Why calorie deficit calculators disagree with each other
Enter identical details into five calorie deficit calculators and you will get five different targets, often spread across 300–400 kcal. None of them is broken. They disagree because each makes three choices, and almost none of them tells you which choices it made.
Knowing the three turns a confusing spread into a readable one.
1. Which BMR equation
Resting burn is estimated from a regression equation, and there are several in circulation.
| Equation | Published | Notes |
|---|---|---|
| Mifflin–St Jeor | 1990 | Most accurate for the general population; the modern default |
| Harris–Benedict | 1919, revised 1984 | Older; tends to read high |
| Katch–McArdle | — | Uses lean mass, so needs a body-fat figure |
Mifflin–St Jeor and Harris–Benedict can differ by 100–150 kcal for the same person. If a calculator does not name its equation, you cannot interpret its output — and the equation itself carries about ±10% even when correctly chosen.
2. Which activity multiplier
This is the largest source of disagreement, and it is mostly your choice rather than the calculator’s. The standard factors run 1.2 to 1.9, and adjacent steps are far apart:
- Sedentary (1.2) on a 1,700 kcal BMR → 2,040 kcal
- Lightly active (1.375) → 2,340 kcal
- Moderately active (1.55) → 2,635 kcal
Nearly 600 kcal separates the first and third, and most people genuinely cannot say which describes them. Calculators also label these bands differently — one site’s “moderate” is another’s “light” — so the same honest self-assessment produces different answers on different sites.
3. Which constant for a unit of fat
To turn a deficit into a date, a calculator divides by the energy in a kilogram or a pound of body fat. Most metric ones use 7,700 kcal per kg; some use 7,000, and US ones work in 3,500 kcal per pound. The spread moves predicted timelines by about 10% — and note that 7,700 ÷ 2.205 is 3,492, not 3,500, so the two conventions do not even agree with each other.
That constant is itself an approximation — real weight loss is not pure fat, and the ratio shifts as you get leaner.
So which number should you use?
None of them, as a final answer. A calculator’s job is to give you a starting hypothesis; your own data gives you the real figure.
- Take any reasonable estimate — ours is the maintenance calculator.
- Eat at that level for two to three weeks, tracking honestly.
- Compare weekly weight averages, not single mornings.
- Adjust: if you lost 0.5 kg (1 lb) over two weeks, you were running roughly 275 kcal/day under, so true maintenance is your intake plus 275.
After three weeks you have a measured number specific to you, and every calculator’s opinion becomes irrelevant. How to calculate a deficit walks through the arithmetic itself, if you would rather do it by hand than trust a form.
What makes a calculator worth using
- It names its equation and its activity bands.
- It explains the activity factor rather than offering bare adjectives.
- It states a margin, or at least admits the output is an estimate.
- It does not demand an account to show you arithmetic.
- It shows the working, so you can check it.
A calculator that returns a single confident number with no error bars is being less honest than one that returns a range, even though the first feels more useful.
Where the app sits
Caldown uses Mifflin–St Jeor and 7,700 kcal/kg — the same figures as our calculators, deliberately — but only as a fallback. Where a Watch supplies resting and active energy to Apple Health, it uses the measured values instead and skips the activity multiplier entirely, which removes the biggest of the three disagreements above.
General information, not medical advice. Estimated figures are not a substitute for clinical assessment.