RMR Calculator

By on Tuesday, August 11, 2026

The RMR Calculator estimates your resting metabolic rate (RMR) — the calories your body burns at complete rest to keep your heart beating, lungs breathing, and organs maintaining homeostasis. RMR usually accounts for 60–75% of total daily energy expenditure, making it a critical baseline for nutrition and weight management.

This tool uses the Mifflin-St Jeor equation, the formula most often recommended in clinical practice for healthy adults, and accepts both metric and imperial inputs. Optionally add a body-fat percentage to compare with the Katch-McArdle lean-body-mass equation and learn which estimate may fit you better.

It is designed for personal trainers, dietitians, students, and anyone tracking performance or body composition. The result is reported in kcal per day, hour, and week, with a clear step-by-step math breakdown underneath.

📊 Results

Step-by-Step Guide

  1. Select your sex and enter your age. The Mifflin-St Jeor equation uses a different constant for men and women, so this field is required.
  2. Enter height and weight in either metric or imperial units. The calculator converts to centimeters and kilograms automatically.
  3. Optionally enter a body-fat percentage if you have a recent measurement from calipers, bioelectrical impedance, DEXA, or hydrostatic weighing.
  4. Click Calculate RMR. Review the estimated calories per day, per hour, and per week, plus the math steps.
  5. Use the Reset button to clear the form and start a new calculation.
For the most consistent results, measure in the morning after fasting and before exercise. Body temperature, caffeine, and prior meals can all raise RMR by several percent.

Formula Explained

RMR is not a simple guess based on weight; it is estimated with predictive equations. The Mifflin-St Jeor equation is the default here because it has been validated across many populations:

Male RMR: 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5
Female RMR: 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) − 161

All values are in kcal/day. The equation was first published by Mifflin et al. in 1990 and remains a common clinical standard (Mifflin-St Jeor equation on Wikipedia).

If you provide body fat, the tool also computes the Katch-McArdle formula: RMR = 370 + 21.6 × lean body mass (kg), where lean body mass = total weight × (1 − body fat/100). This formula ignores sex and age but uses your actual lean tissue, which may be helpful for athletic individuals.

Worked Examples

Example 1 — Male, 30, 178 cm, 80 kg
Mifflin-St Jeor = (10 × 80) + (6.25 × 178) − (5 × 30) + 5 = 800 + 1112.5 − 150 + 5 = 1767.5 kcal/day. That is approximately 73.6 kcal per hour and 12,372 kcal per week.
Example 2 — Female, 45, 165 cm, 70 kg, 28% body fat
Mifflin-St Jeor = (10 × 70) + (6.25 × 165) − (5 × 45) − 161 = 700 + 1031.25 − 225 − 161 = 1345.25 kcal/day. With body fat, lean mass = 70 × 0.72 = 50.4 kg, so Katch-McArdle = 370 + (21.6 × 50.4) = 1458.6 kcal/day. The ~113 kcal difference shows how body composition can change the estimate.

Tips & Best Practices

  • Do not confuse RMR with total daily energy expenditure. After obtaining RMR, multiply by an activity factor (1.2 for sedentary, 1.375 for light, 1.55 for moderate, 1.725 for very active, 1.9 for extra active) to estimate maintenance calories.
  • Track body composition with the same method over time. Switching from calipers to bioimpedance can shift body-fat readings by several percentage points.
  • Retest after meaningful changes in weight, training, pregnancy, or medical conditions. RMR changes as lean mass and hormonal status change.
  • For weight loss, aim for a deficit of 300–500 kcal below maintenance, not below RMR. Very low intake can increase metabolic adaptation and muscle loss.
  • Use the step-by-step breakdown to check your math or to teach clients and students how predictive equations work.

Factors Affecting Accuracy

Predictive equations are population averages, not individual metabolic measurements. The Mifflin-St Jeor equation has a reported standard error of roughly 5–10% in healthy adults, which means a true RMR can be somewhat higher or lower than the estimate. Accuracy depends on many factors:

  • Body composition: More muscle raises RMR; two people with the same weight and height can differ by several hundred calories.
  • Age: Lean mass tends to decline with age, partly why the equation subtracts 5 kcal per year.
  • Hormones: Thyroid function, cortisol, insulin, pregnancy, and menstrual cycle affect metabolic rate.
  • Measurement conditions: The classic resting measurement is done after an 8–12 hour fast, 15–20 minutes of rest, in a thermoneutral room.
This calculator is for educational use and is not a medical diagnosis. If you have a thyroid condition, eating disorder, metabolic disease, or are under medical supervision, discuss energy targets with a registered dietitian or physician.

Frequently Asked Questions

What is the difference between RMR, BMR, and TDEE?

Basal metabolic rate (BMR) is measured under very strict conditions: fasting, rested, in a warm room, immediately after waking. Resting metabolic rate (RMR) is slightly less strict and is usually a few percent higher. Total daily energy expenditure (TDEE) adds physical activity and the thermic effect of food; TDEE is what you use to maintain current weight day to day.

Which activity factor should I use?

If you sit most of the day and exercise 1–3 days per week, use 1.375–1.55. For example, a person with an RMR of 1767.5 kcal/day who trains 3–5 days per week has a maintenance estimate of about 1767.5 × 1.55 ≈ 2740 kcal/day.

Should I use Mifflin-St Jeor or Katch-McArdle?

Use Mifflin-St Jeor when body fat is unknown or measured unreliably. If you have a valid body-composition reading, Katch-McArdle can be more useful because it directly uses lean mass. The two methods can differ by 5–10% in lean individuals.

Why are the results in kcal/day?

A kilocalorie is the standard nutrition energy unit; food labels call it a "Calorie." RMR results are traditionally reported as kcal/day so they can be compared directly to dietary intake.

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