What the research shows
Every calorie calculator works the same way: estimate what you burn at rest from height, weight, age and sex, then multiply by an activity factor. The first half is reasonably well tested. The second half is mostly guesswork.
- Mifflin-St Jeor was the most reliable, landing within 10% of measured resting metabolism in more people than any other equation tested.
- The errors for individuals were still large enough to matter. The authors advise clinical judgement about when to trust the estimate.
- Older adults and ethnic minorities were underrepresented, so the equations are less trustworthy for them.
On a 2,400 kcal day, 10% is 240 kcal. That is half of a typical fat-loss deficit, and the activity multiplier adds its own error on top.
Healthy adults with and without obesity, measured at rest in a lab. Lifters were not studied separately. People carrying more muscle tend to burn more than the equations predict, so for trained people treat the estimate as a floor rather than a ceiling.
Your intake has to change as you go
Hall's group modelled how body weight responds to a change in intake. The response is slow, and it tapers: as you lose weight you burn less, so the same intake produces smaller losses over time. A deficit that worked in month one will be a smaller deficit by month three.
How I set it up with clients
This is my coaching method, not a formula from a paper:
- Find maintenance from your own data. Track everything you eat for two weeks without trying to change it, and weigh yourself most mornings. If the weekly average weight holds steady, your average intake is your maintenance.
- Choose a rate, then a deficit. Aim to lose 0.5–1% of bodyweight per week. As a rough opening figure, each 0.5 kg per week needs about 500 kcal a day below maintenance. Expect that to hold for the first weeks and then slow down.
- Check every 2–4 weeks and adjust from the weekly average weight, not from the calculator.
What to do
- Use Mifflin-St Jeor only if you have no data yet. It is the least-wrong equation, nothing more.
- Two weeks of honest tracking beats any formula. Weigh food, count cooking oils and drinks, include weekends.
- Set the deficit from your target rate, not from a round number like 1,500 kcal.
- Expect to adjust. Your intake will need to come down as you get lighter.
Want this set up around your actual life?
I coach in person in St. Louis and online. First consultation is free.
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