Calorie Calculator

Enter your sex, age, height and weight, pick an activity level, and the calculator estimates how many calories you burn in a typical day. Alongside that maintenance figure it shows three practical targets: a gentle 250 kcal deficit, a standard 500 kcal deficit, and a 500 kcal surplus for deliberate weight gain. Read the maintenance number as the intake that holds your weight steady over time, then eat below it to lose or above it to gain. Everything runs in your browser and updates as you type.

What a calorie actually measures

A calorie is a unit of heat, not something you can see or weigh in a piece of food. The name comes from the Latin calor, meaning heat. Nicolas Clément introduced it as a unit of thermal energy in lectures on calorimetry given between 1819 and 1824, and it worked its way into French and English dictionaries between 1841 and 1867. His calorie was the heat needed to raise one kilogram of water by one degree Celsius, and that choice of mass carried an ambiguity that still confuses people today.

The trouble was scale. In 1852 Pierre Antoine Favre and Jean Thiébault Silbermann attached the same word to a much smaller unit based on a single gram of water. In 1879 Marcellin Berthelot separated the two explicitly, naming them the gram-calorie and the kilogram-calorie and proposing a capital C for the larger one. The food calorie printed on every nutrition label is the large version, properly a kilocalorie, or 1,000 gram calories. Wilbur Olin Atwater, professor of chemistry at Wesleyan University from 1873 until his death in 1907, adopted Berthelot's capital-C convention in an influential 1887 article on the energy content of food and did more than anyone to fix the kilocalorie in American nutrition. So when a label reads 250 calories, it means 250 kilocalories, and this calculator follows the same convention throughout.

From candle flames to canopy hoods

The idea that a living body burns fuel the way a fire does is older than the calorie itself. In the winter of 1782 to 1783 Antoine Lavoisier and Pierre-Simon Laplace built an ice calorimeter, an inner chamber surrounded by packed ice, and placed a live guinea pig inside. By measuring the ice its body heat melted and the carbon dioxide it exhaled, then comparing that with the heat from burning carbon, Lavoisier concluded that respiration is itself a slow form of combustion. Between 1789 and 1790 he extended the work to a human subject, Armand Seguin, before the French Revolution cut the research short.

That insight became the foundation of nutritional science. If the body is a kind of furnace, its fuel needs can be counted in units of heat, and by the late nineteenth century researchers were doing exactly that with room-sized chambers. Two methods still survive: direct calorimetry, which measures the heat a body gives off, and indirect calorimetry, which infers energy use from oxygen consumed and carbon dioxide produced, often under a clear hood placed over a resting subject. Both are slow and costly, so most everyday estimates come instead from prediction equations fitted to measurements from many people.

The best known early equation came from James Arthur Harris and Francis Gano Benedict, reported to the Proceedings of the National Academy of Sciences in 1918 and set out in full in their 1919 Carnegie Institution monograph A Biometric Study of Basal Metabolism in Man. A. M. Roza and H. M. Shizgal reworked its coefficients in the American Journal of Clinical Nutrition in 1984.

This calculator uses the newer Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition in February 1990 by M. D. Mifflin, S. T. St Jeor and four co-authors, with the lead author working in the Nutrition Education and Research Program at the University of Nevada School of Medicine. They measured resting energy expenditure by indirect calorimetry in 498 healthy adults aged 19 to 78, 264 of them of normal weight and 234 obese, and found the Harris-Benedict figures ran about 5 percent high. A 2005 systematic review by David Frankenfield, Lori Roth-Yousey and Charlene Compher, published in the Journal of the American Dietetic Association using that association's evidence analysis method, compared the four equations then common in clinical practice and rated Mifflin-St Jeor the most reliable, landing within 10 percent of measured resting metabolic rate more often than Harris-Benedict, Owen or the WHO/FAO/UNU equation. That verdict is why it has largely replaced the older method.

Where the numbers come from

The starting point is basal metabolic rate, the energy the body spends at complete rest simply to stay alive. Mifflin-St Jeor estimates it from four inputs:

BMR = 10 × w + 6.25 × h − 5 × a + s

Here w is weight in kilograms, h is height in centimeters, a is age in years, and s is +5 for men or −161 for women. The weight and height terms add energy because larger bodies have more tissue to run; the age term subtracts because resting metabolism tends to fall over the years; and the sex constant reflects the fact that men carry more lean mass on average. Multiplying BMR by an activity factor gives total daily energy expenditure, which is the maintenance output.

