How this is worked out
The resting figure uses the Mifflin-St Jeor equation, published in 1990 after measuring resting energy expenditure in nearly 500 healthy adults. It is deliberately simple: ten times your weight in kilograms, plus 6.25 times your height in centimetres, minus five times your age, then plus 5 for the male variant or minus 161 for the female one. That is what your body spends staying alive at rest, and for most people it is somewhere between 60 and 70 per cent of the day.
The daily total multiplies that by an activity factor from 1.2 for a desk-bound week up to 1.9 for physical work or two training sessions most days. These multipliers are conventional values carried through decades of nutrition practice rather than a measurement of you, and they lump together deliberate exercise and all the incidental moving about that fills a day — which differs between two people with the same job by hundreds of kilocalories.
Both numbers are shown separately on purpose. Blending them into one figure hides where the uncertainty is: the equation is the solid half, the multiplier is the guess, and the guess is the one that moves your total by 500 kilocalories if you pick the wrong label.
Why the equation cannot be precise for one person
The equation was fitted to a group, so it describes the average of people who share your weight, height, age and sex. You are not that average. Two people matching on all four inputs can differ in resting expenditure by a couple of hundred kilocalories because of how much of their weight is muscle, how their thyroid is behaving and simple individual variation.
That is why the honest way to use the number is as a hypothesis. Eat around it for two weeks, weigh yourself under the same conditions at both ends, and let the result correct the estimate. After a fortnight your own data beats any equation, because it has already accounted for everything the equation could not see.
Choosing an activity level without flattering yourself
Most people overshoot here, because a gym session feels like it should count for more than it does. Half an hour of moderate exercise is in the region of 200 to 300 kilocalories, which is one step up the ladder at most, and the ladder rungs are worth several hundred each.
The bigger and less obvious contributor is everything you do that is not exercise: walking, standing, carrying, fidgeting, taking stairs. This varies enormously between people and is a large part of why two people with the same job and the same workouts have different daily totals.
- Desk-bound, 1.2 — sitting most of the day and no planned exercise. Choose this if you genuinely commute by car and take the lift.
- Lightly active, 1.375 — light exercise one to three days a week, or a job with a lot of walking.
- Moderately active, 1.55 — moderate exercise three to five days a week.
- Very active, 1.725 — hard exercise six or seven days a week.
- Physical work or two sessions a day, 1.9 — manual labour or training twice most days.
What to do if you want the number to change your weight
The rows below the total show a 15 per cent reduction and a 10 per cent increase. These are common practice, not prescriptions: a modest adjustment that you can hold for months beats an aggressive one you abandon in three weeks, and it also loses less muscle on the way down.
Expect the scale to be noisy. Body weight moves by several hundred grams a day from water, salt and food still in transit, so a single reading tells you almost nothing about fat. Weigh under the same conditions, look at the average across a week, and compare weeks rather than days.
If progress stalls for a month, adjust by 100 to 200 kilocalories rather than by a third. Large cuts tend to be paid for by moving less without noticing, which quietly removes the deficit you just created.
Where a calorie target is the wrong tool
Counting is a measuring instrument, and like any instrument it can be used badly. If tracking is making food anxious rather than clearer, the tracking is the problem to fix — the estimate is not so important that it is worth that.
And there are cases where this page is not the right source at all: eating that is being managed for a health condition, during pregnancy, for a child, or alongside medication that affects appetite or metabolism. Those belong with the professional who knows the case, because the answer depends on details no equation collects.
Common questions
Why do other calculators give me a different number?
Because they use a different equation or different multipliers. The older Harris-Benedict equation tends to read higher, and some sites use body-fat-based formulas that need a measurement most people do not have. A spread of a few hundred kilocalories between calculators is normal and is a fair reflection of the underlying uncertainty.
Should I eat back the calories my watch says I burned?
Cautiously at most. Wearable estimates for calorie burn are the least accurate thing they report, and they are usually generous. If you are already using an activity multiplier, adding the watch figure counts the same exercise twice.
Does the equation work if I am very muscular or carrying a lot of weight?
It is less accurate at the extremes, because it treats all weight the same while muscle burns more at rest than fat does. Muscular people tend to be underestimated and it reads high at very high body weights. The two-week test corrects for both.
Do I need to hit the number every day?
No. The body works on averages across days, not on a daily ledger. Being a few hundred over on Saturday and under on Sunday is the same week as being exactly on target twice.
Is the female variant of the equation just a smaller number?
Effectively yes: the two variants differ by a fixed constant of 166 kilocalories, which stands in for average differences in body composition. It is a population adjustment inside the equation, not a statement about any individual.
HealthGuideTip provides general health information for educational purposes only and is not medical advice. Always consult a qualified healthcare professional about your own health, and before changing any treatment, medication, or lifestyle.