TDEE explained: why your calorie target keeps moving

Total daily energy expenditure is the number every diet plan is built on, and it’s the number almost nobody’s plan accounts for correctly. It isn’t a fixed property of your body. It drifts — sometimes by several hundred calories — in response to what you eat and how much you move.

The four components

Roughly, your daily burn breaks into resting metabolic rate, the thermic effect of food, deliberate exercise, and everything else you do while awake. That last category — fidgeting, pacing, standing, gesturing — is the one that moves most, and the one you have the least conscious control over.

This is why two people with identical training programmes and identical bodyweights can have maintenance figures that differ by four hundred calories. It isn’t a mystery metabolism. It’s that one of them takes twelve thousand steps without thinking about it and the other takes three thousand.

What happens when you diet

Cut calories and expenditure falls for two reasons. The obvious one: a lighter body costs less to move and maintain. The less obvious one: unconscious activity drops. You sit more. You take the lift. Your training sessions get shorter without you deciding to shorten them.

This is why a calculator’s job is to give you a starting point, not a prescription. Estimate, run it for two to three weeks, then adjust from what the scale actually did.

Estimate yours

The tool below is the same component as our full calculator — same equations, same engine. Set your details and note the maintenance figure, then read the next section on what to do with it.

Reading the result

Start by assuming the number is wrong, because for about half of people it is — by more than ten per cent. The Mifflin-St Jeor equation this tool defaults to was derived from 498 adults and explains roughly 71 per cent of the variation between people; comparative studies put its hit rate for landing within ten per cent of measured resting expenditure at somewhere around a half. ¹ It is the most accurate of the widely used equations, and that is still the accuracy on offer.

There is a third limit, and it is the one people find most surprising: the activity multiplier is not a fixed property of you. Non-exercise activity thermogenesis — everything you burn that is not sleeping, eating or deliberate exercise — accounts for the majority of non-resting energy needs, and it rises when you overfeed and falls when you underfeed. ² You fidget less, stand less, and move less around the edges of the day without deciding to.

So the multiplier that described you at maintenance quietly describes you less well three weeks into a deficit. That is not a flaw in the estimate; it is your body doing what bodies do, and it is another reason the weight trend beats the calculation.

Two further limits worth knowing. The derivation sample ran from 19 to 78 years old, so for anyone much older than that the age term is being extrapolated past its data. And the equation knows nothing about your lean mass — which is why entering a body-fat percentage, where you have a reliable one, moves the estimate onto a better footing.

None of that makes the number useless. It makes it a starting estimate, which is what the next paragraph is for.

Track bodyweight daily and take a weekly average; day-to-day swings are water and gut contents, not fat. If the weekly average is flat over three weeks, the number above is close to your true maintenance. If it’s dropping faster than roughly one percent of bodyweight per week, your deficit is more aggressive than intended.

Adjust in small increments — a hundred to two hundred calories — and give each change a fortnight before judging it.