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BMR Calculator

Calculate your Basal Metabolic Rate - the number of calories your body burns at complete rest to maintain vital functions.

BMR Calculation Formulas

Mifflin-St Jeor (Men)

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Mifflin-St Jeor (Women)

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yrs
lb
ft
in
%
For Katch-McArdle formula

What is Basal Metabolic Rate?

Basal Metabolic Rate (BMR) is the number of calories your body requires to perform its most basic life-sustaining functions while at complete rest. This includes breathing, circulating blood, cell production, nutrient processing, protein synthesis, and ion transport across cell membranes.

Your BMR represents approximately 60-75% of your total daily calorie expenditure. Understanding your BMR is fundamental to effective weight management because it establishes the baseline of calories you need just to exist - before accounting for any physical activity.

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Scientific Accuracy

Uses three validated formulas for comprehensive BMR estimation across different body types.

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Instant Results

Get your BMR calculation immediately with detailed breakdown by activity level.

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Multiple Formulas

Compare results from Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle equations.

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Activity Multipliers

See estimated daily calorie needs for different activity levels based on your BMR.

Understanding the BMR Formulas

Several equations have been developed to estimate BMR, each with its own strengths and limitations:

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Mifflin-St Jeor Equation (1990)

The most accurate for most adults according to research. Men: BMR = 10×weight(kg) + 6.25×height(cm) - 5×age + 5. Women: BMR = 10×weight(kg) + 6.25×height(cm) - 5×age - 161. Recommended by the Academy of Nutrition and Dietetics.

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Harris-Benedict Equation (Revised 1984)

The classic formula, originally from 1919, revised for modern populations. Men: BMR = 13.4×weight(kg) + 4.8×height(cm) - 5.7×age + 88.4. Women: BMR = 9.2×weight(kg) + 3.1×height(cm) - 4.3×age + 447.6. Still widely used in clinical settings.

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Katch-McArdle Equation

The only formula that accounts for body composition. BMR = 370 + (21.6 × lean body mass in kg). More accurate for athletic individuals or those with unusual body compositions, but requires knowing your body fat percentage.

BMR vs. RMR vs. TDEE

Understanding the differences between these related but distinct measurements is crucial:

MetricMeasuresConditionsUse Case
BMR Minimum calories for vital functions Complete rest, fasted, thermoneutral environment Theoretical baseline for metabolism
RMR Calories at rest (slightly higher than BMR) Resting but less strict conditions Practical clinical measurement
TDEE Total daily calories including activity Normal daily life Actual calories needed per day

Factors That Affect Your BMR

Many factors influence your basal metabolic rate, some you can control and others you cannot:

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Muscle Mass

Muscle tissue burns more calories than fat tissue, even at rest. Each pound of muscle burns about 6 calories per day versus 2 for fat. This is why strength training can boost your metabolism.

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Age

BMR decreases approximately 1-2% per decade after age 20, primarily due to muscle loss. Maintaining muscle through resistance training can partially offset this decline.

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Genetics

Some people naturally have faster metabolisms due to genetic factors. This can account for 5-10% variation in BMR between individuals of similar size and composition.

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Body Size

Larger bodies require more energy to maintain. Both height and weight contribute to higher BMR - there's simply more tissue to fuel.

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Body Temperature

Fever increases BMR by about 7% for each degree Fahrenheit increase. Cold exposure can also temporarily boost metabolism through thermogenesis.

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Hormones

Thyroid hormones significantly affect metabolic rate. Hypothyroidism slows metabolism, while hyperthyroidism speeds it up. Sex hormones also play a role.

Activity Multipliers Explained

To estimate your total daily calorie needs (TDEE), multiply your BMR by an activity factor:

Activity LevelMultiplierDescriptionExamples
Sedentary 1.2 Little or no exercise Desk job, no regular exercise, mostly sitting
Lightly Active 1.375 Light exercise 1-3 days/week Light walking, casual yoga, occasional gym
Moderately Active 1.55 Moderate exercise 3-5 days/week Regular gym sessions, sports 3-5 times weekly
Very Active 1.725 Hard exercise 6-7 days/week Daily intense workouts, athletic training
Extremely Active 1.9 Very hard exercise + physical job Athletes, construction workers, multiple daily sessions

How to Use Your BMR for Weight Goals

Understanding your BMR is the foundation for any weight management strategy:

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Weight Maintenance

Eat calories equal to your TDEE (BMR × activity factor). Monitor weight weekly - if gaining, reduce slightly; if losing, increase slightly.

