What is BMR and why does it matter?

Basal Metabolic Rate is the minimum calories your body needs to sustain vital functions at complete rest — breathing, circulation, cell repair. It accounts for 60-75% of total daily calorie burn for most people, making it the largest component of your energy budget.

Which BMR formula is most accurate?

The Mifflin-St Jeor formula (used here) outperforms the older Harris-Benedict formula in most studies, with mean error under 10% vs measured metabolic rate. The Katch-McArdle formula is more accurate if you know your lean body mass, since it accounts for muscle mass directly.

Does muscle mass affect BMR?

Yes significantly. Skeletal muscle burns approximately 6 calories per pound per day at rest, compared to 2 calories per pound for fat. A person with 140 lb of lean mass has a meaningfully higher BMR than someone of identical weight with 115 lb of lean mass.

What happens to BMR during a calorie deficit?

BMR decreases. Severe restriction (below 25% of TDEE) can trigger adaptive thermogenesis — metabolic adaptation that reduces BMR by 5-15% beyond what weight loss alone predicts. This is why gradual deficits and diet breaks help long-term fat loss.

Can I increase my BMR?

Yes. Building muscle through resistance training is the most effective method — each pound of muscle raises daily BMR by approximately 6 calories. Adequate protein intake (0.7-1g per lb of body weight) also prevents BMR-reducing muscle loss during weight loss.

Methodology: BMR Calculator

Scope

Computes Basal Metabolic Rate using the Mifflin-St Jeor equation — the calorie cost of staying alive at rest, which is the largest component of TDEE for most people.

Formula

BMR (men) = 10*weight_kg + 6.25*height_cm - 5*age + 5. BMR (women) same minus 166 instead of +5.

Coefficients

Parameter	Value	Note
Weight coefficient	`10 kcal/kg`
Height coefficient	`6.25 kcal/cm`
Age coefficient	`-5 kcal/year`
Sex constant	`+5 (M) / -161 (F)`

Data sources

Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. — PMID 2305711.
Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789. — PMID 15883556. Meta-analysis validating Mifflin-St Jeor as the most accurate predictor.

Assumptions

Subject is awake, fasted, thermoneutral, and resting.

Approximation range

RMSE vs indirect calorimetry in healthy non-obese adults: 150–200 kcal/day.

Limitations

Systematically under-predicts for people with high lean mass; Katch-McArdle using LBM is more accurate there.
Over-predicts for elderly and severely obese populations.

Reproducibility

Female, 65 kg, 165 cm, 35 yrs: BMR = 10*65 + 6.25*165 - 5*35 - 161 = 1345 kcal.

Change log

2026-04-24: methodology page first published.

TDEE Calculator — Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.
Macro Calculator — Convert calorie targets into protein, carbs, and fat grams for your goal.
Protein Intake Calculator — Get daily protein targets based on training level and goal.
Water Intake Calculator — Calculate daily water intake based on weight, activity level, and climate.

Worked example

Computed by the same engine bundle served at /engines/bmr-calculator.js. Re-runnable: the values below are the literal output of compute(engineInput).

Input

tool: bmr_calculator
sex: male
age: 30
weight_kg: 78
height_cm: 178
activity_level: moderate

Output

primaryLabel: Estimated maintenance calories
primaryValue: 2708.63
primaryFormat: calories
summary: Mifflin-St Jeor BMR scaled by selected activity factor.
metrics: [{"label":"BMR","value":1747.5,"format":"calories"},{"label":"Fat-loss target","value":2208.63,"format":"calories"},{"label":"Lean-gain target","value":2958.63,"format":"calories"},{"label":"Protein baseline","value":140.4,"format":"grams"}]
warnings: []
assumptionsEcho: {"sex":"male","age":30,"weight_kg":78,"height_cm":178,"activity_level":"moderate"}

FAQ

What is BMR and why does it matter?: Basal Metabolic Rate is the minimum calories your body needs to sustain vital functions at complete rest — breathing, circulation, cell repair. It accounts for 60-75% of total daily calorie burn for most people, making it the largest component of your energy budget.
Which BMR formula is most accurate?: The Mifflin-St Jeor formula (used here) outperforms the older Harris-Benedict formula in most studies, with mean error under 10% vs measured metabolic rate. The Katch-McArdle formula is more accurate if you know your lean body mass, since it accounts for muscle mass directly.
Does muscle mass affect BMR?: Yes significantly. Skeletal muscle burns approximately 6 calories per pound per day at rest, compared to 2 calories per pound for fat. A person with 140 lb of lean mass has a meaningfully higher BMR than someone of identical weight with 115 lb of lean mass.
What happens to BMR during a calorie deficit?: BMR decreases. Severe restriction (below 25% of TDEE) can trigger adaptive thermogenesis — metabolic adaptation that reduces BMR by 5-15% beyond what weight loss alone predicts. This is why gradual deficits and diet breaks help long-term fat loss.
Can I increase my BMR?: Yes. Building muscle through resistance training is the most effective method — each pound of muscle raises daily BMR by approximately 6 calories. Adequate protein intake (0.7-1g per lb of body weight) also prevents BMR-reducing muscle loss during weight loss.