Calorie calculator: what formulas are inside and why numbers differ

Two calculators using the same height, weight, age, and gender can provide numbers that differ by two or three hundred kilocalories. This is not an error in either of them. They use different formulas derived from different sample groups, and these are multiplied by an activity factor invented long before the advent of pedometers.

What the calculator actually counts

First, it estimates the basal metabolic rate — the energy expenditure at rest, while fasting, at a comfortable temperature. Then, it multiplies this by an activity factor to get the total daily energy expenditure. Both steps are estimates, and both introduce error.

Basal metabolic rate is not "how much a person burns while lying down." It is a standardized measurement performed in research settings in the morning, while fasting, after rest, using indirect calorimetry. The formula merely predicts the result of such a measurement based on anthropometry.

Three formulas and how they differ

• Harris-Benedict (1919, revised 1984). The oldest one. It was derived from a sample that differed in body composition from modern populations and tends to overestimate.
• Mifflin-St Jeor (1990). Currently the default standard in most calculators.
• Katch-McArdle. Calculates based on lean body mass rather than total weight. It requires knowing the body fat percentage, which shifts the uncertainty to the body composition measurement — see the article on smart scales and bioimpedance.

A systematic review by the American Dietetic Association compared the four most commonly used formulas in people with normal weight and obesity. The Mifflin-St Jeor formula proved to be the most reliable: it predicted resting energy expenditure within 10% of the measured value for a larger proportion of people than the others and had the narrowest error range. The authors specifically note that significant errors and limitations persist when applied to an individual. Review in the Journal of the American Dietetic Association.

The key phrasing is: "within 10% for a larger proportion of people." This means that for the others, the deviation is greater than 10%, and you do not know in advance which group you are in.

Subsequent tests on other samples did not replicate the picture exactly. In cross-validation of resting energy expenditure formulas, the smallest systematic bias was shown by Harris-Benedict (−14 ± 378 kcal per day) and the WHO formula (−25 ± 394), while the predictions of all other formulas differed significantly from those measured by indirect calorimetry. At the same time, the limits of agreement for all formulas ranged from 314 to 445 kcal per day. Study in the Journal of the Academy of Nutrition and Dietetics. Accuracy in adults with overweight and obesity was tested separately — Obesity Research and Clinical Practice.

Pay attention to the spread: ±378 kcal per day with an average bias near zero. A formula that is "on average correct" can be off by a third of the daily requirement for a specific individual. Arguing about the choice of formula makes sense only until you see this interval.

Where the error becomes large

The activity factor. It is set in steps: sedentary lifestyle — 1.2, light activity — 1.375, moderate — 1.55, and so on. The step between adjacent levels is about 175 kilocalories for a basal metabolic rate of 1600. Choosing the adjacent row in a dropdown list changes the result more than the difference between the formulas.

At the same time, the definitions of the levels are vague: "workouts 3–5 times a week" does not distinguish between an hour-long walk and interval training. And the largest part of daily expenditure for an office worker is not workouts at all, but daily living activity, which varies significantly between people.

Why individual expenditure does not match the formula

Three reasons, none of which are resolved by choosing a different calculator.

• Body composition. Muscle tissue, organs, and fat consume energy differently. Formulas based on weight do not account for this.
• Individual variation. For two people of the same sex, age, height, and weight, the measured basal metabolic rate differs — this is a property of biology, not a measurement error.
• Adaptation during weight loss. Expenditure drops more than mass loss predicts. What is known about this — in the article on why the counter shows a deficit but weight remains the same.

How to use this, knowing about the error

The number from the calculator is a starting hypothesis, not a daily requirement. It is verified in one way: two to three weeks of consistent eating and weighing, taken under the same conditions, and comparing the actual dynamics with the expected ones. Then, you adjust the number itself, not the formula.

This is precisely why the "Mifflin vs. Harris-Benedict" debate is fruitless: the difference between them is smaller than the discrepancy of either one with the reality of a specific person, and both are corrected by the same verification. How the deficit itself is calculated — in the article on calorie deficit, and where the daily requirement comes from — in the article on daily calorie requirements.

When formulas do not apply

They were derived from healthy adults. Outside of this sample — pregnancy and breastfeeding, childhood and adolescence, thyroid diseases, severe medical conditions, extreme body weight values — the estimate becomes unreliable, and in these cases, the matter should be discussed with a doctor.

Count it from a photo

Frequently asked questions

Which calorie calculation formula is more accurate?

In a systematic review comparing the four most commonly used formulas, the Mifflin-St Jeor formula proved to be the most reliable. In a later cross-validation, the Harris-Benedict and WHO formulas showed the least bias. There is no single answer, and the limits of agreement for all formulas range from 314 to 445 kcal per day.

Why do two calculators give different numbers?

Due to different basal metabolic rate formulas and, primarily, the activity factor. The step between adjacent activity levels is about 175 kilocalories for a basal metabolic rate of 1600 — this is greater than the difference between the formulas.

How much can the calculator be off for me personally?

Even the best formula does not fit within 10% for everyone. For others, the deviation is greater, and it is impossible to determine in advance which group a specific person will fall into.

What is an activity factor and how do I choose one?

A multiplier applied to the basal metabolic rate: from 1.2 for a sedentary lifestyle and up. The definitions of the levels are vague and do not distinguish, for example, between an hour-long walk and interval training.

How can I verify the number from the calculator?

Treat it as a starting hypothesis and compare it with the facts: two to three weeks of consistent eating with the same weighing method, then compare the actual weight dynamics with the expected ones and adjust the number itself.

Read next

• Daily calorie intake — There is no universal norm: it depends on weight, height, age, sex, and activity level.
• Calorie deficit: how to calculate it and how to hold it — A calorie deficit is the one thing without which weight does not come down.
• Metabolism: is it true that it can be "boosted" and that it can "break" — One person is promised a metabolism boost through spices and frequent small meals, another explains weight plateaus by claiming their metabolism is broken from dieting.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.