The BMR Calculator estimates your BMR using three peer-reviewed equations — Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle — and displays the results side by side for comparison.
Basal Metabolic Rate is not the number of calories you need to survive, and confusing BMR with a minimum safe intake is one of the most common and potentially harmful misunderstandings in nutrition. BMR estimates the energy your body uses at absolute rest — lying still in a temperature-controlled room after an overnight fast — just to keep your heart beating, your lungs breathing, and your cells repairing. It does not account for digestion, standing up, walking to the kitchen, or any other activity. Actual daily energy needs are substantially higher, typically 1.2 to 1.9 times BMR depending on lifestyle. Anyone using their BMR figure as a calorie target is almost certainly eating well below their real energy requirements.
Understanding the Calculation
This calculator implements three equations that each take a different approach to predicting resting metabolism. Two of them — Mifflin-St Jeor and Harris-Benedict — use weight, height, age, and sex. The third — Katch-McArdle — ignores all demographic variables and relies entirely on LBM (lean body mass).
The Mifflin-St Jeor equation (1990) applies these coefficients:
- Males: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Females: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
The sex-specific constants (+5 for males, −161 for females) reflect the average difference in lean mass between sexes at equivalent body weights.
The Harris-Benedict equation has the longest history of any metabolic prediction formula. James Arthur Harris and Francis Gano Benedict published the original in 1919, derived from calorimetry data on 239 subjects. By the 1980s, researchers recognised that changes in average body composition and improved statistical methods warranted a revision. Roza and Shizgal published the updated version in 1984 in the American Journal of Clinical Nutrition. The revised equation uses different coefficients for each variable and tends to produce slightly higher estimates than Mifflin-St Jeor, typically by 50–100 kcal/day.
The Katch-McArdle equation (1983) takes a fundamentally different approach: BMR = 370 + (21.6 × LBM in kg). Because it uses lean body mass rather than total weight, it is sex-neutral and age-neutral by design. The logic is straightforward — metabolic rate correlates more closely with muscle and organ mass than with total body weight, so measuring the metabolically active compartment directly should yield a better prediction. The trade-off is that it requires an accurate body fat percentage. If you do not have a recent measurement, tools such as the body fat estimation method using Navy tape or skinfold calipers can provide an approximate figure.
Comparing the Formulas
For an average-build individual, the three equations tend to agree within 50–100 kcal. The divergence grows at the extremes of body composition and age, which is where formula selection starts to matter.
| Scenario | Mifflin-St Jeor | Harris-Benedict | Katch-McArdle |
|---|---|---|---|
| 30M, 80 kg, 178 cm, no body fat data | 1,768 kcal | 1,844 kcal | Not available |
| 32F, 62 kg, 168 cm, 22% body fat | 1,349 kcal | 1,403 kcal | 1,415 kcal |
In the first row, the 76 kcal gap between formulas is small enough that either equation provides a reasonable estimate. In the second row, all three formulas cluster within 66 kcal — a particularly tight grouping that reflects the moderate body composition of the subject. For individuals with very low or very high body fat, the gap between weight-based formulas (Mifflin-St Jeor and Harris-Benedict) and the lean-mass-based Katch-McArdle tends to widen, making the third formula more valuable.
The average of all available formulas — shown in the calculator results — provides a practical single estimate that smooths out the individual biases of each equation.
What This Tool Cannot Do
BMR calculators estimate resting metabolism using regression equations derived from population studies. Several important limitations apply.
No population-based formula accounts for individual genetic variation in metabolic rate. Research suggests that BMR can vary by 5–8% between individuals of the same age, sex, height, and weight due to differences in organ size, thyroid function, and mitochondrial efficiency. Factors such as sleep quality and metabolic rate are closely linked — chronic sleep restriction has been shown to reduce resting metabolic rate by 2–5%, an effect none of these equations can capture. The equations predict the average for a given set of inputs, not the specific value for any one person.
