Your daily target, safe deficit size, and timeline to goal weight
| Activity level | Multiplier | What it looks like |
|---|---|---|
| Sedentary | 1.20 | Desk job, little or no exercise, mostly sitting |
| Lightly active | 1.375 | Light exercise or sports 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.90 | Physical job plus training, or twice-a-day workouts |
Most people overestimate their multiplier. If you're between two, pick the lower one; you can always add exercise calories back later at half their face value.
| Daily deficit | Classic math | Time to lose 20 lb | Best for |
|---|---|---|---|
| 250 | ~0.5 lb/week | ~40 weeks | Last 10 lb, people who hate dieting |
| 500 | ~1 lb/week | ~20 weeks | Standard sustainable cut |
| 750 | ~1.5 lb/week | ~14 weeks | Higher starting weight, deadline pressure |
| 1,000 | ~2 lb/week | ~10 weeks | Only if intake stays above the 1,200/1,500 floors |
The 3,500-calorie rule over-promises after the first month: as you lose weight your TDEE falls and your body adapts, so real-world results typically land 10–20% slower than the table. Recalculate every 10 pounds.
Every diet that works, works through the same mechanism: you eat less energy than you spend, and the difference comes out of stored fat. This calculator makes the arithmetic concrete. It estimates what you burn, subtracts a deficit sized to your goal rate, and tells you how long the trip should take.
BMR comes from the Mifflin-St Jeor equation: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 for men (−161 for women). It's the equation the Academy of Nutrition and Dietetics recommends as most accurate for the general population. TDEE = BMR × activity multiplier. Target = TDEE − (pounds per week × 3,500 ÷ 7). The floor is 1,200 calories for women and 1,500 for men, per NIH guidance; if your deficit would dip below it, the calculator raises the target and flags the trade-off.
Enter your stats and pick an activity multiplier that describes a normal week, not your best week. Choose a rate of loss: 1 lb a week suits most people; 0.5 is better if you're already lean or training hard. Set a goal weight to get a timeline. If the floor note appears, slow the rate; a smaller deficit you sustain beats a big one you abandon in week three.
A 35-year-old man, 180 lb, 70 in, moderately active. BMR = 10 × 81.65 + 6.25 × 177.8 − 5 × 35 + 5 = 1,758. TDEE = 1,758 × 1.55 = 2,724. A 1 lb/week deficit of 500 puts his target at 2,224/day. Twenty pounds to lose at that pace is about 20 weeks, roughly 5 months. Along the way his TDEE will drift down toward 2,600 as he lightens, so he should recalculate at 170 and again at 160.
A 28-year-old woman, 150 lb, 65 in, lightly active. BMR = 10 × 68.04 + 6.25 × 165.1 − 5 × 28 − 161 = 1,411. TDEE = 1,411 × 1.375 = 1,940. At 0.5 lb/week the deficit is 250 and her target is 1,690/day, comfortably above her 1,200 floor. If she tried 2 lb/week instead, the required 1,000-calorie deficit would drop her to 940, far below safe minimums; the calculator would flag it and hold her at 1,200.
The classic rule is 3,500 calories per pound of body fat, so 1 pound a week means a 500-calorie daily deficit (3,500 ÷ 7). Real weight loss runs slower than the math because metabolism adapts as you lose, but 500 a day remains the standard starting point. A 180-pound, 35-year-old moderately active man burns about 2,724 calories a day, so his target would be about 2,224.
Calculate your TDEE (total daily energy expenditure) first: BMR from the Mifflin-St Jeor equation times an activity multiplier from 1.2 to 1.9. Your deficit is TDEE minus what you eat. A 500-calorie deficit loses roughly 1 pound a week; 250 loses about half a pound. Track intake for a week before cutting so you know your true baseline.
The NIH guidance is not to go below 1,200 calories a day for women or 1,500 for men without medical supervision. Below those floors you risk muscle loss, nutrient shortfalls, and a stall as your metabolism adapts. A deficit of 500 to 750 calories, or 10 to 20 percent of TDEE, is the range most people can actually sustain.
It's the ceiling for most people, aimed at roughly 2 pounds a week, and it's only appropriate if your TDEE is high enough to keep intake above 1,200 for women or 1,500 for men. If a 1,000-calorie deficit would push you under those floors, the rate of loss has to slow. Bigger deficits also shed more muscle and trigger stronger metabolic adaptation, which is why crash diets rebound.
Three compounding reasons. You're lighter, so your TDEE drops; your body adapts to low intake by moving less and burning less (adaptive thermogenesis); and early losses include water and glycogen that taper off. That's why the 3,500-calorie rule over-promises after the first month. Recalculate your TDEE every 10 pounds lost and expect the weekly rate to dip as you get leaner.
Usually not all of them. If you picked an honest activity multiplier, your TDEE already includes exercise; adding it back double-counts. A practical approach: set your activity to your typical week without formal workouts, then treat actual workouts as a modest bonus, maybe half the machine's calorie readout, since those estimates run high.