Where the number comes from
Your basal metabolic rate is estimated from height, weight, age and sex using the Mifflin-St Jeor equation, which has been the best-performing of the common formulas in validation studies. That figure is then multiplied by an activity factor to give total daily expenditure.
Both steps are estimates. Individual metabolic rates vary by roughly 10–15% from what any equation predicts, so treat the result as a starting point to adjust from rather than a target to hit.
Why the activity multiplier is where people go wrong
Most people overestimate it. "Moderately active" means genuine exercise most days, not a desk job with a walk to the station. Choosing one level too high adds several hundred calories to the estimate, which is enough to stall the result you were aiming for.
If in doubt, pick the lower option and adjust after two or three weeks based on what actually happens to your weight. Your own trend is better data than any formula.
A deficit that is too large works against you
A deficit of around 500 calories a day is roughly half a kilo a week and is sustainable. Much more than that tends to cost muscle alongside fat, and is difficult to keep up long enough to matter.
Very low intakes are not something to arrange from a web page. If you are considering one, or you have a condition that affects metabolism or appetite, that is a conversation for a doctor or dietitian.
Frequently asked questions
How accurate is the estimate?
Within about 10–15% for most people. Metabolic rate varies individually more than any equation can capture, so use it as a starting point and adjust from your own results.
Which activity level should I pick?
Most people choose one too high. "Moderately active" means real exercise most days. When unsure, take the lower option — it is easier to add calories than to explain a stall.
How big should a deficit be?
Around 500 calories a day, which is roughly half a kilo a week. Larger deficits cost muscle and are hard to sustain long enough to work.
Does this account for medical conditions?
No. Thyroid conditions, medications and several other factors change the picture substantially, and no formula accounts for them.
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