A calorie deficit is one subtraction: what you burn minus what you eat. Burn about 2,724 and eat 2,224, and the 500-calorie gap works out to roughly a pound a week. The craft is in getting the first number honest. Here's the full arithmetic, two worked examples, and where the famous 3,500-calorie rule breaks.
BMR (basal metabolic rate) is what your body burns keeping you alive at complete rest: heart, brain, kidneys, temperature. The standard estimate is the Mifflin-St Jeor equation, which the Academy of Nutrition and Dietetics considers the most accurate simple formula for most people:
For a 180-lb (81.65 kg), 70-inch (177.8 cm), 35-year-old man: 816 + 1,111 − 175 + 5 = 1,758 calories. That's his rent. He'd burn it in a coma.
Nobody lives in a coma, so BMR gets multiplied by an activity factor to produce TDEE, total daily energy expenditure:
| Activity | Multiplier | TDEE for our 1,758 BMR |
|---|---|---|
| Sedentary (desk, no exercise) | 1.20 | 2,109 |
| Light (1–3 workouts/wk) | 1.375 | 2,417 |
| Moderate (3–5 workouts/wk) | 1.55 | 2,724 |
| Very active (6–7 workouts/wk) | 1.725 | 3,032 |
| Extra active (labor + training) | 1.90 | 3,340 |
The multiplier matters more than the formula. A one-notch error (claiming "moderate" for a "light" life) inflates TDEE by 300 calories and quietly erases the whole deficit. When in doubt, round down.
A pound of body fat stores roughly 3,500 calories, which produces the standard menu: 250/day for ~0.5 lb a week, 500 for ~1 lb, 750 for ~1.5, 1,000 for the 2-lb ceiling. Our man at 2,724 TDEE eating 2,224 projects a pound a week, twenty weeks to lose twenty pounds.
Two guardrails apply. First, the NIH floors: don't go below 1,200 calories a day if you're a woman or 1,500 if you're a man without medical supervision. A petite, lightly active woman with a 1,940 TDEE mathematically "can" run a 1,000-deficit diet at 940 calories; she shouldn't, and her body will fight it with muscle loss and a stalled metabolism long before the arithmetic plays out. Second, bigger deficits trade speed for muscle and adherence. The deficit you keep beats the deficit you admire.
28-year-old woman, 150 lb (68.04 kg), 65 in (165.1 cm), lightly active. BMR = 680 + 1,032 − 140 − 161 = 1,411. TDEE = 1,411 × 1.375 = 1,940. At half a pound a week she subtracts 250 and eats 1,690. Modest? Yes, but she'll hold it through birthdays, deadlines, and holidays, and if she later steps up to a full pound a week (about 1,440 at her current size) she still clears the 1,200 floor with room to spare. What she can't do is 2 lb/week: 940 calories is a number her body would veto within a fortnight.
Three effects make real loss slower than the rule projects. Your TDEE falls as you lighten; by the time our man reaches 160 lb, his maintenance has drifted to roughly 2,580. Adaptive thermogenesis squeezes out another few percent as the body economizes on fidgeting and NEAT. And the first weeks of any diet dump water and glycogen, which flatters week one and slanders week four. Recalculate your TDEE every 10 pounds, weigh daily and average weekly, and judge trends over months.
Protein softens all of this: it preserves muscle in a deficit and is the most satiating macro. If you're cutting, 0.7 to 1 g per pound of body weight is the practical range, and our macro split calculator turns that into daily grams alongside your carb and fat targets.
Stats and goal rate in; daily target, deficit, NIH floor check and timeline out.
Calorie Deficit Calculator →When you're ready to go deeper than calories, our calorie calculator gives maintenance and gain targets, and the BMR calculator isolates the resting side of the math.
Step 1: BMR via Mifflin-St Jeor — 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 for men, −161 for women. Step 2: multiply by an activity factor (1.2 sedentary to 1.9 very active) to get TDEE. Step 3: subtract 250 calories to lose ~0.5 lb a week, 500 for ~1 lb, or up to 1,000 for ~2 lb. Step 4: stay above 1,200 calories a day (women) or 1,500 (men).
Only for the first weeks. A pound of body fat stores roughly 3,500 calories, so a 500-daily deficit predicts 1 pound a week. But as you lose weight your TDEE falls and your body adapts by moving less and burning less, so measured losses typically run 10 to 20 percent slower than the rule projects. Recalculate every 10 pounds and treat the rule as a compass, not a contract.
For most people, 500 to 750 calories a day, or 10 to 20 percent below TDEE. That's aggressive enough to see weekly progress but not so aggressive that hunger, poor workouts, and muscle loss derail it. If your intake would fall below the NIH floors of 1,200 (women) or 1,500 (men), the rate of loss has to slow instead.
Both, weighted toward food. It takes about 40 minutes of jogging to burn 400 calories and one pastry to undo it, so intake is the reliable lever. Exercise protects muscle, improves how you feel, and buys some buffer, but a diet built on earning calories through cardio tends to fall apart when training slips.