1 — Choose a formula. The tool supports five equations covering six decades of nutrition science:

Formula	Year	Inputs	Best for
Mifflin–St Jeor	1990	weight, height, age, sex	Default — lowest error in healthy adults
Harris–Benedict Revised	1984	weight, height, age, sex	Slightly better for older adults
Katch–McArdle	1996	lean body mass	Athletes, high/low body-fat outliers
Owen	1986/1987	weight, sex	Simple weight-only estimate
Cunningham	1980	lean body mass	Sports science / athletic populations

Step 2 — Multiply by an activity factor to reach your TDEE (total daily energy expenditure):

TDEE = RMR × activity factor

Activity factors range from 1.2 (desk job, no exercise) to 2.2 (elite twice-daily training).

Step 3 — Apply a calorie target. Subtract 500 kcal/day from TDEE for steady fat loss (~0.5 kg/week), add 250 kcal for a lean gain (~0.25 kg/week), or stay at TDEE to maintain. One kilogram of body fat holds approximately 7,700 kcal.

The Mifflin–St Jeor equation (default)

Published in the Journal of the American Dietetic Association in 1990 and validated against indirect calorimetry on more than 250 subjects, Mifflin–St Jeor is the most cited predictive equation in clinical nutrition:

Men: RMR = 10 × W(kg) + 6.25 × H(cm) − 5 × A + 5
Women: RMR = 10 × W(kg) + 6.25 × H(cm) − 5 × A − 161

The only difference between sexes is the constant (+5 vs −161), reflecting average differences in fat-free mass distribution. The mean prediction error across validation studies is roughly ±8%, which translates to about ±150 kcal for a 1,800 kcal RMR.

The Katch–McArdle equation (lean mass)

When you supply a body fat percentage the calculator can use:

RMR = 370 + 21.6 × LBM (kg)

where LBM = total body weight × (1 − body fat fraction). Because lean mass is the metabolically active tissue, this single-variable formula outperforms weight-based equations for anyone who deviates significantly from average body composition — athletes, strength trainers, and people carrying substantial excess fat. A DEXA scan or calibrated skinfold measurement gives the most accurate body fat input; circumference-based estimates are a reasonable approximation.

Worked example

A 32-year-old woman, 65 kg, 168 cm, 24% body fat, moderately active:

Mifflin–St Jeor:

RMR = 10 × 65 + 6.25 × 168 − 5 × 32 − 161
    = 650 + 1,050 − 160 − 161
    = 1,379 kcal/day
TDEE = 1,379 × 1.55 = 2,137 kcal/day

Katch–McArdle:

LBM  = 65 × (1 − 0.24) = 49.4 kg
RMR  = 370 + 21.6 × 49.4 = 1,437 kcal/day
TDEE = 1,437 × 1.55 = 2,227 kcal/day

The lean-mass formula gives a higher RMR because the woman’s muscle-to-fat ratio is above the population average assumed by Mifflin–St Jeor for that weight. For a high-body-fat individual the relationship reverses.

Goal	Daily calories	Weekly change
Steady fat loss	1,637	−0.46 kg
Maintain	2,137	0
Lean gain	2,387	+0.23 kg

Why RMR matters for fat loss

Most diet calculators show you a calorie number without revealing how it was derived. Understanding that your RMR is the foundation — and that it accounts for up to 75% of the calories you burn — reframes common mistakes. Severe restriction (e.g., 800 kcal/day) eats into the RMR cushion, triggers adaptive thermogenesis (metabolic slowdown of 5–15%), and accelerates lean mass loss. A moderate 15–20% deficit below TDEE preserves muscle, keeps hormonal signals intact, and produces fat loss that is maintained after the diet ends. The goal table in this calculator flags any target below the established safe minima (1,200 kcal for women, 1,500 for men) with a warning symbol.