How the calorie deficit calculator works
The calculator uses the classic estimate that a pound of body fat stores about 3,500 kcal. Dividing the total calories to lose by your daily deficit gives the number of days.
Weekly loss = Daily deficit × 7 ÷ 3,500
The 3,500 rule is a good short-term guide but overestimates long-term loss, because your calorie needs fall as you lose weight and your body adapts. The NIH Body Weight Planner models this more precisely. A deficit of 500 kcal a day, about 1 lb a week, is the pace recommended by the CDC for steady, sustainable loss of 1 to 2 lb a week.
Worked example
Going from 200 lb to 180 lb means losing 20 lb, or 70,000 kcal. At a 500 kcal daily deficit that takes 140 days, about 20 weeks, at 1 lb a week (0.5% of body weight). A 750 kcal deficit would cut it to about 13 weeks.
Frequently asked questions
How big should my calorie deficit be?
Around 500 kcal a day, or 10% to 25% below maintenance, is a common recommendation. Very large deficits are harder to sustain and cost more muscle.
Is 3,500 calories really one pound?
It is a useful approximation for fat tissue, but long-term loss is slower than it predicts because metabolism adapts.
Why have I stopped losing weight?
As you get lighter, your maintenance calories fall, shrinking the deficit. Recalculate TDEE every 10 to 15 lb lost.
Should women and men use different deficits?
The same principle applies. Minimum intakes of about 1,200 kcal for women and 1,500 for men are common guardrails without medical supervision.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.