BMR Calculator

BMR Calculator: Find Your Basal Metabolic Rate and Build Your Calorie Budget

TL;DR: Basal metabolic rate (BMR) is the calories your body burns at complete rest: no movement, no digestion, no deliberate activity. For most adults it falls between 1,300 and 2,000 kcal per day. This calculator runs the Mifflin-St Jeor equation (the most validated formula for healthy adults), compares it to Harris-Benedict, and offers Katch-McArdle if you know your body fat percentage.

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Table of Contents

• The Number Your Body Burns Before You Do Anything
• Who Gets the Most Out of a BMR Calculator
• Three Formulas, One Number: Understanding the BMR Equations
• How to Run the Calculation in Five Steps
• Putting the Formula to Work: Two Profiles Calculated in Full
• Six Errors That Produce a Misleading BMR Result
• FAQ
• Assumptions and Notes
• What to Do With Your BMR Number
• Further Reading

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The Number Your Body Burns Before You Do Anything

Lie completely still. Stop digesting food. Stay at a comfortable temperature. Do not think particularly hard. What remains is basal metabolic rate: the energy your organs draw just to keep running. The liver and brain together account for roughly 40% of resting energy expenditure despite representing a small fraction of body mass. Your kidneys, heart, and lungs claim most of the rest. Skeletal muscle, despite being the body's largest tissue by mass, contributes only 20–25% of BMR at rest because it is largely inactive without a neural signal.

This baseline matters because it is non-negotiable. No diet, however aggressive, permanently reduces resting expenditure to zero. BMR sets the floor below which any calorie target cannot go without triggering catabolism and physiological adaptation. For most adults, BMR represents 60–75% of total daily energy expenditure. The activity on top is the smaller, more controllable portion.

Knowing your BMR gives you the foundation for any calorie target, surplus, or deficit. Everything else in nutrition planning is adjustment from this number.

Skip the arithmetic. The calculator above runs all three formulas and returns your result in under a minute.

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Who Gets the Most Out of a BMR Calculator

• You are starting a fat loss phase and want to set a calorie floor, not just a deficit. A 500 kcal deficit from TDEE is the standard starting point, but if TDEE is low and BMR is 1,350 kcal, the deficit target may fall below the safe minimum of 1,200 kcal for women. Knowing BMR first prevents setting a target that inadvertently restricts intake to below resting expenditure.

• You have hit a plateau after 8–12 weeks of dieting and want to check whether metabolic adaptation has occurred. Extended deficits reduce lean mass, which lowers BMR by approximately 20–30 kcal per kilogram of muscle lost. Recalculating BMR at your current weight after a diet phase quantifies how much your resting expenditure has shifted and informs whether a diet break or calorie refeed is warranted.

• You are comparing the BMR formula output to a measured RMR from a clinical indirect calorimetry test. Mifflin-St Jeor predicts measured BMR within 10% for approximately 80% of healthy adults. If your measured RMR diverges by more than 15% from the formula prediction, a thyroid or metabolic health review with a physician is clinically appropriate.

• You are designing a diet and training plan for someone with a very high or very low body fat percentage. The Mifflin-St Jeor equation uses total body weight, which over-allocates metabolic mass to fat tissue. For a person with 35%+ body fat, Katch-McArdle (which uses lean mass) produces a more accurate result, typically 100–200 kcal lower than Mifflin at equivalent body weights.

• You have recently changed your body composition significantly and want to update your calorie targets. Gaining 3 kg of muscle raises BMR by approximately 60–90 kcal per day. Losing 5 kg of fat lowers total body weight and thus BMR by roughly 50–80 kcal per day. Neither change is captured by a static calorie target set months earlier.

• You are a fitness coach setting up a client programme and need to explain why two people of the same weight need different calorie targets. A 75 kg woman aged 25 has a Mifflin-St Jeor BMR of approximately 1,563 kcal; a 75 kg woman aged 55 has a BMR of approximately 1,413 kcal — 150 kcal less at identical weight. Age-related muscle loss explains the gap, and showing this calculation justifies the different targets clearly.

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Three Formulas, One Number: Understanding the BMR Equations

The three validated BMR formulas differ in their inputs and the populations on which they were tested. Mifflin-St Jeor is the default recommendation; the others are used in specific circumstances.