With the defaults, a 34-year-old man, 172 cm, 74.5 kg, the BMR is 10 × 74.5 + 6.25 × 172 − 5 × 34 + 5 = 1,655 kcal. Moderately active multiplies that by 1.55, giving 2,565 kcal per day to maintain weight. The targets follow directly: 2,315 kcal for mild loss, 2,065 kcal for standard loss, 3,065 kcal for gain.

Basal metabolism is only part of the picture, but it is the largest part, typically 60 to 70 percent of what most people burn in a day. Digesting food accounts for roughly another tenth, and everything else, from structured exercise to fidgeting and walking to the kitchen, makes up the remainder. The activity multiplier is a shorthand for that whole second half.

Choosing an activity multiplier

Most people overrate themselves by one level. When in doubt, start one step lower and let the scale settle the argument over a few weeks.

What the deficit figures assume

The 500 kcal daily deficit maps to roughly 0.5 kg per week because a kilogram of body fat stores about 7,700 kcal, the metric version of the 3,500 kcal per pound rule of thumb that M. Wishnofsky set out in the American Journal of Clinical Nutrition in 1958. That relationship is an approximation, not a law. As weight drops, maintenance drops with it, so real-world loss slows unless the numbers are recalculated every few weeks. The multipliers are estimates too, routinely off by around 10 percent for any individual, and self-reported intake is almost always understated. One firm rule holds: do not eat below your BMR for extended periods without medical supervision.

One limit is built into the calculator itself. The targets only appear when a deficit leaves something to eat, so if age, height, weight and activity combine to give maintenance calories of 500 kcal a day or less, the standard 500 kcal deficit would land at or below zero and the calculator shows a message instead of any figures. Reporting a zero or negative target would be worse than reporting nothing. Combinations that low come from the far corners of the accepted input ranges, such as a very short, very light person near the top of the age range, and they sit well outside the group of adults the equation was fitted to.

Calories, kilojoules and national guidelines

Labels in the US and Canada state energy as Calories, meaning kilocalories. EU and UK labels must print kilojoules and kilocalories together, both jurisdictions still working from Regulation 1169/2011, and Australian and New Zealand panels lead with kilojoules, with kilocalories optional alongside. The ninth General Conference on Weights and Measures resolved in 1948 that the unit of quantity of heat is the joule, and the joule became the SI energy unit when the eleventh conference adopted that system in 1960; one kcal is 4.184 kJ under the thermochemical definition, so the default maintenance of 2,565 kcal is about 10,730 kJ. For context, the NHS reference intake is 2,000 kcal for women and 2,500 kcal for men, and the US Dietary Guidelines give similar ranges, population averages that a personalized estimate like this one is meant to replace.

These figures are statistical estimates for healthy adults, not medical advice for any individual. See the site disclaimer.

Frequently asked questions

How many calories should I eat a day to lose weight?

Take your maintenance number and subtract 250 to 500 kcal. For the default example here — a moderately active 34-year-old man — that means eating 2,065 to 2,315 kcal per day instead of 2,565. Bigger deficits look faster on paper but are much harder to stick to and more likely to cost muscle along with fat.

What is the difference between BMR and maintenance calories?

BMR is what your body burns at complete rest, roughly what you would use lying in bed all day. Maintenance calories, often called TDEE, multiply BMR by an activity factor between 1.2 and 1.9 to cover movement, exercise and digestion. In the worked example the BMR is 1,655 kcal but maintenance is 2,565 kcal.

Is a 500 calorie a day deficit safe?

For most healthy adults, yes — it is the deficit behind the standard advice of losing about 0.5 kg or 1 lb per week, and both the NHS and the CDC describe that pace as sensible. It stops being appropriate if it pushes intake below your BMR or under roughly 1,200 kcal for women and 1,500 kcal for men, in which case talk to a clinician first.

Why am I not losing weight eating my calculated calories?

The activity multipliers are population averages, and individual metabolisms vary by around 10% either way, so your true maintenance could easily be 200 kcal below the estimate. Logging error matters even more — studies consistently find people underreport intake by 20% or so. Track your weight for three to four weeks and adjust by 100-200 kcal rather than trusting any formula on its own.

Which activity level should I pick if I have a desk job but train regularly?

Count the training, not the job title. A desk worker who lifts weights four times a week fits Moderately active at 1.55, not Sedentary. A useful cross-check: if your weight has been stable for a month, whatever level matches your current intake is by definition the right one for you.