حاسبة معدل الأيض الأساسي (BMR) و(TDEE)

BMR, RMR, and TDEE

Basal metabolic rate is measured under strict conditions: complete rest, no food for twelve hours, a thermally neutral room, immediately after waking. Resting metabolic rate is the same idea measured less strictly, and comes out 5–10% higher. In practice the terms are used interchangeably, and the equations above predict something between the two.

Total daily energy expenditure is everything: BMR plus the energy cost of digestion (about 10% of intake), plus deliberate exercise, plus non-exercise activity — fidgeting, walking to the car, standing. That last component varies enormously between people and is the main reason two people of identical size can have TDEEs several hundred calories apart.

Choosing the right activity multiplier

This is where most estimates go wrong, and the error is nearly always in the same direction. The multipliers describe a whole week, not your best day. Three gym sessions on top of a desk job is "light to moderate", not "active" — a 1.375 to 1.55, not 1.725.

"Very active" at 1.9 means manual labour or twice-daily training, not an enthusiastic hour at the gym. If you have a desk job, your baseline is sedentary and exercise moves you up from there. When in doubt, pick the lower option: it is easier to add calories after two weeks of unwanted weight loss than to explain away a stalled cut.

Using the number to lose or gain weight

A pound of body fat stores roughly 3,500 calories, so a 500-calorie daily deficit predicts about a pound of loss per week. That prediction holds well for the first several weeks and then slows, because a lighter body burns less and the body compensates with reduced spontaneous movement.

For gaining, a surplus of 250–500 calories a day supports muscle growth without excessive fat gain. Beyond that, the extra is mostly stored as fat — muscle can only be built so fast regardless of intake.

Never eat below your BMR for extended periods. It rarely accelerates fat loss and reliably costs lean mass, energy, and adherence.

Why the number is only a starting point

The equations were fitted on populations, and individual metabolic rates scatter around the prediction by 10–15% in each direction. Thyroid function, genetics, past dieting history, sleep, and body composition all shift the real value.

The reliable approach is to treat the estimate as hypothesis one. Eat at the predicted maintenance for two to three weeks, weigh yourself consistently, and take the trend line — not any single day. If weight is stable, the estimate was right. If it moved, adjust by 100–200 calories and repeat. Two weeks of your own data beats any equation.