BMR Calculator - Mifflin-St Jeor & Harris-Benedict

Calculate your basal metabolic rate with the two most used formulas side by side, plus a full TDEE table for every activity level. Free and runs entirely in your browser.

Sex

Mifflin-St Jeor

1718

kcal/day at rest

Harris-Benedict

1777

kcal/day at rest

TDEE by activity level (kcal/day)

Activity levelFactorMifflinHarris
Sedentary (little or no exercise)1.220612132
Lightly active (1-3 days/week)1.37523622443
Moderately active (3-5 days/week)1.5526622754
Very active (6-7 days/week)1.72529633065
Extra active (hard training + physical job)1.932633376

This is an indication, not medical advice.

BMR Explained: Mifflin-St Jeor vs Harris-Benedict

Your basal metabolic rate (BMR) is the energy your body burns at complete rest just to stay alive: breathing, circulation, cell repair, and keeping your brain and organs running. It typically accounts for 60-70% of everything you burn in a day. This calculator shows the two most cited estimation formulas side by side so you can see how much (or how little) they disagree for your numbers.

The Mifflin-St Jeor equation (1990) is 10 x weight (kg) + 6.25 x height (cm) - 5 x age, plus 5 for men or minus 161 for women. It was derived from indirect calorimetry on around 500 subjects and a 2005 American Dietetic Association review found it the most accurate general-purpose formula, predicting within 10% of measured BMR for most people. The revised Harris-Benedict equation (Roza and Shizgal, 1984) is an update of the original 1919 formula; it tends to run 5% or so higher, partly because it was calibrated on an early 20th-century population with more physically active lifestyles.

BMR alone is not your calorie target: nobody spends the day motionless. Multiplying BMR by an activity factor gives total daily energy expenditure (TDEE). The standard multipliers run from 1.2 for a desk job with no exercise up to 1.9 for hard daily training plus a physical job. A 75 kg, 178 cm, 30 year old man has a Mifflin BMR of about 1717 kcal, which becomes roughly 2061 kcal sedentary or 2662 kcal at the moderately active level. Most office workers who train a few times a week fall between 1.375 and 1.55.

Both formulas estimate the average person of your size, so treat the spread between them as your uncertainty band. Muscle mass is the biggest hidden variable: muscle burns more at rest than fat, so a lean athlete may run 200+ kcal above formula estimates while someone with low muscle mass may run below. To turn these numbers into a plan, use the Calorie Calculator for goal-based targets or the Macro Calculator to split your TDEE into protein, carbs, and fat.

How the BMR Calculator Works

  1. 01Enter your age, sex, height, and weight in metric or imperial units.
  2. 02The tool computes your BMR with the Mifflin-St Jeor formula (1990).
  3. 03It also computes the revised Harris-Benedict formula (1984) for comparison.
  4. 04A TDEE table multiplies both results by every activity factor from 1.2 to 1.9.

From BMR to Daily Calories

BMR is what your body burns at complete rest, typically 60-70% of total daily burn. To get a usable calorie number, multiply it by an activity factor: 1.2 for sedentary desk work, 1.375 for light exercise 1-3 days a week, 1.55 for moderate exercise 3-5 days, 1.725 for hard exercise 6-7 days, and 1.9 for hard daily training plus a physical job. Most office workers who train a few times per week sit between 1.375 and 1.55. The result, your TDEE, is the intake at which your weight stays stable.

When to Use This Calculator

Use it as the starting point for any calorie plan: the TDEE row matching your lifestyle is your maintenance intake, from which you subtract for weight loss or add for muscle gain. Seeing both formulas side by side is useful as an uncertainty band; if they differ by 100 kcal, do not treat either as gospel. Recalculate after every 5 kg of weight change or when your training routine changes level.

Common Use Cases

Tips

  • Pick the activity factor honestly; most people overestimate and end up 200-300 kcal over their real TDEE.
  • Validate the estimate against reality: track weight for two to three weeks at the calculated maintenance and adjust.
  • Recalculate after every 5 kg of weight change; BMR scales with body mass.

Frequently Asked Questions

Which formula is more accurate?→
A 2005 American Dietetic Association review found Mifflin-St Jeor the most accurate for healthy adults, predicting within 10% of measured BMR for most people. Harris-Benedict tends to estimate around 5% higher because it was calibrated on a 1919 population with more active baseline lifestyles.
What is the difference between BMR and TDEE?→
BMR is energy burned at complete rest; TDEE (total daily energy expenditure) is BMR multiplied by an activity factor between 1.2 and 1.9 to account for movement, exercise, and digestion. TDEE is the number to base calorie targets on, not BMR.
Why do the two formulas give different numbers?→
They were fitted on different populations decades apart with different measurement methods. For most inputs they land within 50-150 kcal of each other. Treat the gap as your uncertainty range rather than an error in either formula.
Does muscle mass change my BMR?→
Yes, and neither formula sees it. Muscle tissue burns more energy at rest than fat, so a lean, muscular person can run 200+ kcal above the estimate while someone with low muscle mass runs below it. The Katch-McArdle formula uses lean mass directly if you know your body fat percentage.
Should I eat at my BMR to lose weight?→
No. Eating at BMR usually means a very aggressive deficit since your real burn is 1.2-1.9 times higher. A sustainable approach is TDEE minus roughly 500 kcal, which produces about 0.5 kg of loss per week without wrecking energy levels.

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