These formulas also cannot detect metabolic adaptation. During prolonged calorie restriction, the body downregulates metabolic rate beyond what changes in body weight would predict. A person who has been dieting for several months may have a BMR 10–15% lower than what any equation estimates. Conversely, after a period of overfeeding, metabolic rate may be temporarily elevated. The calculator reflects static inputs, not metabolic history.
Finally, RMR (Resting Metabolic Rate) is not identical to BMR, though the terms are often used interchangeably. RMR is measured under slightly less restrictive conditions — the subject is rested but not necessarily fasted for 12 hours in a thermoneutral room. RMR typically runs 3–10% higher than BMR. The equations in this calculator estimate BMR specifically, following the original study protocols.
Tips for Better Results
Accurate inputs produce better estimates, and a few practical considerations improve the quality of the data fed into these equations.
- Weigh yourself in the morning after using the bathroom and before eating or drinking. Evening weight can be 1–2 kg higher due to food, water, and fluid retention.
- Measure height carefully. A 2 cm error in height changes the Mifflin-St Jeor estimate by approximately 12.5 kcal — small on its own, but errors compound when combined with other input inaccuracies.
- Use body fat percentage if available. Even an approximate measurement enables the Katch-McArdle equation and provides a useful third data point for cross-referencing.
- Recalculate periodically. A 5 kg weight change, a birthday that crosses a decade boundary, or a significant shift in body composition all warrant a fresh estimate.
For most nutrition planning purposes, BMR alone is not the useful number — it is the starting point for a total daily energy expenditure estimate that accounts for activity, or for splitting calories into a macronutrient split calculator once a daily target is established.
From BMR to a Practical Calorie Target
BMR represents the floor of your energy expenditure — the cost of simply existing. In exercise science, this resting energy cost is standardised as 1 MET; browsing the MET values where 1 MET equals your resting metabolic rate table shows how different activities scale relative to this baseline. Every additional activity layered on top raises total energy needs. To translate BMR into something actionable, multiply it by an activity factor to estimate TDEE, then adjust that TDEE based on goals: a moderate deficit for fat loss, maintenance for weight stability, or a surplus for muscle gain.
The distinction matters because people who skip the activity multiplier and eat at or near their BMR are almost certainly in a steep deficit that risks muscle loss, nutrient deficiency, and metabolic downregulation. A safe and effective deficit typically subtracts 300–500 kcal from TDEE — not from BMR. For guidance on structuring a deficit, the calorie deficit planning tool calculates appropriate targets while respecting minimum calorie floors. Understanding where your body mass index assessment falls can also provide useful context alongside BMR when evaluating overall body composition.
Basal Metabolic Rate
The minimum energy expenditure required to sustain vital organ function in a completely rested, fasted, thermoneutral state. BMR typically accounts for 60–75% of total daily energy expenditure in sedentary individuals and decreases with age, weight loss, and reduced lean body mass.
Lean Body Mass
Total body weight minus fat mass. Lean body mass includes muscle, bone, organs, water, and connective tissue — essentially everything that is not stored body fat. It is the sole input to the Katch-McArdle equation because metabolic rate correlates more strongly with lean tissue than with total weight.
Resting Metabolic Rate
A measurement of energy expenditure taken under resting but less strictly controlled conditions than BMR. RMR is easier to measure clinically because it does not require an overnight stay in a thermoneutral chamber. It typically exceeds BMR by 3–10% and is sometimes used interchangeably with BMR in informal contexts, though the two are technically distinct.
Metabolic Adaptation
A reduction in metabolic rate that exceeds what body weight changes alone would predict, occurring during sustained calorie restriction. Also called adaptive thermogenesis, this phenomenon means that someone who has been dieting for months may burn fewer calories at rest than an equation would estimate for their current weight. Metabolic adaptation is one reason that calculated BMR values become less accurate over time during prolonged energy restriction.