How much energy do you really burn? Why the calculator is only a first guess

Every food app starts the same way: your age, height, weight and how active you are, and out comes a number — the energy you supposedly burn in a day. It feels precise to the last calorie. It is not. For many people that number is off by a couple of hundred calories a day, and for some by much more.

Where the number comes from

Most calculators use an equation such as Mifflin–St Jeor to estimate the energy your body uses at rest, then multiply it by an activity factor. The resting equation is decent on average: in a review of the common formulas it landed within 10% of a measured value for most people of a healthy weight — and missed by more for the rest. The activity factor is cruder still: “lightly active” and “moderately active” cover very different lives. Two people with the same age, height and weight can burn noticeably different amounts.

Your own data knows better

There is a more personal way to find the answer, and it only needs two things you may already have: what you eat and how your weight moves. Over a few weeks, energy in and energy out leave a trace. If you ate around 2,200 calories a day and your weight held steady, you burn about 2,200. If you lost a little over the same weeks, you burn somewhat more — and the size of that trend tells you roughly how much.

The formula guesses what someone like you burns. Your diary and your scale show what you burn.

Why one weigh-in tells you almost nothing

Body weight swings from day to day with water, salt, carbohydrate stored in muscles and what is still in your gut — often by a kilogram or more. A single morning on the scale is mostly noise. What carries the signal is the trend: several weigh-ins spread over at least a couple of weeks. The same goes for the diary. Days logged only halfway would make you look like someone who eats far less than they do, so only complete days should count.

The 7,700 rule — useful, with care

To turn a weight trend into energy, a common shortcut says one kilogram of body weight is worth about 7,700 kcal (3,500 kcal per pound). It is a reasonable average over a few weeks, but the body adapts: as weight changes, so does the energy it uses. That is one more reason to treat the result as an estimate that keeps updating, rather than a figure set once and forgotten.

Moving the target gently

Once you have a better estimate, it is tempting to jump your daily target straight to it. Small steps work better: they leave room for the estimate to settle and are easier to live with. And when there is no recent weigh-in, the honest answer is not a guess — it is to step on the scale.

How foodoo helps

foodoo works out the energy you really burn from the last four weeks: the average of the days you logged in full, set against the trend of your weigh-ins — at least three, spread over ten days or more, the last within two weeks. Without them it does not guess; it asks you to weigh in. If you want, it moves your daily target towards the result, at most 150 kcal every three days and never under 1,200, keeping your protein where it was. Targets you typed by hand are never touched.

Weigh-ins from smart scales arrive through Apple Health by themselves, or you can type your weight in the add bar. And if you read labels in kilojoules, the whole app can follow.