What you actually burn in a day

The calculator gave you a confident number. It is an average of people resembling you — and there is a way to find yours instead.

Around 7 minutes · Calories

Every calorie calculator asks the same five things — height, weight, age, sex, how active you are — and hands back a confident number. Two people can give identical answers to all five and genuinely need a few hundred calories a day apart from each other.

The number isn't wrong, exactly. It is an average of people like you, presented as a fact about you.

Where the number comes from

Almost every calculator runs one of a handful of equations — the Mifflin-St Jeor equation is the usual one — which were built by measuring a lot of people and fitting a line through the results. The line goes through the middle of the scatter. You are somewhere in the scatter, and nobody has told you where.

The published error on these equations is routinely cited at around 10% either way for resting metabolism alone. At a maintenance level near 2,400 kcal, that is a band several hundred calories wide before anyone has said the word "exercise".

The four things you're actually spending

A day's burn is four things, and they are not equally knowable.

That last row is the one that ruins the tidy arithmetic. It has a name — non-exercise activity thermogenesis, which nobody says out loud — and between two similar people it can differ by hundreds of calories a day. One of them takes the stairs, stands on the phone, walks while thinking and cannot sit still. The other doesn't. Neither would describe themselves differently on a form with four activity levels on it.

It also isn't fixed. Eat less for long enough and it quietly falls: you move less, without deciding to. That is a real effect, and it is part of why the calculator's number stops being right partway through a diet even if it was right at the start.

The one measurement that is actually about you

You cannot measure your metabolism at home. You can do something better: measure its consequences.

If you know roughly what you ate over several weeks, and you know what your weight did over those same weeks, then your real maintenance level falls out of the arithmetic. Weight held steady on 2,300 a day means your maintenance is about 2,300 — whatever any equation says. Lost half a kilo a week on 2,000 means you were running about 550 a day under, so maintenance is nearer 2,550.

This is the only method on this page that is about you specifically rather than about people resembling you. It needs nothing but a diary and a scale, and it gets better the longer it runs.

Doing it honestly

• Use trend weight, not a single weigh-in at each end. Comparing one Monday to another Monday measures two days' water balance as much as four weeks of fat. Here's why that matters.
• Log everything, including the bad days. A diary with the weekends missing produces a maintenance figure that is too low, and then a target built from it that you cannot hold.
• Expect it to move. As you get lighter you need less. A figure worked out in January is not a figure for June.
• Don't re-derive it every week. Four weeks of data, updated as more arrives. Chasing it weekly means chasing noise.

What a watch or a tracker adds

A wearable measures what a formula guesses — sort of. It has real data for heart rate and movement, which makes its estimate of activity genuinely better than a four-option dropdown. It is still estimating resting metabolism, and it still cannot see the fidgeting.

Treat a tracker's daily burn as a good estimate rather than a measurement, and it earns its place. Treat it as gospel and you will eat back calories that were never spent — device estimates of calories burned during exercise are the least accurate figure they produce.

So what number should you use?

Start with the calculator, because you have to start somewhere. Then stop trusting it as soon as you have four weeks of your own data, because at that point you have something better: not an estimate of what someone your size burns, but a measurement of what you burned.