A calorie (strictly a kilocalorie, "kcal") is a unit of energy. Food has it, and your body spends it: powering your heart and brain while you sit still, digesting, walking to the shop, climbing stairs. The number printed on a packet says how much energy it holds. The number your body spends each day is your maintenance level, the amount where you neither gain nor lose weight.
Public guidance such as the NHS uses averages of about 2,500 kcal a day for men and 2,000 for women. They are starting points only: the NHS itself says your own figure depends on age, weight, height and how much you move. The calculator below gives a more personal estimate.
The calculator uses the Mifflin-St Jeor equation, published in 1990 and still widely used in dietetics. It first estimates your resting energy (the calories you burn lying still all day):
Then it multiplies by an activity factor: 1.2 for mostly sitting, 1.375 for light activity, 1.55 for moderate, 1.725 for high. Those factors are rules of thumb, not measurements. Most people overestimate how active they are, so if you are between two choices, pick the lower one.
The formula only has two sex options because it was built from measured averages. It can't see your muscle, your sleep, your medication or your genes, which is why two people with the same numbers can differ by a couple of hundred calories.
A calculator gives a starting estimate. Your own week of eating, energy and hunger is the real test.
Eating around your estimate keeps weight steady for most people. If you want to change your weight, that is a conversation with a doctor or dietitian, not a rule from a website, and it should never mean going under the minimums above. Plenty of good reasons to track calories have nothing to do with weight: learning what a portion is, noticing how much is in drinks and sauces, or simply understanding labels.
Two habits help more than precision. First, look at the pattern over a week, not a single day. Second, notice how you feel: steady energy, decent sleep and regular hunger cues beat any decimal on a screen.
This guide is general information, not medical advice. Please talk to a doctor or registered dietitian before changing how you eat if you have a medical condition, are pregnant or breastfeeding, take medication (especially for diabetes or blood pressure), or have or have had an eating disorder. Calorie counting is not a good fit for everyone, and that is fine.