Two acronyms turn up in every calorie calculator, usually without much explanation of which one you are supposed to eat. They measure different things, and confusing them is the most common way a first calorie target ends up several hundred calories too low.
BMR: the floor
Basal metabolic rate is what your body spends keeping you alive while you do nothing at all — breathing, circulation, maintaining tissue, keeping your brain running. It is measured at rest, and for most people it accounts for the largest single share of daily energy use.
The equation most tools use, including ours, is Mifflin-St Jeor:
men: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5
women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161
It is a population fit, not a measurement. Two people with identical inputs can have genuinely different resting rates, and the equation cannot see the difference. It is a good starting estimate and a poor final answer.
BMR is not a calorie target. Eating at your BMR means eating as though you spent the entire day motionless, which you did not.
TDEE: the number you eat against
Total daily energy expenditure is BMR plus everything else: walking, training, digesting food, fidgeting, standing up. Calculators get there by multiplying BMR by an activity factor.
| Activity level | Multiplier |
|---|---|
| Sedentary, little or no exercise | 1.2 |
| Light exercise 1-3 days a week | 1.375 |
| Moderate exercise 3-5 days a week | 1.55 |
| Hard exercise 6-7 days a week | 1.725 |
| Very hard exercise plus a physical job | 1.9 |
The gap between the ends of that table is enormous — the same body can plausibly be assigned anything from 1.2 to 1.9 depending on one dropdown. That single choice moves a target by hundreds of calories, and it is the input people are least accurate about.
Choosing an activity level honestly
Two rules save most of the error here.
- Count what you do in a normal week, not a good one. If training three times a week is the plan rather than the record, pick the level below.
- A desk job with a gym habit is usually "light" or "moderate", not "very active". The multiplier is describing your whole day, and most of a desk day is sitting.
When in doubt, pick lower. An estimate that is slightly too low produces a slightly larger deficit than planned, which the scale reveals within a fortnight. An estimate that is too high produces no progress at all and looks exactly like a broken plan.
So which do you use?
TDEE, always, as the number your intake is compared against. BMR only ever appears as an input to it — and as a rough floor worth staying above, since aggressive deficits that push intake near or below resting expenditure tend to cost sleep, training quality and adherence long before they cost fat.
Once you have a TDEE estimate, subtract a deficit you can hold and correct it from three weeks of weigh-ins. The formula gets you into the right neighbourhood. Your own data gets you the rest of the way.