BMR Calculator
Estimate the energy your body burns at complete rest using the Mifflin-St Jeor equation, and see how that baseline scales into your total daily requirement.
BMR Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Basal Metabolic Rate is the energy your body uses at rest to maintain vital function. The Mifflin-St Jeor equation calculates it as (10 × kg) + (6.25 × cm) − (5 × age) + 5 for men, or −161 for women. It typically accounts for 60–70% of total daily energy expenditure.
Formula
BMRfemale = 10w + 6.25h − 5a − 161
w = weight in kg · h = height in cm · a = age in years
Worked Example
- Weight term. 10 × 82 = 820.
- Height term. 6.25 × 178 = 1,112.5.
- Age term. 5 × 35 = 175, subtracted.
- Sex constant. Add 5 for men: 820 + 1,112.5 − 175 + 5 = 1,763 kcal/day.
- Scale to activity. Sedentary (×1.2) gives a TDEE of 2,115 kcal/day.
Clinical note. The same inputs give 1,842 kcal under the older Harris-Benedict equation — about 4.5% higher. Mifflin-St Jeor has been shown to predict measured resting energy expenditure more accurately in most adults, which is why it is the default here.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Setting a calorie target
BMR is the floor, not the target. Multiply by your activity factor to get maintenance, then adjust from there. The Calorie Calculator and TDEE Calculator handle the full picture.
Understanding why weight loss slows
BMR falls as you lose weight, because there is less tissue to maintain. A deficit that worked at the start becomes maintenance later, which is the main mechanism behind plateaus.
Planning a surplus for muscle gain
A modest surplus of 250 to 500 kcal above maintenance supports lean gain while limiting fat accumulation. Pair it with adequate protein — see the Protein Calculator.
Methodology & Editorial Standards
This engine uses the Mifflin-St Jeor equation, which the Academy of Nutrition and Dietetics identifies as the most reliable predictive equation for resting metabolic rate in non-obese and obese adults. The Harris-Benedict revised equation is shown alongside for comparison. Activity multipliers follow the conventional 1.2 to 1.9 scale. Weekly loss estimates assume roughly 7,700 kcal per kilogram of body tissue, a simplification that overstates loss over long periods. This engine implements the published formula exactly as specified in the source literature, with inputs validated for physiological plausibility before evaluation. Values outside the domain of the formula return an em-dash rather than a misleading figure.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
BMR Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is BMR?
Basal Metabolic Rate is the energy your body expends at complete rest to sustain vital functions — breathing, circulation, cell production and brain activity. It is the minimum energy required to stay alive, measured over 24 hours.
What is the difference between BMR and RMR?
BMR is measured under strict conditions: fasted, fully rested, in a thermally neutral environment. RMR, resting metabolic rate, is measured under less restrictive conditions and typically comes out 10 to 20% higher. In everyday use the terms are often interchanged.
Which BMR equation is most accurate?
Mifflin-St Jeor, published in 1990, is generally the most accurate predictive equation for adults and is the default here. Harris-Benedict tends to overestimate slightly. For very lean or very muscular individuals, the Katch-McArdle equation, which uses lean body mass, is better still.
How do I use BMR to lose weight?
Multiply BMR by your activity factor to find maintenance energy, then subtract a deficit. A 500 kcal daily deficit corresponds to roughly 0.45 kg per week. Never target intake below your BMR without clinical supervision.
Can I increase my BMR?
Modestly. Building muscle raises it because lean tissue is metabolically more active than fat, though the effect is often overstated — roughly 13 kcal per kg of muscle per day. Severe calorie restriction lowers BMR, which is why crash diets tend to backfire.
Why does BMR decline with age?
Primarily through loss of lean muscle mass, which begins in the third decade and accelerates later. The age term in the equation captures this average decline. Resistance training substantially slows it.
Does BMR differ between men and women?
Yes, on average. Men typically have greater lean mass and larger body size, which is why the equation adds 5 for men and subtracts 161 for women. The difference largely disappears when compared at equal lean body mass.
Should I eat below my BMR?
Generally no. Sustained intake below BMR risks nutrient deficiency, muscle loss and metabolic adaptation. Very low calorie diets have legitimate clinical uses but should only be undertaken under medical supervision.
How accurate is a predicted BMR?
Predictive equations are typically within about 10% of measured values for most people, but individual variation is real. Indirect calorimetry is the reference standard if you need a precise figure.
Does thyroid function affect BMR?
Considerably. Hypothyroidism lowers metabolic rate and hyperthyroidism raises it, sometimes well outside the range any predictive equation would suggest. If your measured energy needs differ markedly from this estimate, that is worth discussing with a clinician.
How many calories does my brain use?
Roughly 20% of BMR in adults, around 300 to 400 kcal a day. It is remarkably constant and does not increase meaningfully with mental effort, despite the popular belief that hard thinking burns calories.
What is the thermic effect of food?
The energy cost of digesting and absorbing what you eat, typically 10% of intake. Protein has the highest thermic effect at 20 to 30%, carbohydrate 5 to 10% and fat 0 to 3%. Activity multipliers already incorporate this approximately.
Is this bmr calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.