Which BMR formula is more accurate?

The Mifflin-St Jeor equation (1990) is generally considered the most accurate for the general population, typically within 10% of measured BMR for most healthy adults. The Harris-Benedict equation (originally 1919, revised 1984) tends to slightly overestimate BMR, often by 5%. Both are useful starting estimates; precise individual measurement requires indirect calorimetry.

Why does BMR decrease with age?

BMR declines roughly 1–2% per decade after age 20, primarily due to loss of lean muscle mass (sarcopenia). Muscle is metabolically active even at rest; less muscle means lower resting calorie burn. Strength training is the most effective intervention to slow or partially reverse the decline.

Is BMR the same as resting metabolic rate?

Very close but not identical. BMR is measured under strict laboratory conditions (12+ hours fasted, complete physical and mental rest, thermoneutral environment). Resting Metabolic Rate (RMR) is measured under less strict conditions and is typically 5–10% higher than BMR because it includes minimal activity and digestion effects. For practical calorie planning, the two are usually used interchangeably.

How do I increase my BMR?

The most effective approach is building lean muscle mass through resistance training — muscle is metabolically active at rest. Even modest gains (5–10 lb of lean muscle over 1–2 years) raise BMR by 50–150 calories/day. Other factors: getting adequate sleep, eating sufficient protein, avoiding chronic large calorie deficits (which trigger metabolic adaptation), and managing stress.

Should I eat exactly at TDEE?

For weight maintenance, yes — eat approximately at TDEE. For weight loss, eat 250–500 calories below TDEE for sustainable loss (1/2 to 1 lb/week). For weight gain (especially muscle), eat 250–500 calories above. Larger deficits or surpluses are usually counterproductive — body composition outcomes worsen at the extremes.

BMR Calculator

Basal Metabolic Rate (BMR) is the number of calories your body needs at complete rest. This calculator compares two widely used formulas: the Mifflin-St Jeor equation (considered most accurate) and the original Harris-Benedict equation.

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just keeping the lights on. Heart pumping, brain functioning, kidneys filtering, body maintaining temperature. For a typical adult, BMR represents about 60–75% of total daily calorie burn. The remainder comes from physical activity (the more variable component) and the thermic effect of food (small but real).

Knowing your BMR is the starting point for any calorie planning — weight loss, weight gain, athletic performance, or simply staying at a healthy weight. Total Daily Energy Expenditure (TDEE) is BMR multiplied by an activity factor (typically 1.2 for sedentary, up to 1.9 for very active). Eating at TDEE maintains weight; eating below creates a deficit (loss); eating above creates a surplus (gain). The math is simple in principle, complicated by individual variation and the body's remarkable ability to adjust to chronic over- or under-eating.

This calculator uses two well-known formulas — Mifflin-St Jeor (developed 1990) and the original Harris-Benedict (developed 1919) — to estimate BMR. Modern research has shown Mifflin-St Jeor to be more accurate for the general population, typically within 10% of measured BMR for most healthy adults. Harris-Benedict tends to slightly overestimate. Both are useful as starting estimates; precise individual BMR requires laboratory measurement (indirect calorimetry).

Inputs

Age

Gender

Weight (lbs)

Height (inches)

Results

Mifflin-St Jeor

1,706 cal/day

Harris-Benedict

1,780 cal/day

Average BMR

1,743 cal/day

Difference

74 cal

BMR Formula Comparison

Last updated: May 26, 2026Reviewed by the CalcMountain editorial team

Formula

Mifflin-St Jeor equation (most accurate for general population): 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 equation (revised 1984): 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 Unit conversions: weight(kg) = weight(lbs) × 0.4536 height(cm) = height(inches) × 2.54 Total Daily Energy Expenditure (TDEE) — applying an activity multiplier: Sedentary (little/no exercise): TDEE = BMR × 1.2 Lightly active (1–3 days/wk): TDEE = BMR × 1.375 Moderately active (3–5 days/wk): TDEE = BMR × 1.55 Very active (6–7 days/wk): TDEE = BMR × 1.725 Extra active (athlete): TDEE = BMR × 1.9 Weight change math: 3,500 calorie deficit ≈ 1 lb fat loss (theoretical) 3,500 calorie surplus ≈ 1 lb body weight gain (varies by composition) Example: 30-year-old male, 170 lb, 5'8" (68 inches). weight(kg) = 77.1 height(cm) = 172.7 Mifflin-St Jeor: 10 × 77.1 + 6.25 × 172.7 − 5 × 30 + 5 = 771 + 1,079 − 150 + 5 = 1,705 calories/day Harris-Benedict: 88.362 + 13.397 × 77.1 + 4.799 × 172.7 − 5.677 × 30 = 88 + 1,033 + 829 − 170 = 1,780 calories/day The two formulas agree to within about 4%. At a moderate activity level (1.55), TDEE ≈ 2,640 calories/day.

How to use this calculator

Enter your current age in years.
Select your biological sex assigned at birth. BMR formulas are calibrated based on physiological differences and weight body composition tendencies.
Enter your weight in pounds. Use a recent weighing — the formula is sensitive to weight changes of even a few pounds.
Enter your height in inches. Use 12 × feet + inches if you usually think in feet. Be reasonably precise — a 1-inch error shifts BMR by roughly 16 calories.
Review the BMR estimate. This is calories burned at complete rest — the floor of your daily metabolic needs.
To get Total Daily Energy Expenditure (TDEE), multiply by an activity factor: 1.2 (sedentary, office work, no exercise), 1.375 (light exercise 1–3 days/week), 1.55 (moderate 3–5 days), 1.725 (vigorous 6–7 days), 1.9 (athlete training twice daily).
For weight loss, eat 250–500 calories below TDEE for slow (1/2 to 1 lb/wk) loss. Aggressive deficits over 1,000 calories/day are usually counterproductive — the body adapts metabolically.
For weight gain (muscle building), eat 250–500 calories above TDEE. Faster gains usually add fat without much extra muscle.

