Estimate the calories a day that hold your weight steady, and the deficit that would take one or two pounds off a week. The equation behind it is the one most widely used in clinical practice.
These are population estimates from an equation, not measurements of you. Real metabolic rate varies between people of identical build and shifts day to day. Do not go below 1,500 calories a day (men) or 1,200 (women) without clinical supervision.
A calorie deficit is the only mechanism by which fat is lost: over time, less energy coming in than going out. Everything else — the diet you pick, the training you do, the medication you may be on — is a way of arriving at that deficit and staying there without being miserable.
Your resting metabolic rate is estimated with the Mifflin-St Jeor equation, the one most widely used in clinical practice. It takes weight in kilograms, height in centimetres, age and sex and returns the calories your body would burn at complete rest. That figure is then multiplied by an activity factor between 1.2 and 1.9 to give total daily energy expenditure — the number that holds your weight steady.
From there the arithmetic is simple. A pound of body fat stores roughly 3,500 calories, so a deficit of about 500 calories a day works out to a pound a week, and 1,000 a day to two.
One to two pounds a week is the range most clinical guidance settles on. Below about 1,500 calories a day for men and 1,200 for women it becomes difficult to get adequate protein and micronutrients, and the deficit starts costing you more lean tissue than it needs to. If the two-pound figure above would take you under that line, this calculator holds it at the floor instead.
Every equation here is a population average. Real metabolic rate varies between people of identical build by a few hundred calories, and it moves within one person from day to day with sleep, stress, illness, training load and when you last ate. That is why a deficit that worked in week one can quietly stop working by week five, and why the honest instruction is to treat this figure as a hypothesis and adjust it against what actually happens.
Every calculator on this page works the same way: it takes your height, your weight, your age and a guess about how much you move, and returns the average answer for people who look like that on paper. It is a reasonable place to start. It is not a reading of you, and it cannot change when you do.
Rysflo measures instead. One sixty-second exhale each morning reads three molecules in your breath, and the day’s protein, carbohydrate, fat and fibre come out in grams — against the goal you set, from what your metabolism is actually doing rather than from a formula on your dimensions.