🥗 Nutrition · 4 min read

Calorie formulas: what an energy requirement estimate can do

What a formula estimates

Daily energy expenditure is made up of several parts. The largest is basal metabolic rate — the energy the body needs at complete rest for organs, circulation and cell maintenance. Added to this are the energy for digestion, movement in everyday life and deliberate training.

The well-known formulas first estimate only basal metabolic rate and then multiply it by an activity factor. Both steps contribute their own uncertainty, and these add up.

The result is nonetheless displayed to the kilocalorie. That presentation is misleading: a number like 2,347 suggests a precision the calculation does not support. A range would be more appropriate — but most calculators show a point value because it makes planning easier.

The common equations

Several formulas are in circulation, all derived from measurement series on groups.

• Harris-Benedict. The oldest of the widespread equations, created in the early 20th century and later revised. It uses weight, height, age and sex.
• Mifflin-St Jeor. Developed in the 1990s on a larger and more contemporary sample. In comparative studies it is considered the more accurate of the two for many adults.
• Katch-McArdle. Calculates not with weight but with fat-free mass — and therefore needs an estimate of body fat percentage, whose uncertainty it inherits.

None of these equations fits everyone equally well. Which comes closest depends on build.

In practice the choice of equation matters less than often assumed. The differences between the formulas usually fall within a range smaller than the spread between two people with identical figures. Anyone hesitating between two calculators should therefore not dwell on it — the decision about the starting value matters less than the later correction.

Where the deviation comes from

Even at the same weight, height, age and sex, actual basal metabolic rate differs between people. Deviations are described on an order of magnitude that can amount to several hundred kilocalories over the day.

The reasons lie partly in body composition — muscle tissue uses more at rest than fat tissue — in the size and activity of organs, and in differences that simple figures cannot capture. A formula that knows only four values cannot represent this.

Everyday activity is the least certain item

Even larger than the spread in basal metabolic rate is the range in movement outside training — walking, standing, stairs, housework, fidgeting while seated. This share differs considerably between people and varies from day to day in the same person.

This is exactly where the formula is at its coarsest: it offers three to five activity levels to choose from, behind which lies a continuous range. Anyone who sees themselves between two levels already holds an uncertainty of several hundred kilocalories.

On top of this: a calorie deficit changes this share. Many people move less unconsciously under energy scarcity — the estimated expenditure then no longer matches the actual one after a few weeks.

Why the number is still useful

An estimate with known uncertainty is considerably better than none. It provides a starting point in the right order of magnitude — and that is the point: not to hit expenditure to the kilocalorie, but not to be off by a multiple.

The error lies not in the formula but in how it is used. Anyone who takes the output for a measurement and doubts their own discipline when results fail to appear draws the wrong conclusion. The number was always an estimate.

From estimate to your own value

The most reliable route is through observation. You start with the estimated value, keep intake reasonably constant for two to three weeks and see what weight and circumferences do. From this response comes a personal value superior to any formula — because it already contains your own physiology and your own daily life.

After that, adjust in small steps rather than large ones. And the corrected value does not hold forever: if weight, training volume or life circumstances change, it shifts with them.

For the observation itself, what applies to weighing applies here too: individual days fluctuate too much to draw anything from them. Only the course over several weeks shows whether the assumed intake matches actual expenditure.

Last reviewed: 14.08.2026

Created by AI