BMR Calculator: Estimate Your Basal Metabolic Rate

Your basal metabolic rate is the energy cost of staying alive with nothing else going on. Breathing, circulation, kidney and liver function, cell repair, immune activity, and the constant electrical work of the brain all run whether you move that day or not. In most adults that baseline is the largest single share of daily energy expenditure, typically well over half of the total, before any deliberate exercise is added on top.

It is worth being precise about what the number is. It describes energy your body spends. It is not a quantity of food, and this page does not turn it into one.

BMR vs. RMR

Basal metabolic rate is defined under strict laboratory conditions: measured after an overnight fast, at complete rest, in a thermally neutral room, before the person has gotten out of bed. Resting metabolic rate is measured under looser conditions, so it usually reads slightly higher, often by a few percent. The two terms are used interchangeably in most consumer tools, including this one. The equation behind this calculator was developed against measured resting energy expenditure, so the figure labeled BMR here is closer to RMR in practice.

BMR vs. TDEE

Total daily energy expenditure is your basal rate plus everything else: walking, training, fidgeting, and the energy it costs to digest and absorb food. The activity multipliers in the table above are a coarse approximation of that additional load. If you want the total rather than the resting figure, use the TDEE calculator (your total daily burn).

Five fields, in the order they appear. Every one of them feeds the published equation directly.

1. Sex. This selects the published constant the equation applies at the end, +5 or −161. It is a math input, nothing more.
2. Age in whole years. The equation subtracts five calories for every year.
3. Height in feet and inches. The calculator converts to centimeters before it does anything else.
4. Weight in pounds. Converted to kilograms with the exact factor, 0.45359237.
5. Activity level. Choose the band that matches the last four weeks of your actual behavior, not the week you intend to have.

The result appears as soon as every field holds a valid number. There is no submit gate, no account, and no email step. Everything runs in your browser, and nothing you type is sent anywhere.

Both forms of the equation, exactly as this calculator runs them:

The equation itself comes from Mifflin MD, St Jeor ST, et al., A new predictive equation for resting energy expenditure in healthy individuals, American Journal of Clinical Nutrition (1990). A later systematic review, Frankenfield D, Roth-Yousey L, Compher C, Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults, Journal of the American Dietetic Association (2005), found that it predicted resting metabolic rate more reliably than the older equations it was compared against, which is why it became the common default.

Worked Example — Male, 35, 5'10", 180 lb

Worked Example — Female, 40, 5'4", 150 lb

Notice what the activity rows do with that second figure. The light band is 1,335.388555 × 1.375 = 1,836.16, which the table shows as 1,840. The multiplier is applied to the unrounded value, never to the 1,335 on screen, because rounding twice introduces error that compounds.

The multipliers are the coarsest part of the whole calculation. Height, weight, and age are measured. This one is guessed.

Self-reported activity is the single biggest source of error in the output. A desk job plus three gym sessions is usually Light to Moderate rather than Heavy, and the gap between those two multipliers is roughly a 13 percent swing in the final number. The Centers for Disease Control and Prevention physical activity guidelines for adults give a concrete reference point to measure yourself against: 150 minutes of moderate-intensity activity per week plus two muscle-strengthening sessions. If you are unsure which band you sit in, the lower one is usually closer to the truth.

The Harris-Benedict equation, published in 1919 and revised by Roza and Shizgal in 1984, was the standard for most of the twentieth century. It reads the same four inputs used here (weight, height, age, sex) and tends to run slightly high in modern populations, which is part of why newer equations displaced it.

Katch-McArdle takes a different input entirely: BMR = 370 + (21.6 × lean body mass in kg). Because it works from lean mass rather than from sex and total weight, it can be more accurate for people who have a reliable body-fat measurement, and it makes no assumption about sex at all. Its weakness is the same as its strength: it is only as good as the body-composition figure you feed it, and consumer bioimpedance scales are not a dependable source for that.

Neither equation is computed on this page. This calculator uses Mifflin-St Jeor only.

Predictive equations carry roughly a 10% margin in either direction for most adults, per Frankenfield, Roth-Yousey and Compher, Journal of the American Dietetic Association (2005). On a 1,758-calorie estimate that is a band running from roughly 1,580 to 1,930. The estimate is a starting point that real-world data can correct, not a fixed truth.

The only actual measurement is indirect calorimetry. You breathe through a mask or under a hood while the equipment measures oxygen consumed and carbon dioxide produced, and resting energy expenditure is derived from that gas exchange. It is done in a clinical or laboratory setting, not by a web page.

What pushes an estimate off: unusually high or low muscle mass, since the equation never sees body composition; older age, where tissue composition shifts; larger body size, where the equation is less well calibrated; thyroid and other endocrine conditions; and certain medications. The equation was derived in adults roughly 19 to 78 years old. Outside that range the estimate is less reliable.

If you want a model that accounts for the way energy needs change as body weight changes, rather than a single static figure, the Body Weight Planner from the National Institute of Diabetes and Digestive and Kidney Diseases is a more sophisticated tool than any one equation.

The equation was published with two constants derived from data grouped by biological sex, +5 and −161. Selecting an option tells the calculator which published constant to use. It is a math input, not a statement about identity, and the selection is never stored, transmitted, or attached to you in any way.

The 166-calorie gap between those two constants is the equation’s shorthand for average differences in lean mass and body composition at a given height and weight. It is a population average, and averages describe groups rather than individuals. People on hormone therapy, people with atypical body composition, and anyone whose lean mass sits far from the average for their size may find that a lean-mass equation such as Katch-McArdle fits them better. A clinician can help interpret which figure is closer to your situation.

Lean muscle mass. Muscle tissue is metabolically more expensive to maintain than fat tissue, so two people at the same weight and height can have meaningfully different resting rates. This is the single largest reason an equation misses an individual, because it never sees body composition.

Age. Resting rate tends to drift down across adulthood, largely tracking changes in tissue composition rather than age itself.

Body size. A larger body has more tissue to maintain and more surface area to keep warm, so bigger bodies generally have higher resting rates.

Sleep. Short or fragmented sleep is associated with changes in appetite-regulating hormones and daily movement. The effect on measured resting rate itself is smaller and less consistent than the effect on how much you move and eat.

Chronic under-eating. Sustained low intake is associated with a downward shift in resting expenditure that is larger than body-weight loss alone would predict. This is one of several reasons very low intakes are a clinical conversation.

Thyroid and other endocrine factors. Thyroid hormone is a primary regulator of resting metabolic rate, and both under-active and over-active thyroid states can shift it meaningfully. That is a diagnosis a physician makes from labs and history, not something a calculator can indicate.

Protocol MD does not offer any therapy to raise or lower your basal metabolic rate. Nothing on this page is a claim that peptide therapy changes the number above.

Protocol MD is a physician-guided peptide therapy practice. Licensed physicians evaluate performance, longevity, and body composition, alongside recovery, sleep, energy, and cellular health.

If you want the background before anything else, here is what peptide therapy is and how it is used. A calculator estimates energy needs. It cannot tell you whether any treatment is appropriate for you. Only a licensed physician can, after a full assessment.