Mifflin-St Jeor (1990) — recommended for healthy adults:
  Male:   BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
  Female: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

Harris-Benedict (Roza & Shizgal 1984 revision):
  Male:   BMR = 88.362 + 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age
  Female: BMR = 447.593 + 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age

Katch-McArdle — use when body fat percentage is known:
  BMR = 370 + 21.6 × lean_mass(kg)
  where lean_mass = weight(kg) × (1 − body_fat% / 100)

Formula Comparison at a Sample Profile (Female, 35 years, 70 kg, 168 cm, 28% body fat)

The three formulas converge within 30–50 kcal for most healthy adults at average body fat. Divergence grows at the extremes: for someone at 40% body fat, Katch-McArdle will produce a result 150–200 kcal below Mifflin-St Jeor because it removes fat mass from the calculation. For a very lean athlete at 8% body fat, Katch-McArdle may run slightly higher, as lean tissue is metabolically more active than the population average the Mifflin formula assumes.

Reference BMR Ranges by Age Group (Female, 165 cm, 65 kg — Mifflin-St Jeor)

Genetic variation is real and measurable. Twin studies show that individuals with identical age, sex, height, and weight can differ in true resting metabolic rate by up to 15%. Part of this is explained by thyroid hormone activity, mitochondrial efficiency, and sympathetic nervous system tone differences that no formula captures. The practical implication: if consistent food tracking produces results that persistently diverge from formula predictions after 4 weeks, adjust the BMR estimate by 100–150 kcal rather than assuming the tracking is wrong.

The formula's main limitation is that it treats all weight as metabolically equivalent. A kilogram of fat burns approximately 4.5 kcal per day at rest; a kilogram of muscle burns roughly 13 kcal per day. Total weight inputs average this difference across the body, which is acceptable for typical populations but introduces error at compositional extremes.

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How to Run the Calculation in Five Steps

1. Enter your weight in kilograms (or pounds — the calculator converts automatically). Use your current weight on a consistent day-of-week morning measurement. Do not use goal weight; BMR is a description of the body you have now.

2. Enter your height in centimetres (or feet and inches). Measure barefoot. A 2 cm height error shifts the BMR result by approximately 12 kcal for men and 12 kcal for women. Small, but worth measuring accurately.

3. Enter your age in whole years. Each decade after 30 lowers BMR by approximately 50 kcal at fixed weight and height, driven by age-related muscle loss averaging 0.5–1% per year.

4. Select your sex. The Mifflin formula uses a constant of +5 for males and -161 for females, reflecting the average BMR difference from the validation population. This is a population adjustment; individual variation means some women have higher BMRs than some men at identical stats.

5. Optionally, enter your body fat percentage to unlock Katch-McArdle. If you have a DEXA, Bod Pod, or reliable skinfold result from the past 6 months, enter it. The calculator will show all three formula results side by side. Use Katch-McArdle as your primary figure if your body fat percentage is outside the 15–35% range where Mifflin-St Jeor is most accurate.

Non-obvious insight: BMR does not equal the minimum safe calorie intake. BMR represents resting expenditure only and excludes the energy cost of digestion (roughly 10% of intake) and any movement. The practical minimum safe intake for sedentary adults is BMR plus approximately 10–20% for the thermic effect of food and basic daily movement: typically 1,300–1,600 kcal for women and 1,600–2,000 kcal for men.

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Putting the Formula to Work: Two Profiles Calculated in Full

Example 1: Postmenopausal Woman, Age 61, Moderate Activity

Helen is 61 years old, 162 cm tall, and weighs 74 kg. She walks daily and does yoga twice a week. Her GP has asked her to understand her calorie baseline before starting a supervised weight loss programme.

BMR (Mifflin-St Jeor, female):
  = 10 × 74 + 6.25 × 162 − 5 × 61 − 161
  = 740 + 1012.5 − 305 − 161
  = 1,286.5 kcal

TDEE estimate (Lightly Active, 1.375):
  = 1,286.5 × 1.375 = 1,769 kcal

Helen's BMR of 1,287 kcal is the non-negotiable floor. A moderate 400 kcal daily deficit from her TDEE (1,369 kcal intake) stays 82 kcal above her BMR floor and 169 kcal above the 1,200 kcal safe minimum. Her actionable plan: target 1,350–1,400 kcal daily, with a protein target of at least 90–100 g to minimise muscle loss, and recalculate BMR after every 4 kg of weight reduction.

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Example 2: Male Shift Worker Starting Resistance Training, Age 38

Marcus works nights and has recently joined a gym. He is 182 cm, weighs 93 kg, and wants to understand his resting calorie needs before setting up a moderate lean-gain surplus. He does not know his body fat percentage.

BMR (Mifflin-St Jeor, male):
  = 10 × 93 + 6.25 × 182 − 5 × 38 + 5
  = 930 + 1137.5 − 190 + 5
  = 1,882.5 kcal

Harris-Benedict check (1984 revision):
  = 88.362 + 13.397 × 93 + 4.799 × 182 − 5.677 × 38
  = 88.362 + 1245.921 + 873.418 − 215.726
  = 1,991.975 kcal

The two formulas diverge by 109 kcal for Marcus. This is within the expected range for Harris-Benedict's tendency to run slightly higher on larger-framed males. His actionable approach: use Mifflin-St Jeor (1,883 kcal) as the conservative baseline, multiply by his activity factor once training frequency is established (likely Moderately Active at 1.55, giving TDEE 2,919 kcal), and apply a 300 kcal surplus (3,219 kcal) to support muscle gain without excessive fat accumulation.

