BMR Calculator
Calculate your Basal Metabolic Rate using the Mifflin-St Jeor and Harris-Benedict formulas — the calories your body burns at complete rest.
What is BMR?
Your Basal Metabolic Rate (BMR) is the number of calories your body needs to sustain its basic life-support functions — breathing, circulation, cell repair, and temperature regulation — while at complete rest. Think of it as your body's idle fuel consumption: the energy cost of simply being alive, with no movement at all.
BMR is the foundation of all calorie calculations. Multiply it by an activity factor and you get your Total Daily Energy Expenditure (TDEE) — the actual calories you burn each day including movement and exercise. Knowing your BMR gives you a precise anchor for any diet or fitness goal.
The BMR Formulas: Mifflin-St Jeor vs Harris-Benedict
This calculator uses two well-established BMR formulas, both shown side by side so you can compare results and understand where differences come from.
A 2005 meta-analysis in the Journal of the American Dietetic Association found Mifflin-St Jeor to be the most accurate for the general population, predicting measured resting metabolic rate within 10% for about 82% of people. The revised Harris-Benedict formula (not the original 1919 version) is a solid alternative and often gives results within 50–100 calories of Mifflin.
Mifflin-St Jeor vs Harris-Benedict: Which Is More Accurate?
Both formulas estimate the same thing but were derived from different study populations and regression methods. Mifflin-St Jeor (1990) used a sample of 498 adults aged 19–78 and has become the standard recommendation because it outperforms Harris-Benedict on average across diverse body types.
Harris-Benedict (revised 1984 by Roza and Shizgal) is not wrong — it simply tends to run slightly higher for most people. For individuals with higher than average muscle mass, Harris-Benedict may actually be more accurate. Showing both gives you a practical range rather than a false sense of precision from a single number.
How BMR Changes With Age
BMR typically falls 1–2% per decade after age 30, driven primarily by sarcopenia — the gradual, age-related loss of skeletal muscle mass. Since muscle tissue burns roughly three times more calories at rest than fat tissue, less muscle means a lower idle metabolic rate.
The practical implication is that the same calorie intake that maintained weight at age 25 will often produce slow weight gain at 45 or 55, even without any change in behavior. Resistance training is the most effective intervention: preserving — or rebuilding — muscle mass counteracts the BMR decline more than any other lifestyle factor.
BMR vs TDEE: What's the Difference?
BMR measures your resting calorie burn — zero movement, no digestion, just baseline biology. Total Daily Energy Expenditure (TDEE) adds everything else: the thermic effect of food (roughly 10% of calories eaten), non-exercise activity thermogenesis (NEAT — fidgeting, standing, walking), and deliberate exercise.
For most sedentary adults, TDEE runs about 20–30% above BMR. For very active individuals, TDEE can be 80–90% above BMR. To get your TDEE, multiply BMR by an activity factor ranging from 1.2 (sedentary) to 1.9 (extra active). Use the Calorie Calculator if you want TDEE and calorie targets calculated directly.
Static Example — 35-Year-Old Female, 5'5", 140 lbs
A 35-year-old woman, 5'5" (165 cm), 140 lbs (63.5 kg):
- Mifflin-St Jeor BMR ≈ 1,397 cal/day
- Harris-Benedict (rev.) BMR ≈ 1,435 cal/day
- Estimated TDEE at moderate activity (×1.55) ≈ 2,165–2,224 cal/day
- Typical BMR range for 30–39-year-old females: 1,300–1,500 cal/day
The two formulas differ by only 38 calories here — less than 3%. In practice, treat either result as a calibrated estimate; real-world BMR can vary ±10% from prediction depending on lean body mass and hormonal factors.
About These Parameters
- Age
- Both formulas subtract a fixed calorie amount per year of age. Mifflin subtracts 5 cal/year; Harris-Benedict subtracts approximately 5.7 cal/year for males and 4.3 for females. This reflects the gradual decline in lean body mass and basal metabolic activity as we age.
- Gender
- Males on average have 10–15% higher BMR than females of the same age, height, and weight — primarily because of greater lean muscle mass. Muscle tissue is metabolically more expensive than fat tissue. The formulas capture this with gender-specific constants and coefficients.
- Height
- Height has a positive correlation with BMR: taller people have a larger body surface area and more tissue to maintain. The coefficient is smaller than weight, but still meaningful — 10 cm of extra height adds roughly 47–64 calories to the estimate depending on the formula.
- Weight
- Weight is the strongest driver of BMR variance among the measurable inputs. However, body composition matters too: 10 lbs of muscle raises BMR more than 10 lbs of fat, because lean mass is more metabolically active at rest. BMR formulas use total body weight as a proxy and cannot distinguish between lean mass and fat without additional measurements.
Frequently Asked Questions
What is a normal BMR?
There is no single "normal" — BMR varies widely with body size, age, gender, and body composition. Rough ranges for adults: males typically 1,500–2,100 cal/day, females typically 1,200–1,700 cal/day. Athletes with high muscle mass can exceed 2,500 cal/day. Small-framed older females may be below 1,200. What matters is your number relative to your own intake and output — not a comparison to population averages.
Does muscle mass affect BMR?
Yes — significantly. Skeletal muscle burns approximately 6–10 calories per pound per day at rest, compared to about 2 calories per pound for fat tissue. This is why two people of identical weight, age, height, and gender can have meaningfully different BMRs if one has significantly more muscle. Formula-based BMR calculators cannot capture this directly; for a body-composition-adjusted estimate, the Katch-McArdle formula (which uses lean body mass) is more accurate if you know your body fat percentage.
How do I use BMR to lose weight?
BMR alone is not enough — you need TDEE (Total Daily Energy Expenditure), which accounts for activity. Multiply your BMR by your activity factor to get TDEE. Then eat below TDEE to create a deficit: a 500-calorie daily deficit produces approximately 1 pound of fat loss per week (3,500 cal ≈ 1 lb of fat). Do not eat below your BMR long-term — that creates too large a deficit and risks muscle loss, nutrient deficiency, and metabolic adaptation. Use the Calorie Calculator to set specific targets for every goal.
Why do the two formulas give different results?
Mifflin-St Jeor (1990) and the revised Harris-Benedict (1984) were derived from different samples and use different regression coefficients. Harris-Benedict was originally created in 1919 and revised in 1984; Mifflin validated its formula on a more diverse modern population. For most people the difference is 50–200 calories. When in doubt, use Mifflin-St Jeor — it is the current clinical recommendation — but tracking either formula consistently over time matters more than which formula you pick.
Does BMR change when I lose weight?
Yes. BMR decreases as you lose weight because there is less body mass to maintain. If you lose 10 pounds, your BMR will drop by roughly 30–60 calories. This is why calorie targets need to be recalculated periodically during a weight loss phase — a target set at 200 lbs will be slightly too high by the time you reach 180 lbs. Recalculate every 10–15 lbs of change to keep targets accurate.