MathBench

Calorie Calculator

Find your resting metabolic rate, your maintenance calories, and a target for losing or gaining weight — using the equation the research supports, with the arithmetic shown and the uncertainty stated rather than hidden.

Estimates, not medical advice. These calculators apply published equations to the numbers you enter. They describe averages across large groups, and any individual can differ from the average by a wide margin — metabolism, body composition, medication, and medical conditions all shift the result. Nothing here diagnoses, treats, or replaces advice from a doctor or registered dietitian, and no target produced by a formula should override how you actually feel. If you are managing a health condition, are pregnant, or have a history of disordered eating, talk to a clinician before acting on any of it.

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Your numbers

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Where the calories go

If your activity level is different

A general-wellness estimate, not a diagnosis. It cannot tell you whether you are healthy — only where a measurement falls on a published scale. Everything is calculated in your browser: nothing you type is sent anywhere or stored. If counting is making things worse rather than better, the National Eating Disorders Association keeps a current list of support options.

How this calculator works

The calculation runs in three steps. First it estimates your resting metabolic rate — the energy your body spends staying alive at complete rest, which is the largest part of most people's daily total. Then it multiplies that by an activity factor to reach total daily energy expenditure, usually shortened to TDEE, which is the intake that would hold your weight steady. Finally it adds or subtracts an amount sized to the rate of change you asked for.

For the resting step, the default is the Mifflin-St Jeor equation, published in 1990 and repeatedly found in validation studies to be the most accurate of the simple predictive equations. Harris-Benedict is offered for comparison — it dates from 1919 and was revised in 1984, and it tends to read slightly high. Katch-McArdle takes a different route, working from lean body mass rather than height and weight, which makes it the better choice for people who are unusually muscular or unusually lean, provided the body fat figure feeding it is trustworthy.

The activity multiplier is where most of the error enters. It is a convention rather than a measurement, and people reliably place themselves a level too high: three gym sessions a week around an otherwise seated day is closer to lightly active than to moderately active. If your results disagree with reality after a few weeks, the multiplier is the first thing to revisit.

The formula

Resting metabolic rate, Mifflin-St Jeor (metric):
    men     BMR = 10 × kg + 6.25 × cm − 5 × age + 5
    women   BMR = 10 × kg + 6.25 × cm − 5 × age − 161

Revised Harris-Benedict:
    men     BMR = 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age
    women   BMR = 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age

Katch-McArdle, from lean body mass:
    BMR = 370 + 21.6 × lean kg          lean kg = kg × (1 − body fat %)

Daily total and target:
    TDEE   = BMR × activity multiplier
    target = TDEE ± (weekly rate × 7,700 kcal per kg) ÷ 7

Macronutrients:
    protein g = g per kg × body weight        4 kcal per gram
    fat kcal  = 25% of target                 9 kcal per gram
    carb kcal = whatever remains              4 kcal per gram

Mifflin-St Jeor comes from M. D. Mifflin and S. T. St Jeor, "A new predictive equation for resting energy expenditure in healthy individuals," American Journal of Clinical Nutrition 51 (1990) 241–247, fitted on 498 healthy adults and validated many times since. The tidy coefficients above — 10, 6.25 and 5 — are not a later rounding by calculator authors: they are the paper's own simplified equations, offered alongside the full regression whose coefficients are 9.99, 6.25 and 4.92 with a sex term of 166. The gap between them is exactly 0.08 × age − 0.01 × weight calories a day, which for the worked example below comes to 1.6 — far inside the equation's own error. This page uses the simplified form the authors published for practical use. The revised Harris-Benedict coefficients are those of Roza and Shizgal, American Journal of Clinical Nutrition 40 (1984) 168–182. The equation everyone calls Katch-McArdle is really Cunningham's: it was published by J. J. Cunningham in the American Journal of Clinical Nutrition 54 (1991) 963–969, and acquired the popular name through the McArdle, Katch and Katch textbook Exercise Physiology. It works from lean mass because fat tissue is metabolically far less active than muscle and organ tissue, and it carries no sex term at all — Cunningham's argument was that the apparent difference between men and women disappears once you account for fat-free mass. Note that a different Cunningham equation from 1980 (500 + 22 × lean kg) also circulates and gives answers around 150 calories higher; this page uses the 1991 form.

