Free tool
Calorie deficit calculator
Pick a daily deficit (or a weekly rate) and see how long it takes to reach your goal weight - plus whether that pace is sane.
Time to goal
20 weeks
about 140 days · reach 180 lb around Nov 2, 2026
- Total to lose
- 20 lb
- Daily deficit
- 500 kcal
- Weekly loss
- 1 lb/wk
- Rate of body weight
- 0.50% / wk
Pace check
At 0.50% of body weight per week, this sits inside the sustainable band (under ~1%/week). Steady wins.
This uses the linear 3,500 kcal-per-lb rule, so it's a best-case estimate. Early weeks drop faster (water), later weeks slower (your maintenance burn falls as you shrink). Re-check your numbers every few weeks.
Formula & assumptions
The arithmetic everyone uses comes from a single rule of thumb: a pound of body fat stores roughly 3,500 kcal, and a kilogram roughly 7,700 kcal. Run a deficit, divide, and you get a timeline:
weekly_loss = daily_deficit × 7 / kcal_per_unit total_to_lose = current_weight - goal_weight days_to_goal = (total_to_lose × kcal_per_unit) / daily_deficit goal_date = today + days_to_goal kcal_per_unit = 3,500 (lb) or 7,700 (kg)
If you'd rather pick a rate, the calculator runs it backward: a target of 1 lb/week implies a 500 kcal/day deficit, 0.5 kg/week implies about 550 kcal/day, and so on. Either way you land on the same three outputs - total to lose, time to goal, and a calendar date.
The honest caveat. Real weight loss is not the clean straight line this math draws. Early drops are mostly water and glycogen, not fat, so week one flatters you. And as you get lighter your body fights back: adaptive thermogenesis lowers your maintenance burn, so a fixed-calorie deficit shrinks over time and the curve flattens. The 3,500-kcal rule itself, traced back to Wishnofsky in 1958, systematically over-predicts long-term loss because it ignores this. Treat the date as a best-case target, not a promise - then re-check your maintenance every few weeks.
A note on rate. Faster is not better. We flag any plan losing more than about 1% of your body weight per week, the rough ceiling above which you start sacrificing more lean mass and risk gallstones, fatigue, and rebound. For most people 0.5-1.0% per week (often 0.5-1 lb or 0.25-0.5 kg) is the sustainable band.
References
Wishnofsky, M. (1958). Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition, 6(5), 542-546. The origin of the "3,500 kcal = 1 lb" rule used here - and now known to over-predict long-term loss because it assumes a constant, linear energy deficit.
Hall, K. D. (2008). What is the required energy deficit per unit weight loss? International Journal of Obesity, 32(3), 573-576. Shows why the fixed 3,500-kcal rule fails over months and motivates a dynamic model.
NIH Body Weight Planner (Hall et al.). A dynamic simulation that accounts for adaptive thermogenesis and the shifting deficit as you lose - a more accurate (if less tidy) alternative to the linear math on this page.
Garthe, I. et al. (2011). Effect of two different weight-loss rates on body composition: support for keeping the weekly rate near ~0.5-1% of body weight to preserve lean mass.