How Many Calories Do You Need Each Day?
Any calculator that gives you a daily calorie number is doing two things: estimating what your body burns at rest, then multiplying by a guess at how active you are. The Calorie Calculator does both, and it is worth knowing how approximate each step is.
Where the number comes from
The first part is your basal metabolic rate - what you would burn lying still all day. It is estimated from your height, weight, age and sex using an equation fitted to measurements of real people, and the BMR Calculator shows it on its own.
The second part is an activity multiplier, typically between 1.2 for sedentary and 1.9 for very active. Multiply the two and you get total daily energy expenditure, which the TDEE Calculator breaks out separately.
How wide the error bar is
This is the part worth taking seriously. The BMR equations are accurate to roughly ten per cent for most people, which on a 1,600 calorie BMR is 160 calories either way. The activity multiplier is a much cruder guess - the gap between "lightly active" and "moderately active" can be 300 calories, and there is no objective test for which one you are.
So a calculated figure of 2,400 calories honestly means somewhere around 2,100 to 2,700. That is useful as a starting point and useless as a target to hit precisely, which is why chasing a calculated number to the calorie is a frustrating way to spend a month.
The equations are also fitted to population averages, so they are least accurate for the people furthest from average - the very muscular, the very light, the very tall, and anyone with a thyroid condition or taking medication that affects metabolism.
The figure you measure beats the figure you calculate
The reliable approach is to treat the calculation as a hypothesis and test it. Eat at roughly that level for two or three weeks, track your weight, and see what happens. If the weight is stable, the estimate was close. If it moves, your real number is different and now you know by roughly how much.
Two weeks matters because daily weight swings of a kilogram or more are normal - water, salt, and what is still in transit. A single morning's reading tells you nothing; a two-week trend tells you a lot.
Common questions
Why do two calculators give different answers?
They use different equations - Mifflin-St Jeor, Harris-Benedict, Katch-McArdle - fitted to different populations. A spread of a hundred calories or so between them is normal and none is the true one.
Does muscle change the number?
Yes, muscle burns more at rest than fat does, which is why the equations that include body fat percentage are more accurate for very lean or very muscular people. The Body Fat Percentage Calculator estimates the input.
Do I need to count calories to use this?
No. Plenty of people use the figure simply as context for whether a day's eating was unusually high or low, without tracking anything.
Should I eat below my BMR?
That is a question for a doctor or dietitian rather than a calculator, and the answer depends on far more than arithmetic.
Want a starting figure for your own numbers?