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Six Errors That Produce a Misleading BMR Result

Entering goal weight instead of current weight. BMR is a function of the body you have today. A 90 kg person entering 75 kg (their target) reduces the formula result by approximately 150 kcal and sets calorie targets based on a body composition that does not yet exist. The practical consequence: the "deficit" from TDEE is 150 kcal smaller than intended, slowing fat loss by roughly 0.15 kg per week invisibly.

Using the original 1919 Harris-Benedict formula from an older online calculator. The original formula overestimates BMR by an average of 5% compared to measured values. For someone with a true BMR of 1,700 kcal, this produces a reading of approximately 1,785 kcal. Many online BMR tools still use the 1919 version without labelling it. Always check which formula version is being applied; the correct one is the 1984 Roza-Shizgal revision.

Ignoring the BMR-age relationship when setting a calorie target months after initial calculation. A 2 kg fat loss and 0.5 kg muscle loss over 16 weeks reduces BMR by approximately 30–45 kcal. This sounds small, but combined with the adaptive reduction in non-exercise activity thermogenesis (NEAT) that typically accompanies prolonged deficits, the effective deficit can shrink by 100–150 kcal below its original value. Recalculating at 6–8 week intervals prevents this drift from stalling progress silently.

Applying Katch-McArdle with an inaccurate body fat percentage. The Katch-McArdle formula is more accurate than Mifflin-St Jeor when body fat percentage is known, but only if the body fat measurement is accurate. A bioelectrical impedance reading taken in the evening after eating can overestimate body fat by 3–5 percentage points compared to a fasted morning reading, shifting lean mass input by 2–4 kg and the resulting BMR by 43–86 kcal. Use DEXA or a 3-site skinfold reading for Katch-McArdle inputs, not a consumer BIA scale.

Treating BMR as a daily calorie target rather than a floor. BMR does not include the energy cost of digestion (approximately 10% of calorie intake) or any movement whatsoever. Eating at BMR leaves the body in an energy deficit even on a completely sedentary day. Safe minimum intake for sedentary adults is typically 15–20% above BMR; for women this usually means at least 1,200 kcal, and for men at least 1,500 kcal, regardless of what the raw BMR number is.

Using a single BMR calculation for years without updating. BMR declines approximately 1–2% per decade after age 30 primarily due to muscle loss, but in practice changes occur continuously. A person who gained 5 kg of muscle over 2 years has a measurably higher BMR (roughly 100 kcal) than the original calculation suggests. A person who lost 8 kg during a diet has a lower BMR. The formula needs current inputs to remain useful. Stale numbers produce stale targets.

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Assumptions and Notes

• Margin of error: The Mifflin-St Jeor formula predicts measured BMR within 10% for approximately 80% of healthy adults. For the remaining 20%, particularly those with thyroid disorders, significant obesity (BMI above 40), or very high lean mass, the formula may diverge by 10–20% from true resting expenditure. If tracked calorie data consistently shows no expected change after 4 weeks of accurate logging, adjust the BMR estimate by 100–150 kcal in the direction the results indicate.
• Professional disclaimer: The BMR values produced by this calculator are estimates based on validated population equations and are for informational and planning purposes only. They do not constitute medical or nutritional advice. Individuals with metabolic conditions, eating disorder history, or specific dietary medical requirements should work with a registered dietitian or physician before applying any calorie target derived from this calculator.

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What to Do With Your BMR Number

Helen's result put her BMR at 1,287 kcal and her fat loss intake target at 1,350–1,400 kcal, a specific, safe number with a defined protein floor. Marcus's comparison of two formulas explained a 109 kcal gap and gave him a conservative baseline for his surplus calculation. In both cases, the BMR result is not the answer in itself. It is the starting point for the actual calculation: TDEE, then goal adjustment, then weekly tracking to confirm the formula's prediction matches reality.

Run the calculator, note the number, and then go one step further: multiply it by your activity factor and set a date 6 weeks from today to re-enter your updated weight.

Enter your details above and get your BMR result now.

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Further Reading

• Calorie and TDEE Calculator: Multiply Your BMR by an Activity Factor to Get Your Daily Target
• Macro Calculator: Distribute Your TDEE Across Protein, Carbohydrates, and Fat
• Lean Body Mass Calculator: Use Lean Mass as the Input for the Katch-McArdle Formula