Total daily energy expenditure is the number of calories your body uses in a day. It has three parts.
Basal metabolic rate is what you burn at complete rest, keeping your organs running. For most people this is 60 to 70 per cent of the total.
Activity covers both deliberate exercise and everything else you do, from walking to fidgeting. This varies enormously between people.
The thermic effect of food is the energy used digesting what you eat, roughly 10 per cent, and it is higher for protein than for fat or carbohydrate.
This calculator uses the Mifflin-St Jeor equation, which is the most accurate of the widely used formulas for estimating basal rate in the general population.
Any equation is an estimate. Individual metabolic rate varies by 10 to 15 per cent from what a formula predicts, driven by muscle mass, genetics, thyroid function and past dieting.
So treat the number as a starting point rather than a fact. The useful method is to eat at your estimated maintenance for two to three weeks, track your weight, and adjust. If weight is stable, the estimate was right for you. If it moves, adjust by 200 to 300 calories and repeat.
Choosing your activity level honestly matters more than the equation. Most people overestimate it, which is the single largest source of error. If you train three times a week but sit at a desk otherwise, you are lightly active, not very active.
For fat loss, a deficit of 15 to 20 per cent below maintenance is sustainable for most people. Larger deficits work faster and cost more muscle, which matters more the older you are.
For muscle gain, a surplus of 10 to 15 per cent is enough. Larger surpluses add fat rather than accelerating muscle growth, which is limited by other factors.
Protein deserves separate attention. Around 1.6 to 2.2 g per kg of body weight supports muscle retention in a deficit, and this becomes more important with age. The calculator includes a protein range alongside the calorie target for that reason.
Two things change. Muscle mass falls, which lowers basal rate, and unconscious daily movement tends to decline, which lowers the activity component.
The result is that the same eating pattern that maintained your weight at thirty-five may exceed your needs at fifty. That is not a failure of willpower and it is not your metabolism collapsing; the largest study of human energy expenditure found metabolic rate stable from twenty to sixty when adjusted for body composition.
The lever is body composition. Resistance training preserves the muscle that keeps the number up.
Related reading: Losing weight in menopause · Why weight gathers around your middle · Protein intake calculator · Strength training in midlife
Educational only, not medical advice.