How many calories do you actually burn in a day?

Everyone knows the numbers. Two thousand calories a day for a woman, two and a half thousand for a man. They are printed on the side of cereal boxes and repeated so often they feel like a law of physics. They are not. They are a rough average, and your own daily burn can sit a long way from either figure depending on how big you are, how old you are and how much you move about. The good news is that working out your real number takes about a minute.

Most of your calories are gone before you move

The biggest surprise for most people is how little of their daily energy goes on exercise. The largest slice by far is your basal metabolic rate, or BMR: the calories your body spends just keeping you alive and idling. Your heart beats, your lungs work, your brain hums along, your body holds its temperature, cells repair themselves. For most adults that background running cost is somewhere around 60 to 70 per cent of the whole day's total, and you spend it lying completely still.

You can estimate BMR with a formula that has held up well since it was published in 1990, the Mifflin-St Jeor equation. It uses your weight, height, age and sex. For men it is ten times your weight in kilograms, plus 6.25 times your height in centimetres, minus five times your age, plus five. For women the last number is minus 161 instead of plus five. It looks fiddly written out, but it is just addition, and it is the engine inside most calorie tools, including the TDEE calculator.

Then you add the moving about

BMR is you switched off. To get the figure that actually matters, your total daily energy expenditure or TDEE, you multiply BMR by an activity factor. A desk job with little exercise sits around 1.2. Light activity a few days a week is about 1.375. Training hard most days pushes towards 1.7 and beyond. Somebody who is on their feet all day on a building site burns in a different league to somebody who commutes by car to a chair.

This is why two people of the same weight can need very different amounts of food. A tall, young, active man might genuinely burn well over 3,000 calories a day. A smaller, older, sedentary person might sit comfortably under 1,600. Feeding both of them the same 2,000 calorie template and expecting the same result was never going to work, and it does not.

Why the number drifts over time

Your TDEE is not fixed for life. It falls slowly as you age, partly because muscle, which is metabolically busy, tends to fade if you do nothing to keep it. It changes when you lose or gain weight, because a smaller body costs less to run, which is one of the reasons weight loss slows down the further you go. And it responds to how you live, not just how you train. Fidgeting, standing, walking to the shop, taking the stairs: this everyday, non-exercise movement adds up to more than most gym sessions, and it is where a lot of the day-to-day variation between people actually hides.

What the number is good for

Knowing your TDEE turns vague calorie advice into something you can use. Eat roughly at your TDEE and your weight tends to hold steady. Eat consistently below it and you lose, above it and you gain. That is the whole of the arithmetic, even if appetite, sleep, hormones and the sheer difficulty of measuring food make the day-to-day messier than the sum suggests.

A single figure will not capture all of that, and no formula should be treated as gospel. Treat your TDEE as a well-informed starting point, watch what actually happens over a few weeks, and adjust. If you also want to see where your weight sits against your height while you are at it, the BMI calculator uses the same measurements you have already typed in.

This is general information, not medical or dietary advice. Calorie estimates are approximations; if you have specific health goals or conditions, speak to a GP or a registered dietitian.