This estimates how many calories your body uses in a day: the amount it burns at complete rest, and the larger figure once your week is taken into account. It is a starting point for planning, not a measurement — the only way to know your real number is to watch what happens over several weeks.
Key terms
- BMR — basal metabolic rate. What you would burn lying still all day, keeping your heart, brain and organs running. For most people it is the largest single part of the total.
- Activity multiplier — a factor applied to BMR to account for movement, from a desk job to physical work.
- Calories a day — BMR × the multiplier. Roughly what you would eat to stay the same weight.
How it works
The resting figure comes from the Mifflin-St Jeor equation, published in 1990 after measuring 498 healthy adults by indirect calorimetry. The same study found the older Harris-Benedict equations overestimated resting expenditure by about 5%, which is why this one is used here.
Resting energy expenditure
REE = 9.99 × weight + 6.25 × height − 4.92 × age + 166 × sex − 161
Weight in kilograms, height in centimetres, age in years, and sex as 1 for men and 0 for women. Imperial measurements are converted first, using the exact international definitions.
The multipliers are a convention, not part of the equation
The equation above is measured science. The activity factors — 1.2 for a desk job through to 1.9 for very physical work — are a widely used convention, and they are the least precise part of the answer. Two people who both describe their week as “moderate” can differ by several hundred calories a day.
That is why every level appears in the table rather than only the one you picked. Read it as a range. If your weight is steady, the calories you currently eat are a better estimate of your maintenance level than any equation, and you can plan changes from there.
What the equation cannot see
It uses weight, height, age and sex, so it cannot tell muscle from fat. A muscular person of a given weight burns more at rest than the equation predicts, and someone with little muscle burns less. It also does not account for pregnancy, breastfeeding, thyroid conditions, or medication that changes metabolism.
How to use this calculator
- Choose metric or imperial, then enter your age, height and weight.
- Pick the activity level that matches a typical week, not your best one.
- Read the daily figure, then look at the table to see how much the answer moves with the level above and below.
- Treat it as a hypothesis. Eat around that figure for two or three weeks and adjust against what the scale actually does.
Frequently asked questions
Why does my answer differ from another calculator?
Most differences come from the equation used. Harris-Benedict, Mifflin-St Jeor and Katch-McArdle give different numbers for the same person, and Katch-McArdle needs a body fat measurement the others do not ask for. The one here is Mifflin-St Jeor, which the 1990 study found more accurate than Harris-Benedict.
Should I eat my BMR?
No. BMR is what you would burn asleep and still — it does not include getting up, working or exercising. Eating at BMR is a substantial deficit for almost everyone. The daily figure is the one that reflects a normal week.
How many calories should I cut to lose weight?
That is a question for a doctor or dietitian rather than a calculator, because a sensible answer depends on your health, your medication and how much you have to lose. What this page can tell you is the maintenance figure to plan from.
Does exercise get counted twice?
Only if you let it. The multiplier already includes your usual training, so adding calories back for the same workouts double-counts them. Pick the level that describes the whole week and leave it there.
Why does the number fall as I get older?
The equation subtracts about 4.92 calories for each year of age, so a decade is roughly 49 calories a day at the same height and weight. Real ageing also tends to bring a loss of muscle, which the equation cannot see, so the fall is often larger in practice than on paper.