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Weight Loss

Create a calorie deficit of 500-1000 calories below TDEE for 1-2 pounds of loss per week. Never eat below your BMR for extended periods - this can slow metabolism.

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Weight/Muscle Gain

Eat 300-500 calories above TDEE. Combine with resistance training to ensure weight gained is primarily muscle rather than fat.

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Never Go Below BMR

Eating fewer calories than your BMR is counterproductive and dangerous. It triggers metabolic adaptation, muscle loss, hormonal disruption, and often leads to weight regain.

Boosting Your Metabolism Naturally

While you can't dramatically change your BMR, these evidence-based strategies can help optimize your metabolic rate:

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Build Muscle

Strength train 2-4 times weekly. More muscle mass directly increases BMR because muscle requires constant energy to maintain.

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Eat Adequate Protein

Protein has a higher thermic effect (20-30%) than carbs (5-10%) or fat (0-3%). You burn more calories digesting protein. Aim for 0.7-1g per pound of bodyweight.

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Prioritize Sleep

Sleep deprivation disrupts metabolic hormones. Aim for 7-9 hours. Poor sleep is linked to lower BMR and increased appetite.

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Cold Exposure

Brief cold exposure activates brown fat and increases metabolic rate temporarily. Cold showers or lowering thermostat slightly may help.

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Caffeine & Green Tea

Both can temporarily boost metabolism by 3-11%. The effect is modest but adds up over time when combined with other strategies.

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Stay Hydrated

Drinking water can temporarily increase metabolic rate by 10-30% for about an hour. Cold water may be slightly more effective.

How to use this BMR calculator

  1. Pick a unit system — Imperial (lb / ft-in) or Metric (kg / cm) — using the toggle above the form.
  2. Enter your Age in years. BMR drops about 1–2% per decade after 20, so this input matters more than people expect.
  3. Select Gender (Male or Female). The Mifflin-St Jeor equation uses a different constant for each (+5 vs -161) to reflect average differences in lean mass.
  4. Enter Weight — pounds for Imperial, kilograms for Metric. Use your morning weight after the bathroom for a consistent reading.
  5. Enter Height — feet plus inches for Imperial, or a single centimeter value for Metric.
  6. Choose a Formula: Mifflin-St Jeor (recommended default), Harris-Benedict (older, classroom-standard), or Katch-McArdle (requires Body Fat % and is best for lean, athletic users).
  7. Click Calculate BMR. The result panel shows your BMR in cal/day plus a TDEE breakdown for each of the five activity multipliers (1.2 → 1.9).

Examples

Basic: 30-year-old male, moderately active

Software engineer who lifts and runs three to five times per week, looking for a maintenance calorie target.

ResultBMR ≈ 1,783 cal/day. At the Moderately Active multiplier (1.55), TDEE ≈ 2,763 cal/day.

Convert to metric: 180 lb × 0.4536 = 81.65 kg; 70 in × 2.54 = 177.8 cm. Plug into Mifflin-St Jeor (men): BMR = 10(81.65) + 6.25(177.8) − 5(30) + 5 = 816.5 + 1111.25 − 150 + 5 ≈ 1,783. Multiplying by 1.55 gives ~2,763 cal/day for maintenance with moderate training.

Comparison: 25-year-old female, lightly active

Office worker walking on lunch breaks and doing yoga twice a week, planning a modest fat-loss phase.

ResultBMR ≈ 1,336 cal/day. At Lightly Active (1.375), TDEE ≈ 1,836 cal/day. A 500-cal deficit lands near 1,336 — exactly at BMR, so a 300-cal deficit (~1,536) is the safer cut.

130 lb = 58.97 kg; 65 in = 165.1 cm. Mifflin-St Jeor (women): BMR = 10(58.97) + 6.25(165.1) − 5(25) − 161 = 589.7 + 1031.9 − 125 − 161 ≈ 1,336. This example shows why aggressive deficits hit BMR fast for smaller bodies — calculate first, then choose a deficit that keeps intake above BMR.

Edge case: lean lifter using Katch-McArdle

Same 30-year-old male as Example 1, but with a measured body fat of 15%. Mifflin-St Jeor underestimates BMR for very lean individuals because it doesn't know about extra muscle mass.

ResultLean body mass ≈ 69.4 kg; BMR ≈ 1,869 cal/day — about 86 cal/day higher than Mifflin-St Jeor's 1,783 estimate for the same person.

