Free tool
TDEE calculator
Work out your maintenance calories with the Mifflin–St Jeor equation, then a daily target for losing fat, holding steady or building muscle. Same code as the Formcast Fitness app. No sign-up.
| Goal | Adjustment | Daily kcal |
|---|---|---|
| Lose fat | −500 | — |
| Maintain | 0 | — |
| Build muscle | +300 | — |
The same numbers, one set at a time
Formcast Fitness carries this calculation into the app: it tracks what you actually eat against the target, and once you have a few check-ins it suggests an adjustment when your weight trend and your goal disagree.
Download on the App StoreiPhone only · free trial · subscription after the trial
How this calculator works
This page runs the same two functions the Formcast Fitness app runs, ported line for line from
src/services/calorieGoal.ts and src/services/macroGoal.ts. If you compute a number here
and then install the app and enter the same details, you should see the same number.
The formula: Mifflin–St Jeor
Your basal metabolic rate — roughly the energy you would burn lying still all day — comes from the Mifflin–St Jeor equation:
- Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
Total daily energy expenditure is that BMR multiplied by an activity factor. Formcast asks how many days a week you train rather than asking you to pick from vague labels like “lightly active”, and maps the answer to three factors:
- 1–2 training days → ×1.375
- 3–5 training days → ×1.55
- 6–7 training days → ×1.725
The TDEE is rounded to a whole calorie before the goal adjustment is applied, then the goal delta (−500 for fat loss, 0 for maintenance, +300 for muscle gain) is added and the result is floored at 1,200 kcal so a small person on a deep cut is never handed a dangerous number.
The macros
Protein is set per kilogram of body weight and depends on the goal — 2.2 g/kg when cutting (protein is protective of lean mass in a deficit), 2.0 g/kg when building, 1.8 g/kg at maintenance. Fat is a share of total calories (28% cutting, 27% maintaining, 25% building) at 9 kcal/g. Carbohydrate is whatever calories are left, at 4 kcal/g. Fiber is 14 g per 1,000 kcal, which is the Institute of Medicine recommendation.
What this assumes, and where it is wrong
- It is a population average, not you. Mifflin–St Jeor was fitted to a group. Individual resting metabolic rates scatter roughly ±10% around the prediction, so a 2,600 kcal estimate could honestly be 2,350 or 2,850 for a given person.
- It ignores body composition. Two people the same height, weight, age and sex get the same answer whether they are 10% or 30% body fat, even though muscle is more metabolically active. If you are very lean or very heavy, expect the estimate to be further off.
- The activity factor is coarse. Three buckets cannot separate a desk job from a job on a building site. Non-exercise activity varies more between people than training does.
- Your TDEE is not a constant. It drifts down as you lose weight, and it moves with sleep, illness, stress and how much you fidget.
The honest way to use this: take the number as a starting point, eat at it for two weeks, weigh yourself daily and compare weekly averages. If the average did not move and you wanted it to, adjust by 10–15%. Your own two-week trend beats any equation.
This is not medical advice. These figures are estimates produced by a formula, not a measurement, and nothing here diagnoses, treats or supervises anything. If you are pregnant, have a medical condition, or are working with a clinician or dietitian, follow their guidance rather than this page.
More free calculators
All four use the same code the Formcast Fitness app uses, so the numbers match what you would see in the app.
- TDEE & maintenance calorie calculator — Mifflin–St Jeor.
- Barbell plate calculator — what to load per side.
- One-rep max calculator — Epley.
- Warm-up set calculator — the ramp to your working set.