Worked examples

30-year-old woman, moderate exercise

Age 30, female, 140 lb, 5'5" (65 inches). Mifflin-St Jeor BMR: 10(63.5) + 6.25(165.1) − 5(30) − 161 = 635 + 1,032 − 150 − 161 = 1,356 cal/day At moderate activity (1.55 multiplier): TDEE ≈ 2,102 cal/day For 1 lb/week weight loss: eat 1,600 calories/day (500-calorie daily deficit). For maintenance: 2,100 calories/day.

45-year-old man, sedentary office job

Age 45, male, 200 lb, 6'0" (72 inches). Mifflin-St Jeor BMR: 10(90.7) + 6.25(182.9) − 5(45) + 5 = 907 + 1,143 − 225 + 5 = 1,830 cal/day At sedentary activity (1.2 multiplier): TDEE ≈ 2,196 cal/day A sedentary 45-year-old often needs surprisingly few calories. Aiming for "average" intake recommendations (~2,500–2,800 cal/day) without matching activity levels often leads to slow weight gain over years.

Why BMR declines with age

Same person at 25 vs at 65 (170 lb, male, 5'10"): Age 25 BMR: 10(77.1) + 6.25(177.8) − 5(25) + 5 = 771 + 1,111 − 125 + 5 = 1,762 cal/day Age 65 BMR: 10(77.1) + 6.25(177.8) − 5(65) + 5 = 771 + 1,111 − 325 + 5 = 1,562 cal/day Difference: 200 calories/day, or about 73,000 calories/year — roughly 20 lb of theoretical weight gain over a year if eating habits don't adjust. The decline is mostly due to loss of lean muscle mass (sarcopenia). Strength training is the most effective intervention to slow or reverse this decline.

When to use this calculator

Use this calculator any time you're starting a weight management plan, training for a specific athletic goal, planning recovery from injury or illness, or just trying to understand what your body actually needs to function. The BMR number is the starting point for almost all calorie-related decisions.

It's also useful for revisiting periodically. BMR drops with age (about 1–2% per decade for typical adults), changes with weight (a 200-pound person has higher BMR than a 150-pound person), and decreases with weight loss (one of the reasons sustained weight loss is hard — your body needs fewer calories as you shrink). Re-running every 5–10 lb of weight change keeps your calorie targets calibrated.

Pair this with the TDEE calculator (which applies the activity factor directly), the calorie calculator (which builds in your weight goal), and the macro calculator (which divides total calories into protein, carbs, and fat targets).

A real-world caveat: these formulas estimate BMR for a typical body composition. Athletes with high lean muscle mass typically have higher BMR than the formula suggests; individuals with lower muscle mass (sedentary lifestyle, older age, post-prolonged-illness) typically have lower BMR. For precise measurements, indirect calorimetry (measuring oxygen consumption at rest) is the gold standard but requires specialized equipment — usually only available at fitness research centers or specialty clinics.

Another reality: the body adapts to chronic calorie intake. Sustained large deficits trigger metabolic adaptation — BMR drops, NEAT (non-exercise activity thermogenesis) decreases, and the body becomes more efficient at maintaining weight with less food. This is why aggressive cutting plateaus and why "diet breaks" (eating at maintenance periodically) can paradoxically support continued progress.

Common mistakes to avoid

Treating BMR as exact when it's an estimate. The formulas are typically within 10% of measured BMR for most healthy adults — meaning your real BMR could be 150 calories higher or lower than the formula suggests. Adjust based on actual results after 4–6 weeks.
Forgetting to multiply by activity factor. BMR alone is what you burn at rest. TDEE (BMR × activity multiplier) is what you actually need to maintain weight. Eating at "BMR" without the multiplier is a 30–90% deficit — far too aggressive.
Using outdated formulas. The original 1919 Harris-Benedict has been updated; the revised 1984 version is more accurate. Mifflin-St Jeor (1990) is more accurate still. Many older online calculators still use the original Harris-Benedict.
Ignoring body composition. Two people of the same weight, height, and age can have meaningfully different BMRs if one has substantially more muscle (higher) or more body fat (lower). The formulas can't account for individual composition.
Not adjusting after weight loss. Losing 20 lb reduces BMR by roughly 200 calories/day on Mifflin-St Jeor. Continuing to eat at the pre-loss TDEE causes regain. Recalculate after meaningful weight changes.
Crash dieting based on BMR. Chronic intake below BMR triggers metabolic adaptation, increased hunger, and muscle loss. Sustainable weight loss happens at modest deficits (250–500 cal/day) below TDEE, not at or below BMR.

Frequently Asked Questions

Sources & further reading

Dietary Reference Intakes for Energy — U.S. National Academies / USDA National Agricultural Library
Dietary Guidelines for Americans 2020–2025 — U.S. Department of Agriculture / U.S. Department of Health and Human Services
Physical Activity Guidelines for Americans — U.S. Department of Health and Human Services

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