LBM = 81.65 × (1 − 0.15) = 69.4 kg. Katch-McArdle: BMR = 370 + 21.6 × 69.4 ≈ 370 + 1499 ≈ 1,869. The gap widens as body fat drops further: at 10% BF the same lifter clears ~1,956 cal/day. Use Katch-McArdle only when you have a real body-fat measurement (DEXA, hydrostatic, or skinfolds done by a trained tester) — not a smart-scale guess.

How the BMR formulas work

BMR is the energy your body spends at complete rest — fasted, awake, lying still in a thermoneutral room — to keep organs running, ion pumps firing, and core temperature stable. It is not the same as your daily calorie burn; physical activity and the thermic effect of food sit on top of it.

Mifflin-St Jeor (1990) is the modern default. It fits a linear regression on weight (kg), height (cm), and age (years) with a sex offset of +5 for men or −161 for women. Each kilogram adds 10 cal/day, each centimeter adds 6.25 cal/day, and each year of age subtracts 5 cal/day. Validation studies place its accuracy within roughly ±10% for healthy non-obese adults — the best of the population-based equations.

Harris-Benedict (revised by Roza & Shizgal in 1984) is the older standard that you still see in textbooks. It uses heavier weight and height coefficients with a larger constant, and tends to overestimate BMR for obese individuals. Use it when you need to match historical clinical work; otherwise prefer Mifflin-St Jeor.

Katch-McArdle takes a different approach: it ignores height, age, and sex and depends entirely on lean body mass (LBM = weight × (1 − body fat %)). Because muscle drives most resting energy use, this formula outperforms the others for very lean, very muscular, or very high-body-fat individuals — but only if your body-fat measurement is accurate.

When to use a BMR calculator

  • Setting a fat-loss calorie target. BMR sets the floor — never eat below it for more than a short, supervised window. Calculate BMR, multiply by your honest activity factor to get TDEE, then subtract 300–750 cal for a sustainable 0.5–1.5 lb/week loss.
  • Planning a lean bulk or recomposition. Add 200–400 cal above TDEE with progressive resistance training. Knowing BMR helps you spot when your tracked intake is too low to support recovery, which is the most common reason 'lean bulks' stall.
  • Diagnosing why a diet has plateaued. If you're tracking food carefully and weight has stalled for 3+ weeks, compare your current intake to your BMR. Eating only slightly above BMR for months can cause adaptive thermogenesis — the fix is usually a diet break at maintenance, not a deeper cut.
  • Comparing formulas for special body types. Run Mifflin-St Jeor and Katch-McArdle side by side if you're very lean (< 12% male / < 18% female) or carry significantly more muscle than average. A 100–200 cal gap between formulas is a useful planning signal.
  • Checking a registered dietitian's targets. If a coach or RD has given you a calorie goal, calculating your own BMR is a quick sanity check. Targets that fall below your BMR for weeks at a time deserve a conversation.

Common BMR mistakes

  • MistakeEating at or below your BMR to lose weight faster.
    FixBMR is what your organs need at rest. Sustained intake at or below BMR triggers muscle loss, hormonal disruption, and adaptive thermogenesis. Subtract calories from your TDEE (BMR × activity factor) instead, and keep daily intake above BMR.
  • MistakeTreating BMR as the calories you should eat per day.
    FixBMR is one slice of your daily burn — usually 60–75%. Daily intake for maintenance should match TDEE, not BMR. Use the calculator's activity multipliers to bridge from BMR to TDEE.
  • MistakeOver-rating your activity level.
    FixMost people overestimate. 'Moderately Active' (1.55) means deliberate exercise 3–5 days/week, not a few walks. If weight rises faster than expected at your calculated TDEE, drop one tier (e.g., from Moderately to Lightly Active) and recalculate.
  • MistakeUsing Katch-McArdle with a smart-scale body-fat estimate.
    FixConsumer bio-impedance scales can be off by 5–10 percentage points, which throws Katch-McArdle off by hundreds of calories. Use Katch-McArdle only with DEXA, hydrostatic, or carefully measured skinfolds — otherwise stay on Mifflin-St Jeor.
  • MistakeRecalculating BMR every day chasing small weight fluctuations.
    FixBMR barely changes day to day. Recalculate after a real change — 5+ lb of weight gained or lost, a birthday, a body-composition shift — not after a single bad scale reading.
  • MistakeForgetting that medical conditions and medications shift BMR.
    FixHypothyroidism, GLP-1 medications, chronic stress, and severe calorie restriction all change resting metabolism. Population formulas can't see those. If your real-world intake doesn't match the math by more than ~15%, talk to a clinician.
  • MistakeComparing your BMR to a friend's and concluding one is 'wrong'.
    FixBMR scales mostly with lean body mass, height, and age. A taller, heavier, or more muscular person will have a higher BMR — that's the formula working correctly, not a sign of a 'fast' or 'slow' metabolism.

