RMR Calculator - Resting Metabolic Rate and TDEE

An RMR calculator is a health planning tool that estimates how many calories your body burns at complete rest, then turns that number into a total daily energy estimate you can use to plan meals, workouts, and weight goals.

• • Set a baseline for a calorie target: start a weight loss, maintenance, or weight gain plan with a defensible baseline calorie number before deciding on a surplus or deficit.
• • Compare RMR equations in one pass: switch between Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle with the same inputs to see how each estimates the same adult.
• • Plan a TDEE range with activity factors: see the same RMR scaled by the 1.2 to 1.9 activity factors so you can pick a maintenance range that matches your real training load.

Resting metabolic rate is the energy your body uses to keep the heart, brain, lungs, and other organs running. It covers 60 to 75 percent of total daily energy expenditure for most adults, which is why it is the first number a weight or training plan needs. The RMR calculator on this page reuses the same Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle equations that appear in the BMR calculators.

For the same Mifflin-St Jeor and Harris-Benedict results without the activity-tier range, the BMR Calculator runs the equations as a pure resting-calorie number.

The RMR calculator runs one of three equations on age, sex, height, weight, and body fat, then multiplies the result by an activity factor for a TDEE range. Mifflin-St Jeor is the default because it is the most accurate for most adults in published validation studies.

• sex: Used by Mifflin-St Jeor and Harris-Benedict. Katch-McArdle is sex-neutral.
• age: Age in years, between 15 and 100. Both equations reduce the estimate by a fixed amount per year of age.
• weightKg: Body weight in kilograms. Pounds entered in the imperial path are converted to kilograms at the NIST 0.453592 ratio.
• heightCm: Standing height in centimeters. Feet+inches entered in the imperial path are converted to centimeters at 2.54 cm per inch.

Mifflin-St Jeor was validated against indirect calorimetry in 498 adults and is the most accurate weight-height-age equation in the published comparison studies.

According to Mifflin et al. (American Journal of Clinical Nutrition, 1990), the new predictive equation for resting energy expenditure was (10 x weight kg) + (6.25 x height cm) - (5 x age) + 5 for men and the same expression minus 161 for women, validated against indirect calorimetry in 498 healthy adults.

According to Roza and Shizgal (American Journal of Clinical Nutrition, 1984), the Harris-Benedict equation reevaluated to 88.362 + 13.397*weight + 4.799*height - 5.677*age for men and 447.593 + 9.247*weight + 3.098*height - 4.330*age for women, after comparing the original 1919 coefficients to body cell mass.

When the user wants a deeper dive on lean mass, body fat percentage, and the Katch-McArdle constants, the Katch-McArdle BMR Calculator shows the same 370 + 21.6 x lean mass equation with a maintenance calorie range.

Four ideas keep the RMR result from being read as a clinical measurement. Naming them up front turns the calculator from a black box into a planning tool.

RMR and BMR are often used interchangeably in consumer apps, but a strict BMR measurement requires a 10 to 12 hour fast, no exercise for 12 hours, and a thermoneutral room. The published equations were developed against these stricter conditions.

Lean mass is the driver of the Katch-McArdle result, and the Lean Body Mass Calculator estimates the same value from weight, height, sex, and an optional body fat measurement.

The form is organized as a short setup, a single equation pick, and a single activity pick. Read the result panel top-to-bottom: RMR, TDEE range, and lean body mass.

1. 1 Pick sex and enter age: sex changes the Mifflin-St Jeor and Harris-Benedict coefficients but not Katch-McArdle. Age is capped at 15 to 100 so the equation stays in the validated adult range.
2. 2 Pick Metric or Imperial and enter weight, height, and body fat: the Metric path shows kilograms and centimeters, and the Imperial path shows pounds and feet+inches that are converted to metric at the NIST ratios before the equation runs. Body fat is only needed for Katch-McArdle.
3. 3 Pick an RMR equation and an activity factor: Mifflin-St Jeor is the default. Harris-Benedict uses the 1984 revised coefficients. Katch-McArdle is the best choice when body fat is reliable. The result panel shows the RMR plus all five activity factors so the chosen activity is anchored against a full range.
4. 4 Read the result panel and the TDEE range: the panel shows the RMR, the moderate TDEE, the lean body mass, and the full TDEE range from sedentary to very active. Re-evaluate the chosen activity every 4 to 6 weeks against real weight change.

A 35-year-old woman who is 165 cm, 60 kg, 22 percent body fat, and trains four days per week enters sex female, age 35, weight 60 kg, height 165 cm, body fat 22, and activity 1.55. Mifflin-St Jeor returns 1295 cal/day, and Katch-McArdle returns 1381 cal/day once the same inputs are run against lean body mass.

Once the RMR is settled, the TDEE Calculator multiplies it by the same 1.2 to 1.9 activity factors and adds a maintenance calorie range to plan a daily target.

The RMR calculator is most useful when the result is used as a starting range, not a single prescription. Four specific uses come up in published nutrition guidance.

• • Three equations in one form: Mifflin-St Jeor, Harris-Benedict revised, and Katch-McArdle are all available so the same inputs can be cross-checked against the most-cited RMR formulas.
• • Built-in TDEE range with the 1.2 to 1.9 activity factors: the result panel shows the RMR scaled by sedentary, light, moderate, active, and very active factors, anchored against the full range from the Harris-Benedict 1984 paper.
• • Metric and Imperial paths both supported: the unit toggle switches between kilograms+centimeters and pounds+feet+inches; the imperial values are converted to metric at the NIST ratios before the equations run.
• • Comparable to the BMR and TDEE calculators: the result is a calories-per-day number, the same unit used by the BMR, TDEE, calorie, calorie-deficit, and maintenance-calorie calculators.

When the goal is fat loss, the Calorie Deficit Calculator takes the RMR or TDEE result and applies a 250 to 750 kcal deficit to estimate a weekly weight change target.

The RMR result is a planning estimate, not a clinical measurement. Three factors change it most, and the calculator exposes them in the result panel so the user can see which lever is doing the work.

• • Predictive RMR equations are not a substitute for indirect calorimetry. A registered dietitian or clinical exercise physiologist can run a metabolic test for adults with medical conditions, eating-disorder recovery, pregnancy, or athletic fueling needs where a calculator estimate is not enough.
• • Calorie targets set below resting energy expenditure should be supervised. Sustained intake well below the RMR can affect hormonal balance, training recovery, and bone health, especially in adults under 25 or anyone with a history of restrictive dieting.

The Mifflin-St Jeor equation has a typical prediction error of about 10 percent of measured RMR. A calculated RMR is a starting point: a calorie plan should be adjusted against real weight change.

According to Frankenfield et al. (Journal of the American Dietetic Association, 2005), the Mifflin-St Jeor equation was the most accurate predictor of resting metabolic rate in healthy nonobese and obese adults, while Katch-McArdle (370 + 21.6 x lean body mass) was more accurate when body-fat data was reliable and worse when it was estimated.

The Katch-McArdle RMR is only as good as the body fat percentage it is given, so the Body Fat Calculator is the natural next stop when the user wants a fresh body fat estimate.