Protein recommendations follow the International Society of Sports Nutrition position stand on protein and exercise (Jäger et al., Journal of the International Society of Sports Nutrition, 2017), which supports 1.4 to 2.0 grams per kilogram for people doing resistance training, with evidence for the upper end and slightly beyond when eating at a deficit. The 7,700 kcal per kilogram figure (3,500 per pound) is the conventional energy density of body fat; its limitations are discussed in the FAQ and come from the dynamic modelling work of Kevin Hall and colleagues at the National Institutes of Health.

Worked example

A 30-year-old man, 180 cm and 80 kg, training three to five times a week — the calculator's default:

  1. Mifflin-St Jeor: 10 × 80 = 800, plus 6.25 × 180 = 1,125, minus 5 × 30 = 150, plus 5
  2. Resting metabolic rate: 800 + 1,125 − 150 + 5 = 1,780 kcal a day
  3. Moderately active multiplier: 1,780 × 1.55 = 2,759 kcal to maintain
  4. To lose 0.5 kg a week: 0.5 × 7,700 ÷ 7 = 550 kcal a day below maintenance, so a target of 2,209 kcal
  5. Protein at 1.6 g/kg: 128 g, which is 512 kcal — about 23 percent of the budget
  6. Fat at 25 percent: 552 kcal, or 61 g. Carbohydrate takes the remaining 1,145 kcal, about 286 g

The honest caveat sits alongside those figures: the ±10 percent accuracy band on the equation alone puts maintenance somewhere between about 2,483 and 3,035 calories, and the activity multiplier widens that further. The number to trust is not the one the equation prints but the one your weight trend reveals after three weeks of eating near it.

Assumptions & tips

  • Pick the lower activity level. This is the most common error by a wide margin. The multiplier is meant to capture your whole week — including how much you move outside training — and a desk job with a few gym visits is lightly active, not moderately active. Guessing one level high inflates maintenance by roughly 300 calories.
  • Judge by the trend, not the equation. Weigh yourself under the same conditions several times a week and compare weekly averages. Two or three weeks of data tells you more about your real maintenance than any formula can, and the formula's job is only to give you a sensible place to start.
  • Expect the target to move. A lighter body needs fewer calories, so the deficit that produced steady loss at the start produces slower loss later. Recalculate every 4 to 6 kilograms, and treat a genuine plateau as information rather than failure.
  • Protein first, then fat, then carbohydrate. Protein protects lean mass while you are losing weight and is the most satiating macronutrient per calorie. Set it from body weight, keep fat above roughly 20 percent of calories, and let carbohydrate take the remainder — it is the one that fuels hard training.
  • Slower is usually better. Deficits beyond about 20 to 25 percent of maintenance cost lean mass, worsen training quality, and are harder to stick to. The rate that you can maintain for months beats the rate that looks impressive for two weeks.
  • This is not a clinical tool. Pregnancy, breastfeeding, thyroid conditions, diabetes, and many medications all change energy needs in ways no predictive equation captures. If any of those apply, or if you have a history of disordered eating, take these numbers to a professional rather than acting on them alone.

Frequently asked questions

How accurate is a calculated calorie target?

Less accurate than the confident-looking number suggests. The Mifflin-St Jeor equation predicts measured resting metabolism to within 10 percent for about 82 percent of people who are not obese, and about 70 percent of those who are — and the model explains roughly 71 percent of the variance, meaning nearly a third is left to individual differences it cannot see. On an 1,800 calorie resting rate, a 10 percent band is a spread of nearly 400 calories. The activity multiplier adds more uncertainty still, because self-reported activity is notoriously optimistic. Treat the result as a starting hypothesis: eat at that level for two or three weeks, track your weight trend rather than daily readings, and adjust by 100 to 200 calories based on what actually happens. Your own data beats any equation.

Which equation should I choose?

Mifflin-St Jeor unless you have a specific reason otherwise — it was derived more recently than Harris-Benedict, on a more representative population, and validation studies consistently find it the most accurate of the predictive equations for people who are not extremely lean or extremely heavy. Choose Katch-McArdle only if you have a reliable body fat measurement, because it works from lean mass and therefore handles very muscular or very lean bodies better than the height-and-weight equations. If your body fat number came from a bathroom scale, prefer Mifflin-St Jeor; those scales are not reliable enough to improve the estimate.