Frequently asked questions

What's the difference between BMR and RMR?

BMR is measured under strict conditions — fully rested, fasted 12 hours, lying still in a thermoneutral room right after waking. RMR (Resting Metabolic Rate) is measured under more relaxed conditions and runs about 10% higher because it captures small movements, digestion, and slight temperature variation. The two numbers are close enough that most calculators and clinicians use them interchangeably.

Which BMR formula should I use?

Start with Mifflin-St Jeor — it's the modern default and the Academy of Nutrition and Dietetics' recommended population formula. Switch to Katch-McArdle only if you have an accurate body-fat measurement (DEXA, hydrostatic) and are notably lean or muscular. Harris-Benedict is mainly useful for matching older clinical references; it tends to overestimate BMR for obese individuals.

Why is my BMR lower than I expected?

Three usual suspects. First, your weight is lower than you guessed, or your lean mass is — population formulas key off bodyweight and assume an average muscle/fat ratio. Second, age: BMR drops about 1–2% per decade after 20. Third, the formula itself: real-world BMR varies ±10% from any equation due to genetics, thyroid status, and recent dieting history.

Can I increase my BMR?

Yes, modestly. The single most reliable lever is building muscle — each pound of added lean mass adds roughly 6 cal/day at rest. Adequate sleep, sufficient protein (0.7–1 g/lb bodyweight), and avoiding chronic large deficits also help by protecting muscle and preventing adaptive thermogenesis. Pills, 'metabolism boosters', and frequent small meals do not meaningfully move BMR.

How does muscle mass affect BMR?

Muscle is the most metabolically active tissue outside your organs. At rest, a pound of muscle burns roughly 6 cal/day versus about 2 cal/day for a pound of fat. That's why Katch-McArdle — which keys off lean body mass instead of total weight — is more accurate for athletes and lifters than Mifflin-St Jeor.

Does the menstrual cycle affect BMR?

Yes, slightly. BMR drops to its lowest point about a week before ovulation and rises 2.5–11.5% during the luteal phase (after ovulation, before the next period). Most people don't need to adjust calorie targets for this — the day-to-day noise from food tracking and water weight is larger. If you track meticulously and see a predictable shift, adjust intake by 100–200 cal in the luteal phase.

How does aging affect BMR?

BMR declines about 1–2% per decade after age 20, then accelerates after roughly age 60. Most of that drop is loss of lean mass, not 'aging metabolism' itself — research published in Science (Pontzer et al. 2021) found that energy expenditure is remarkably stable from age 20 to 60 once you adjust for body composition. Resistance training is the most effective countermeasure.

Why is my BMR higher in winter or when I'm sick?

Cold exposure activates brown fat and shivering thermogenesis, which can raise BMR by a few percent for hours. Fever raises BMR roughly 7% per degree Fahrenheit (13% per degree Celsius), which is why illness leaves you hungry and tired. Neither effect is large enough to plan a diet around, but it can explain a short-term shift in appetite.

Is the calculator's BMR exact?

No — and no calculator is. Population-based equations have a typical error of ±10% versus indirect calorimetry, the gold-standard measurement. Use the calculator's BMR as a starting estimate, track real-world results for 2–3 weeks, and adjust your calorie target up or down by 100–200 cal/day if your weight moves faster or slower than planned.

Should I use BMR or TDEE to set calories?

TDEE. BMR is the floor — the calories you'd burn lying in bed all day. TDEE = BMR × activity multiplier and represents your real daily burn. Set fat-loss, maintenance, or muscle-gain targets from TDEE, and use BMR only as the lower bound you should not cross.

Sources

Methodology

Defaults to the Mifflin-St Jeor equation, which validation studies (Frankenfield 2005) identify as the most accurate population-based BMR predictor for healthy adults. Harris-Benedict (1984 revision) and Katch-McArdle are offered for backwards compatibility and lean-mass-based estimates. Inputs are normalized to kilograms and centimeters internally; imperial values are converted using 1 lb = 0.45359237 kg and 1 in = 2.54 cm. TDEE multipliers (1.2 sedentary → 1.9 extra active) follow the values used in ACSM and USDA Dietary Reference Intakes. The calculator returns BMR with no rounding bias and a TDEE breakdown for all five activity tiers so users can choose the multiplier that matches their honest week.

Learn more about our methodology →

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