Calorie calculator
A kilogram of body fat holds roughly 7,700 kcal, so losing half a kilogram a week means eating about 550 kcal a day less than you burn. That arithmetic is the whole basis of every calorie target, and it is an approximation: the body adapts as weight falls.
An estimate from a population equation. Individual metabolisms vary by roughly 10% either way, and the figure ignores illness, medication and pregnancy. Anyone eating below about 1,500 kcal a day should be doing it with medical supervision.
The panel estimates resting metabolic rate with the Mifflin–St Jeor equation, multiplies it by an activity factor between 1.2 and 1.9, then shifts the result by whatever your chosen rate of change costs. A kilogram of body fat is treated as 7,700 kcal, so half a kilogram a week works out at 550 kcal a day. On the default figures that gives a target of 1,480 kcal against a maintenance level of 2,030.
How to use this calculator
Activity level is the largest error term in the calculation and the only field nobody can measure. The five bands run from 1.2 to 1.9, so one band too high adds several hundred calories to the target before any of the measurements matter. People routinely pick "moderate" on the strength of three gym sessions while sitting still for the other hundred and sixty hours of the week. If the scale has not moved in a fortnight, drop a band before you cut the food further.
The deficit arithmetic assumes two things that hold for a few weeks and then stop. It fixes a kilogram of fat at 7,700 kcal, and it assumes the gap between intake and expenditure stays the same size indefinitely. A lighter body costs less to run, so a target set 550 kcal below maintenance in January is much closer to maintenance by spring. Kevin Hall and colleagues, modelling energy imbalance for the NIH, put the half-time of the body's adjustment to a change in intake at roughly one year. Held flat for twelve months, 550 kcal a day predicts a loss of twenty-six kilograms; the real figure is a fraction of that. Over four to six weeks the straight line is a reasonable approximation, which is the horizon this tool is useful over.
There is also a floor, and the default settings sit on it. A 35-year-old woman of 170 cm and 75 kg has a resting rate of 1,477 kcal, so a half-kilogram-a-week deficit lands the target at 1,480: essentially her resting requirement, and just under the 1,500 kcal the disclaimer names. When the target comes out at or below resting rate, the correct response is a slower rate of loss rather than a smaller plate. Choosing 0.25 kg a week instead moves it to 1,755.
What people use it for
- Turning a rate of loss in kilograms per week into a daily calorie figure
- Recalculating the target after a few kilograms have already gone
- Seeing what a slower, more sustainable rate of loss costs per day
- Checking an intake an app suggested against the underlying equation
- Working out the surplus needed to gain weight rather than lose it
- Spotting when a chosen rate would push the target below resting rate
Questions
Mifflin–St Jeor, published in 1990 from indirect calorimetry on 498 adults. A 2005 systematic review for the American Dietetic Association compared it against Harris–Benedict, Owen and the WHO/FAO/UNU equations and found it landed within 10% of measured resting rate more often than any of them, with the narrowest error range.
The equation carries a fixed offset, plus 5 for men and minus 161 for women, standing in for the average difference in body composition at the same height and weight. It is a constant, not a variable, so it cannot describe an individual whose composition is unusual for their sex.
For a while, and then it costs muscle and adherence. Half to one kilogram a week is the usual recommendation. Intakes below about 1,500 kcal belong under medical supervision rather than under a calculator.
Not if training is already in the activity band, or it gets counted twice. Trackers and cardio machines also tend to report more than the session cost.
Because the same intake becomes a smaller deficit as the body gets lighter, and because expenditure falls a little further than size alone predicts. Recalculating every few kilograms is what keeps the target honest.
Weeks, not months. The model behind it is linear and the body is not, so the projection drifts steadily optimistic the longer you extend it.
Lower the rate of loss. At 0.25 kg a week the default target rises from 1,480 to 1,755, which is a far more workable number for the same direction of travel.
Less well. It reads only weight, height, age and sex, so it cannot tell a heavy person carrying muscle from a heavy person carrying fat, and lean mass is what actually drives resting expenditure.
It subtracts rate × 7,700 ÷ 7 calories a day. Half a kilogram a week is 550, a quarter is 275. The imperial tab converts pounds to kilograms first, so a pound a week is 499 a day and not 550.
Because the requested rate needs a deficit larger than your whole maintenance figure, and the panel floors the answer at zero instead of printing a negative number. On the defaults, asking for 2 kg a week does it: that is a 2,200 calorie deficit against 2,030 of maintenance. Nothing is wrong with the calculator; the rate is impossible.