AkCalculators

Resting Metabolic Rate (RMR): what it is, how to estimate it, and practical use

Resting metabolic rate (RMR) is the energy your body requires at rest to maintain basic physiological functions — breathing, circulation, thermoregulation and cellular processes. RMR is usually expressed in kilocalories per day (kcal/day) and is a key component in energy-balance calculations. Basal metabolic rate (BMR) is a more strictly defined laboratory measurement taken under controlled conditions; in practice RMR and BMR are often used interchangeably with small differences.

Why estimate RMR?

Knowing an estimate of RMR helps with planning diets, setting calorie targets for weight loss or gain, and calculating medication dosing or clinical energy needs. While indirect calorimetry is the gold standard, predictive equations are useful, low-cost alternatives for everyday use.

Common predictive equations

This calculator implements three widely used equations:

• Mifflin–St Jeor (1990):
  • Male: RMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + 5
  • Female: RMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) − 161
• Harris–Benedict (revised):
  • Male: RMR = 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age(y) + 88.362
  • Female: RMR = 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age(y) + 447.593
• Katch–McArdle (lean-mass based):

  Requires lean body mass (kg). RMR = 370 + 21.6 × lean_mass(kg). If you don't know lean mass, compute it using a body fat % (Lean = weight × (1 − BF%/100)).

From RMR to TDEE

To estimate total daily energy expenditure (TDEE), multiply RMR by an activity factor that reflects daily physical activity and exercise. The calculator includes standard multipliers (1.2 sedentary up to 1.9 for extremely active). Remember this yields an estimate — actual energy needs vary with non-exercise activity, thermic effect of food, and metabolic adaptations.

Worked example

Consider a 30-year-old female, 65 kg, 165 cm, body fat 22%:

1. Mifflin: 10×65 + 6.25×165 − 5×30 −161 = 650 + 1031.25 −150 −161 ≈ 1370 kcal/day.
2. Harris–Benedict: 9.247×65 + 3.098×165 −4.33×30 +447.593 ≈ 1410 kcal/day.
3. Katch–McArdle: lean mass = 65×(1−0.22)=50.7 kg → RMR = 370 + 21.6×50.7 ≈ 1454 kcal/day.
4. TDEE (moderate activity ×1.55) ≈ 1450 × 1.55 ≈ 2250 kcal/day.

Accuracy and limitations

Predictive equations typically estimate RMR within ±10–15% for many adults but can misestimate for extremes of body composition or clinical states (e.g., severe obesity, cachexia, thyroid disease). For precise needs (clinical nutrition, research), measure RMR via indirect calorimetry.

Practical tips

• Use measured weight and height and a recent body fat estimate if possible.
• Compare multiple equations; the average is often a robust starting point.
• When setting calorie targets, account for goals (weight loss vs maintenance) and monitor progress — adjust as needed.

This calculator is for informational and general-purpose use only. It does not replace personalized medical or dietary advice. For individualized planning consult a registered dietitian or clinician.