Calorie and TDEE calculator

How to use it

1. Pick metric or imperial, then enter age, height, current weight and goal weight.
2. Choose the activity level that matches a normal week, not your best one.
3. Either set the weekly change you want and read the intake it needs, or set an intake and read what it does.
4. Follow the curve: it shows the week you reach your goal, or the weight the plan levels off at if that intake never gets there.

Why the line bends, and why that matters

Almost every calorie calculator projects weight loss as a straight line: a 500 kcal daily deficit, half a kilogram a week, repeated indefinitely. That is arithmetically wrong, and it is the reason so many people conclude their diet "stopped working" around month three. The resting metabolic rate depends on body mass — the Mifflin–St Jeor equation adds ten calories for every kilogram — so as you get lighter you burn less, and a fixed intake that started as a 500 kcal deficit is a 350 kcal deficit twelve kilograms later.

This projection recomputes the burn at every week’s weight, so the curve flattens the way a real one does and approaches the weight at which your chosen intake simply is maintenance. If that settling point is above your goal, the page says so plainly and names the weight, because "you will plateau at 78 kg on this intake" is more useful than a date that will never arrive. Reaching the goal from there means eating less again or moving more — not trying harder at the same plan.

Where the error in these numbers actually lives

Mifflin–St Jeor is the best-validated of the simple predictive equations and lands within about 10% of measured resting expenditure for most adults, which is respectable for a formula needing only height, weight, age and sex. It is a population regression, though: it knows nothing about your body composition, thyroid function, medication or genetics, and two people matching on all four inputs can genuinely differ by a few hundred calories a day.

The much cruder step is the activity multiplier. Choosing "moderate" over "light" moves the answer by roughly 200 kcal on typical inputs, and honest self-assessment here is rare — most people select the level matching their best week rather than their average one. If your results consistently undershoot the projection, the multiplier is the first thing to lower, before doubting the equation.

What to expect in the first fortnight

The scale will move faster than this model predicts in week one, then appear to stall. Neither is fat. Cutting carbohydrates depletes glycogen, and each gram of stored glycogen holds about three grams of water, so a few kilograms can leave in days and return just as quickly after one large meal. Gut contents, salt and — for women — the menstrual cycle add a swing of one to two kilograms that has nothing to do with energy balance. Weigh daily if it does not bother you and read the weekly average; weigh weekly if it does.

One number in the model deserves its own caveat: the 7,700 kcal-per-kilogram conversion is a textbook approximation derived from the energy density of adipose tissue, and it systematically overstates long-run loss because real weight change includes lean tissue and water, and because activity tends to fall as intake does. Recalculating the burn each week corrects part of that, not all of it. Treat the timeline as optimistic by a margin.

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