TDEE calculator that shows its working
Most calculators give you one number and hide the formula. This one runs Mifflin-St Jeor, Harris-Benedict and Katch-McArdle side by side, so you can see how much they disagree — and why. No signup, no email, nothing leaves your browser.
How the three formulas compare
| Formula | BMR | TDEE | Best for |
|---|
Show the math
What TDEE actually means
BMR (basal metabolic rate) is what your body burns doing nothing at all — keeping your heart beating, your brain running, your temperature steady. It's roughly 60–70% of what you burn in a day.
TDEE (total daily energy expenditure) is BMR multiplied by an activity factor, to account for movement, exercise and digestion. It's the number that matters: eat below it and you lose weight over time, eat above it and you gain.
Why every calculator gives you a different number
This is the question that sends people from one calculator to the next, and the answer is boring: they're using different equations, each derived from a different study population decades apart.
- Mifflin-St Jeor (1990) — the modern default. Validated on a broad population and generally the most accurate for people of average body composition.
- Harris-Benedict (1919, revised 1984) — the original. Tends to read slightly higher, and was derived from a smaller, less representative sample.
- Katch-McArdle — ignores your total weight and works from lean mass. If you know your body fat percentage with reasonable confidence, this is the one to trust, especially if you're very lean or very heavy.
A 100–300 kcal spread between them is normal. None of them is wrong; they're all approximations of something that varies between individuals more than the formulas vary between themselves.
The activity multiplier is where people go wrong
Formula choice matters less than honesty about activity. Jumping from "moderate" to "very active" adds ~11% to your TDEE — usually far more than the difference between equations. Three gym sessions a week and a desk job is light to moderate, not very active. If your weight isn't moving the way the number predicts, this is almost always the reason.
Turning the number into macros
The split above uses defaults that suit most people: protein at 1.8 g per kg of bodyweight (preserves muscle in a deficit and keeps you full), fat at 25% of calories (hormonal health floor), carbohydrate filling the rest (training fuel). Athletes, low-carb eaters and people with medical conditions have good reasons to deviate.
FAQ
Why does every TDEE calculator give a different number?
Different formulas, derived from different study populations. Mifflin-St Jeor, Harris-Benedict and Katch-McArdle can differ by 100–300 kcal for identical inputs. None is "correct" — they're population averages, and real expenditure varies with genetics, spontaneous movement and body composition.
Which formula should I use?
Mifflin-St Jeor unless you know your body fat percentage, in which case Katch-McArdle is better because it works from lean mass. Harris-Benedict is included mainly for comparison — it's the historical one.
How big should my deficit be?
15–20% below TDEE suits most people, roughly 0.5–1% of bodyweight per week. Bigger deficits cost muscle and adherence without proportionally faster fat loss.
Do I need to recalculate as I lose weight?
Yes. TDEE falls as you get lighter. Recalculate every 4–5 kg, or whenever progress stalls for more than two or three weeks.
Is this free? Do you store my data?
Free, and no. The whole calculation runs in your browser — nothing is sent anywhere, and there's no signup or email capture.
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