A pound of body fat is commonly estimated at about 3,500 calories (roughly 7,700 per kilogram). Cutting 500 kcal a day should therefore lose about a pound a week. That works for a few weeks, but the rule overpredicts over months because your calorie burn falls as you lose weight: for one example woman it overstates a year's loss by about 10 lb.
Key takeaways
- The 3,500 kcal-per-pound rule (7,700 per kg) is a useful short-term approximation for planning.
- It assumes your calorie burn stays the same, but a lighter body burns less, so the deficit shrinks over time.
- For our example woman eating 1,903 kcal a day, the flat rule predicts 34.9 lb lost in 52 weeks; a dynamic model predicts 24.9 lb.
- Published modeling shows the rule overestimates long-term loss, which is why plateaus are normal rather than a failure.
- Use the rule for a few weeks at a time, then re-measure with your weight trend.
For a personal timeline rather than a rule, use the weight loss calculator, which recalculates your burn as you lose. Here is the rule, and why it breaks.
How many calories are in a pound of fat?
The common figure is about 3,500 kcal per pound of body fat, equal to roughly 7,700 kcal per kilogram. It is the basis of most weight-loss advice: if you burn 3,500 kcal more than you eat, you lose about a pound. A 500 kcal daily deficit adds up to 3,500 per week, so it is said to lose a pound a week.
Treat it as a rounded average. Body weight also changes through water and lean mass, and these carry far less energy per pound than fat (Hall et al., 2011).
Why does the 3,500-calorie rule fail over time?
The rule treats your TDEE as a constant. It is not. A lighter body needs less energy to move and maintain, so every pound lost shrinks your daily burn, and the same intake gives a smaller deficit (Hall et al., 2011; Thomas et al., 2014). On top of that, some people see expenditure fall a bit further than body size alone predicts (Rosenbaum & Leibel, 2010).
Published dynamic models, including one built into the NIH Body Weight Planner (NIDDK Body Weight Planner), therefore predict slower long-term loss than a straight line does. The rule is a fair estimate for a few weeks and a poor one for a year.
Worked example: 52 weeks, flat rule vs a dynamic model
Our example woman (35, 5′5″, 165 lb, moderately active) has a TDEE of 2,239 kcal and eats 1,903 kcal a day (15% deficit, 336 kcal below TDEE). The flat rule predicts 336 × 7 ÷ 3,500 = 0.67 lb lost every week. The dynamic model recalculates her TDEE each week from her new weight using Mifflin–St Jeor (Mifflin et al., 1990). Both use the same intake.
| Week | Flat rule: weight (lb) | Dynamic model: weight (lb) | Dynamic model: TDEE (kcal) |
|---|---|---|---|
| 0 | 165.0 | 165.0 | 2,239 |
| 4 | 162.3 | 162.4 | 2,220 |
| 8 | 159.6 | 159.9 | 2,203 |
| 12 | 156.9 | 157.5 | 2,186 |
| 16 | 154.3 | 155.3 | 2,170 |
| 20 | 151.6 | 153.2 | 2,156 |
| 26 | 147.5 | 150.3 | 2,135 |
| 36 | 140.8 | 145.9 | 2,104 |
| 52 | 130.1 | 140.1 | 2,056 |
After a year the flat rule says she has lost 34.9 lb; the dynamic model says 24.9 lb. The straight line overshoots by 10 lb, about 40% too much. By week 52 her TDEE has fallen to 2,056 kcal, so her deficit has shrunk from 336 to 153 kcal a day.
The dynamic model is also a planning approximation. It lowers TDEE only through body size and does not include any extra adaptive slowdown, so real-world loss can be slower still.
How long does it take to lose 5, 10, 15 or 20 pounds?
The same example woman, with the weeks needed under each approach. The flat column divides the total energy by her daily deficit and rounds up; the dynamic column uses the projection model.
| Pounds to lose | Total deficit (3,500 rule) | Flat rule at 1,739 kcal (500 deficit) | Dynamic at 1,739 kcal | Flat rule at 1,903 kcal | Dynamic at 1,903 kcal |
|---|---|---|---|---|---|
| 5 lb | 17,500 kcal | 5 weeks | 6 weeks | 8 weeks | 8 weeks |
| 10 lb | 35,000 kcal | 10 weeks | 11 weeks | 15 weeks | 17 weeks |
| 15 lb | 52,500 kcal | 15 weeks | 17 weeks | 23 weeks | 27 weeks |
| 20 lb | 70,000 kcal | 20 weeks | 24 weeks | 30 weeks | 39 weeks |
The gap is small for the first 5 pounds and widens with every pound after that. At the gentler 1,903 kcal target, the last 5 of 20 pounds take about 12 weeks by the dynamic model instead of the 7 or 8 the flat rule implies. That is a normal slowdown, not a stall caused by anything you did wrong.
How should you use the 3,500 rule?
- For quick sizing. To lose roughly 1 lb a week, aim for a 500 kcal deficit. To lose 0.5 lb a week, aim for 250.
- For short windows. Over 2–4 weeks it is close enough to compare your results to a plan.
- Not for forecasting a year. Use a dynamic projection or the calorie deficit calculator to estimate a goal date.
- To correct course. If your trend is off the plan, adjust intake by 100–200 kcal, or find your real maintenance with adaptive TDEE.
Read more about shrinking deficits in the calorie deficit guide, and why plateaus happen in weight loss plateaus.
Frequently asked questions
How many calories are in a pound of fat?
The usual figure is about 3,500 kcal per pound of body fat, or roughly 7,700 kcal per kilogram. It is a rounded average, not an exact constant, and the real figure for your weight change depends on what you lose.
Does a 500 calorie deficit really equal 1 pound a week?
For the first few weeks, roughly yes. Over months, no: as weight falls, your TDEE falls, so a fixed intake gives a smaller deficit. Dynamic modeling has shown the 3,500 rule overestimates loss over longer periods. Try the calorie deficit calculator, which models this slowdown.
Why am I losing less weight than the 3,500 rule predicts?
Usually because the rule assumes constant expenditure. Your TDEE fell as you lost weight, you may be moving less, your logging may be loose, or water is masking fat loss. Compare weekly averages and, if the gap persists, measure your real maintenance with adaptive TDEE.
Is it 3,500 calories per pound of weight or per pound of fat?
The rule is applied to body-weight change, but the energy content is specific to fat tissue. Weight also includes water, glycogen and lean mass, which carry far fewer calories per pound. That is one reason short-term scale changes can look much bigger or smaller than the math predicts.
How many calories to lose 10 pounds?
By the flat rule, a 35,000 kcal deficit. At 500 kcal a day that is about 10 weeks. In practice, the later pounds take longer because the deficit narrows, so plan on re-measuring and adjusting rather than relying on the straight line.
How many calories in a kilogram of fat?
About 7,700 kcal, which is the figure TDEECalc uses for planning and for adaptive TDEE. Dividing 3,500 by 0.4536 gives about 7,716, so the two forms of the rule are the same number in different units.
References
- Hall KD, Sacks G, Chandramohan D, et al. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet 378(9793):826–837.
- Thomas DM, Gonzalez MC, Pereira AZ, Redman LM, Heymsfield SB (2014). Time to correctly predict the amount of weight loss with dieting. Journal of the Academy of Nutrition and Dietetics 114(6):857–861.
- National Institute of Diabetes and Digestive and Kidney Diseases (2024). Body Weight Planner. NIDDK, U.S. National Institutes of Health.
- Rosenbaum M, Leibel RL (2010). Adaptive thermogenesis in humans. International Journal of Obesity 34(Suppl 1):S47–S55.
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 51(2):241–247.