BMR Calculator: Your Resting Metabolic Rate, Three Ways

How this BMR calculator works

You get up to three numbers. Here is exactly what each one does.

1. Mifflin-St Jeor (the headline number). Built from 498 healthy adults aged 19-78, normal weight and obese, measured by indirect calorimetry (Mifflin et al., 1990):

• Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
• Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

2. Harris-Benedict, revised. The 1919 equations re-fitted by Roza and Shizgal in 1984, who reported a precision of about ±14% in normally nourished people and found it unreliable in malnourished patients (Roza and Shizgal, 1984):

• Men: 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
• Women: 447.593 + (9.247 × kg) + (3.098 × cm) − (4.33 × age)

3. Katch-McArdle (only if you enter body fat). It ignores sex, age and height and works from lean mass alone: 370 + (21.6 × lean mass in kg), where lean mass = weight × (1 − body fat % / 100). That 370 + 21.6 form is the general equation Cunningham proposed after pooling studies on fat-free mass and resting energy expenditure (Cunningham, 1991).

Worked example: a 25-year-old man, 70 kg, 172 cm, 15% body fat. Mifflin gives about 1,655 kcal, revised Harris-Benedict about 1,710 kcal, and Katch-McArdle (59.5 kg lean mass) about 1,655 kcal. Close, not identical. That is normal.

Which number should you trust?

Default to Mifflin-St Jeor. In a systematic review of the four most commonly used clinical equations, it landed within 10% of measured resting metabolic rate in more people than any other, with the narrowest error range (Frankenfield et al., 2005). The same review flagged real errors for individuals. Ten percent of 1,700 kcal is 170 kcal either way.

Athletes are a special case. In 50 young athletes (28 men, 22 women), all five equations tested underestimated measured RMR; Harris-Benedict did best in the men and Cunningham’s lean-mass equation did best in the women (Jagim et al., 2018). That is one small study of young athletes, so do not over-read it. If you train hard and carry plenty of muscle, the higher of your numbers may be closer to reality.

What to do with your BMR

BMR alone does not tell you what to eat. It leaves out walking, training and digesting food. The useful next step:

1. Get your maintenance calories from the calorie calculator, which multiplies Mifflin-St Jeor by an activity factor.
2. Split that target into protein, fat and carbs with the macro calculator.
3. Track your weekly average weight for two to three weeks and adjust from real data.

Want the full breakdown of where daily calories go? Read TDEE deconstructed.

Limitations and who this is not for

• Under-18s. The form accepts ages from 15, but Mifflin-St Jeor was built on adults aged 19-78. Teenagers should treat it as a rough guide only.
• Very muscular or very lean people. Weight-based equations can miss. Use Katch-McArdle if your body fat estimate is decent; a guessed body fat % just moves the error elsewhere. The body fat calculator gives a tape-measure estimate.
• Indian users. Most equations were validated mainly on Western samples. A Bangalore study of 121 men found no clear BMR gap versus Americans of the same weight and age (Soares et al., 1993), but data on Indian women are thin.
• Medical conditions. Pregnancy, thyroid disease, recent major weight loss, illness or medications that affect metabolism all break these equations. A doctor or registered dietitian can arrange a measured test if it matters.