Is the 3,500 calories per pound rule true?

It is a useful approximation for the short term and wrong over the long term. The figure comes from the energy density of body fat, and it predicts that a 500 calorie daily deficit produces a pound a week indefinitely. It does not, because losing weight makes you smaller: a lighter body burns fewer calories at rest and costs less to move, so the same intake becomes a smaller deficit as you go. Kevin Hall and Carson Chow set out the problem directly in "Why is the 3500 kcal per pound weight loss rule wrong?" (International Journal of Obesity 37, 2013, 1614), and the accompanying analysis by Thomas and colleagues quantified it: over 65 days the static rule predicted 27.6 pounds of loss where 20.1 pounds actually occurred, an overestimate of roughly a third. Each pound lost also lowers maintenance needs by around 10 calories a day, so loss decelerates and eventually plateaus. This calculator uses the 3,500 rule for the initial target and says plainly that you should expect to recalculate as your weight changes.

How much protein do I actually need?

The RDA of 0.8 grams per kilogram is a minimum to prevent deficiency, not an optimum for someone training. The International Society of Sports Nutrition position stand recommends 1.4 to 2.0 grams per kilogram for people doing resistance training, and the evidence for preserving lean mass during a calorie deficit supports the upper part of that range or slightly above — roughly 1.6 to 2.4 grams per kilogram. Higher intakes are not harmful in healthy people but stop adding benefit. The calculator lets you pick a level and shows what it costs in calories, since protein has to fit inside the same budget as everything else.

Why does the calculator refuse some inputs?

It declines to produce a target below 1,200 calories a day for women or 1,500 for men, and it will not generate a weight-loss target at all for someone whose weight is already below the healthy range. Those floors are not arbitrary: the NHLBI Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults (1998) place low-calorie diets at 1,000 to 1,200 calories a day for women and 1,200 to 1,500 for men, and anything below that belongs under clinical supervision. Very low calorie diets can be medically appropriate, but they belong under clinical supervision rather than being handed out by a web page, and a formula cannot tell whether the person entering the numbers is safe to follow them. If your calculation lands at the floor, that is a signal to talk to a dietitian rather than to eat less.

Sources

  1. A new predictive equation for resting energy expenditure in healthy individuals — M. D. Mifflin, S. T. St Jeor and colleagues, American Journal of Clinical Nutrition 51(2), 241–247, 1990. doi.orgThe default resting metabolic rate equation, including the simplified 10 / 6.25 / 5 coefficients and the +5 / −161 sex terms this page uses.
  2. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass — A. M. Roza and H. M. Shizgal, American Journal of Clinical Nutrition 40(1), 168–182, 1984. doi.orgThe revised Harris-Benedict coefficients (88.362 / 13.397 / 4.799 / 5.677 for men, 447.593 / 9.247 / 3.098 / 4.330 for women) offered as the second equation option.
  3. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation — J. J. Cunningham, American Journal of Clinical Nutrition 54(6), 963–969, 1991. doi.orgThe lean-mass equation this page offers under its popular name, Katch-McArdle: 370 + 21.6 × lean kg, with no sex term.
  4. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report — National Heart, Lung, and Blood Institute, NIH Publication No. 98-4083, September 1998. nhlbi.nih.govSource of the calorie floors this calculator refuses to go below: the report places low-calorie diets at 1,000 to 1,200 kcal a day for women and 1,200 to 1,500 for men.
  5. International Society of Sports Nutrition Position Stand: protein and exercise — R. Jäger and colleagues, Journal of the International Society of Sports Nutrition 14:20, 2017. pmc.ncbi.nlm.nih.govEvery option in the protein selector: the 0.8 g/kg RDA baseline, the 1.4 to 2.0 g/kg training range, and the higher intakes evidenced for preserving lean mass in a deficit.
  6. Why is the 3500 kcal per pound weight loss rule wrong? — K. D. Hall and C. C. Chow, International Journal of Obesity 37(12), 1614, 2013. doi.orgWhy the 7,700 kcal per kg (3,500 per pound) conversion behind the goal adjustment overstates long-run loss, and why the page tells you to recalculate as your weight changes.

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