Calorie deficit

What it's for

To lose weight you need a calorie deficit: eating less than you burn. About 7700 kcal equals 1 kg of fat. Here you get target calories and time to the goal.

Safety

A healthy deficit is 300-500 kcal per day (max ~1 kg per week). Too aggressive deficits backfire. Not medical advice: for serious diets see a nutritionist.

Where the number 7700 comes from

It's the classic estimate: a kilo of fat tissue holds about 7700 kilocalories, so a deficit of 500 a day should in theory lose half a kilo a week. It's the right sum to start from, but it should be taken for what it is: a linear approximation of a system that isn't linear, and over long stretches it always errs in the same direction, promising more than actually happens.

Why weight loss slows by itself

And it's why nearly every diet «works for a month». A lighter body burns less: every kilo lost lowers the requirement by a few dozen calories a day, and the starting deficit thins out on its own. A second effect adds to it, the drop in spontaneous movement — you walk less, gesture less, sit more without noticing — which is worth more calories still.

The practical consequence isn't «it doesn't work», it's that maintenance calories have to be recalculated every 4-5 kilos. Anyone carrying on with the same starting number ends up halfway through with a deficit that no longer exists, and concludes their metabolism is broken when all they have is an old calculation.

The first two kilos aren't fat

In the first week weight drops fast, and nearly all of it is water: cutting carbohydrates empties the glycogen stores, and every gram of glycogen holds about three grams of water with it. That's two or three kilos that go immediately and come back identically as soon as you eat normally again, and it's why crash diets look miraculous and the rebounds look like disasters.

For the same reason the scales should be read as a weekly average, not day by day: they go up and down by a kilo or two for salt, the menstrual cycle, digestion and yesterday's training, and reading those swings as results is the quickest way to get discouraged for no reason.

How much is too much

A sensible deficit sits between 300 and 500 calories a day, which means half a kilo a week. Going below 1200 total calories for a woman and 1500 for a man, without medical supervision, speeds up nothing good: muscle is lost along with the fat, and muscle is precisely what keeps the burn high. For serious goals, or where there are health conditions, the sums should be done with a professional: this page is a tool for orientation, not a diet.

Nearby tools

For the requirement to start from there is Basal metabolic rate, and for how to divide the calories Macronutrients. For weight against height BMI calculator and Waist-to-height ratio, and for the calories of what you eat Food calories.