🔥 How many kilojoules a day does a person need?
← Health & FitnessPut in a sex, an age, a height, a weight and how much you move, and see what a day of energy looks like for a body like yours — as a band rather than a number, in kilojoules and in calories, with the resting figure underneath it that most of the day is actually spent on.
The day
The same body at every level
| Activity level | kJ a day | Calories a day | Over sitting still |
|---|
An estimate, and not a target, a diet plan or medical advice. The sum is the Mifflin-St Jeor equation, which estimates what a body uses at rest and multiplies it by an activity level; the band either side of the answer is there because the equation is an average of a great many people rather than a measurement of one. It knows nothing about body composition, so a very muscular person and an overweight person of the same height, weight, age and sex get the same figure from it — and the people the equation was built from were in the normal and overweight weight ranges, so it is not necessarily right outside them either. Pregnancy, breastfeeding, competitive training, illness and anybody under 18 all sit outside its arithmetic, and each of them deserves a professional rather than a page.
Why a band and not a number
Anybody who tells you a single figure for how much energy you need is guessing more finely than the science allows. What exists is a set of equations built from measurements of thousands of people, and the best of them lands within about a tenth either way for most of those people. So this page gives the middle of a band and then says how wide the band is: a figure with its error bars attached is more useful than a figure pretending not to have any.
The second reason for the band is what the equation cannot see. Two people of the same height, weight, age and sex get the same answer from it, even if one of them is carrying twenty kilos of muscle and the other twenty kilos of fat, and even though muscle costs more energy to keep than fat does. The figure is a fair starting point for a body in the ordinary range and a poor one for a body outside it — which is why the page says so in the fine print rather than in a footnote nobody reads.
How it's worked out
The equation first estimates what the body uses at complete rest, which is most of the day's total, and then an activity level multiplies it:
at rest (kcal) = 10 × kg + 6.25 × cm − 5 × years + 5 (men) or − 161 (women)
the day = the resting figure × the activity level
kilojoules = calories × 4.184
- The resting figure is the energy of lying still: breathing, a heartbeat, a brain, a body kept at 37 °C. It is usually the largest part of the day, which is the fact most diets ignore.
- The activity levels are the conventional ones: 1.2 for sedentary, 1.375, 1.55, 1.725 and 1.9 for a day of hard exercise on top of physical work. They are broad descriptions rather than measurements, and picking the one that describes an ordinary week matters more than the decimal place.
- The band is 10% either side of the answer, which is roughly how closely an equation like this can be expected to track one person rather than a population.
- Kilojoules are the calories multiplied by 4.184, the same conversion the kilojoules page does in the other direction.
Where the equation comes from
Estimating a body's energy needs from a tape measure and a scale is a century-old problem, and the equation here is the third generation of an answer to it. Harris and Benedict published the first version in 1919, from measurements of a few hundred people; Roza and Shizgal revised its coefficients in 1984; and Mifflin and St Jeor produced the version this page uses in 1990, which was better at predicting for the people of that decade and has stayed the most-used estimate since.
The reason the estimation exists at all is clinical: a hospital patient who cannot be weighed accurately, or a study of a thousand people, needs a figure that can be had from a form rather than from a metabolic chamber. It was never meant to tell one person what to serve themselves at dinner, and it is at its most honest when it is presented the way this page presents it — as the middle of a band.
Where these numbers come from
- The equation, its coefficients and the history of the versions before it are set out in this reference on the equations for resting energy, which also carries the two caveats the page repeats: the equations ignore body composition, and the latest one was built from people in the normal and overweight weight ranges.
- The activity levels are the conventional multipliers used with equations of this kind. The page states them rather than hiding them: 1.2, 1.375, 1.55, 1.725 and 1.9.
- The conversion between calories and kilojoules — 4.184 per calorie — is the same one used on the kilojoules page, where it is set out in more detail.
None of it is medical advice, and no figure here knows anything about your health, your medication or your training. If the number matters to you for a reason — a diagnosis, a pregnancy, a sport, a growing child — it is a conversation with a dietitian or a doctor rather than with a calculator.
Frequently asked questions
Is this the number of calories I should eat?
No. It is an estimate of what a day uses, which is a different sentence from what anybody should eat — the page has no idea what you are trying to do, and no view about it either. It is also the middle of a band roughly 10% wide, so treat it as the centre of a territory rather than a line to land on.
Why is my figure different from a fitness app's?
Three common reasons. Apps often use the older Harris-Benedict equation rather than the Mifflin-St Jeor one this page uses. Many of them add exercise on top of an activity level, which counts it twice. And some are built around a goal rather than a day, so something has already been subtracted before you see the number. This page does one sum: a resting figure multiplied by the activity level you chose.
How do I choose an activity level?
Pick the level that describes an ordinary week, not a good one. Sedentary means a desk and little else; lightly active is exercise on a few days; moderately active is three to five days of proper exercise; very active is hard exercise most days; extra active is hard training on top of a physical job. Between the first and the last is about 60% of the whole figure, so the choice matters more than the decimals suggest.
Does the page count exercise twice?
No, and that is deliberate. The activity level already contains whatever exercise your week holds, so there is no separate exercise box to add on top of it. Typing gym sessions into a formula that has already multiplied for them is the single most common way these estimates end up too high.
The equation does not know my body — does that matter?
It matters most outside the ordinary ranges. Because the equation works from height and weight alone, it cannot tell muscle from fat: a lean, muscular person needs more than it says, and for somebody carrying a lot of fat the arithmetic overstates what the body it is counting costs to run. The page says so plainly, and it is the strongest reason to read the band as a band.
What if I am pregnant, training hard, or under 18?
Then this is not your sum, and the page will not pretend otherwise: under 18 is refused outright, and pregnancy, breastfeeding, hard training and illness all sit outside an equation built from ordinary adults. Each of those deserves a person rather than a page — a dietitian, a doctor or a coach — and the calculator is at its best for an ordinary adult on an ordinary week.
Why does the resting figure take up most of the day?
Because that is what being alive costs. A body at rest spends most of its energy on the heart, the liver, the brain and keeping itself warm, and movement adds a share on top rather than replacing it. The bar on this page makes that visible: on a moderately active day it is usually around two thirds rest and one third movement, which is worth knowing before believing a walk has earned a large lunch.
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