To reach a goal weight by a set date, eat your maintenance calories minus a daily deficit that depends only on how much you lose and how fast. Losing 10 lb in 60 days needs about 739 kcal/day below maintenance, not the 582 kcal the 3,500-calorie rule suggests, because your needs fall as you lose weight.
The calculator estimates your current maintenance calories with the NASEM 2023 energy equations, then works out the daily deficit you need with Hall & Chow's (2011) energy-balance model, which accounts for your metabolism slowing as you lose weight.
How to use this calculator
- Enter sex, age (18 or older) and height.
- Enter your current weight and goal weight in lb or kg.
- Pick a goal date. The default is 120 days from today.
- Choose your usual activity level (see the EER calculator for what each level means).
- Read "Eat about": your daily calorie target. Check the rate per week and the maintenance calories you will need at your goal.
How it's calculated
Hall & Chow write the energy balance as ρ · dBW/dt = ΔEI − ε · (BW − BW₀), where ρ = 9,100 kcal/kg is the energy density of weight change and ε = 22 kcal/kg/day is how much daily energy expenditure falls per kilogram lost. Solving it for a constant calorie cut gives ΔBW(t) = (ΔEI / ε)(1 − e^(−εt/ρ)), so the deficit needed to lose L kg in t days is ε · L / (1 − e^(−εt/ρ)). The solved form is our derivation from their equation.
- Maintenance now is your NASEM 2023 Estimated Energy Requirement for your current weight and activity.
- Target intake = maintenance − deficit.
- Maintenance at goal = maintenance − 22 kcal × kg lost, the steady state the model settles at.
The model uses average parameters and is for adults who are not pregnant or breastfeeding. Real results vary with adherence, water shifts and individual metabolism.
Daily deficit needed by goal
| Lose | In | Hall model | 3,500 kcal/lb rule |
|---|---|---|---|
| 5 lb | 30 days | 713 kcal/day | 582 kcal/day |
| 10 lb | 60 days | 739 kcal/day | 582 kcal/day |
| 10 lb | 90 days | 510 kcal/day | 388 kcal/day |
| 15 lb | 90 days | 766 kcal/day | 582 kcal/day |
| 20 lb | 120 days | 793 kcal/day | 582 kcal/day |
| 20 lb | 180 days | 566 kcal/day | 388 kcal/day |
| 30 lb | 180 days | 848 kcal/day | 582 kcal/day |
| 30 lb | 365 days | 511 kcal/day | 287 kcal/day |
The table is independent of your size: the deficit depends only on how much you lose and how fast. What you should eat is your maintenance minus that deficit. The gap between the two columns grows with longer timelines, because the fall in energy needs has more time to add up.
Worked examples
Woman, 40, 5′5″, 180 lb → 165 lb in 120 days, low active
Maintenance 2,376 kcal, deficit 594 kcal, so eat about 1,782 kcal/day. That is 0.9 lb a week. At 165 lb her maintenance drops to about 2,227 kcal.
Man, 35, 5′10″, 220 lb → 200 lb in 150 days, inactive
Maintenance 2,937 kcal, deficit 656 kcal (the 3,500 rule would say 466), so eat about 2,281 kcal/day, or 0.9 lb a week.
Woman, 28, 5′4″, 150 lb → 140 lb in 6 weeks, active
Maintenance 2,417 kcal, deficit 1,034 kcal, so eat about 1,383 kcal/day. At 1.7 lb a week this sits at the fast end of the CDC's 1–2 lb guide and needs a daily cut of over 1,000 kcal. Moving the date out a few weeks shrinks the cut.
How fast should you lose weight?
The CDC says people who lose weight gradually, about 1 to 2 pounds a week, are more likely to keep it off than people who lose weight faster. Even modest loss helps: the CDC notes that a 5% reduction (10 lb for a 200 lb person) can improve blood pressure, cholesterol and blood sugar. NIDDK adds that fast weight loss raises the chance of gallstones, and that experts suggest beginning with a loss of 5–10% of starting weight over 6 months.
If your result is close to or below about 800 kcal a day, it is in the range NIDDK calls a very low-calorie diet, which should be supervised by a health care professional. Choose a later goal date instead.
Common mistakes
- Using the 3,500 calorie rule. It assumes each pound always costs the same deficit, so it promises results that come too fast (Thomas et al., 2013).
- Overstating activity. An activity level that is too high inflates maintenance and hides the deficit. When in doubt, pick one level lower.
- Reading one weigh-in. Water and food cause daily swings. Compare weekly averages.
- Going back to old intake at the goal. Maintenance at your new weight is lower, about 10 kcal/day per pound lost (Hall et al., 2011).
- Large goals. The linear model is validated for changes of about 10% of body weight. For bigger goals use the NIDDK Body Weight Planner.
When not to use this calculator
This is general information, not medical advice. Do not use it if you are under 18, pregnant or breastfeeding, and talk to a doctor before cutting calories if you have diabetes (especially on insulin or other glucose-lowering medicine), heart or kidney disease, a history of eating disorders, or take medicines for weight loss. See a doctor if you are losing weight without trying. People with diabetes can track progress with the A1C calculator; if you lift weights and care more about fat than scale weight, try the body recomposition calculator.
Frequently asked questions
How many calories should I eat to lose 10 pounds in 2 months?
About 739 kcal a day below your maintenance, using the Hall & Chow model. The old 3,500 kcal-per-pound rule suggests only 582 kcal, which underestimates the cut because your energy needs fall as you lose weight.
How many calories should I eat to lose 20 pounds in 4 months?
About 793 kcal a day below maintenance. That works out to about 1.2 lb a week, inside the 1–2 lb a week the CDC says is more likely to last.
Is the 3,500 calorie rule accurate?
Not for predicting weight loss over time. Hall (2008) put the energy content of weight change at about 32.2 MJ/kg (≈3,500 kcal/lb) for people with more body fat, but using it as a fixed rule ignores the fall in energy expenditure as weight drops; Thomas et al. (2013) found it grossly overestimated actual loss in seven supervised studies.
Why does weight loss slow down?
As you lose weight you burn fewer calories, so a fixed calorie cut does less over time. In the linear model a 500 kcal/day cut loses about 13.3 kg (29 lb) in the first year and levels off near 22.7 kg after several years. Hall et al. (2011) summarise it as roughly 10 kcal/day per pound, with half the change in about a year and 95% in about three years.
How many calories will I need after reaching my goal?
Roughly 22 kcal/day less per kilogram lost (about 10 kcal per pound). Losing 20 lb lowers maintenance by about 200 kcal/day.
How fast is it safe to lose weight?
The CDC says people who lose about 1 to 2 pounds a week are more likely to keep the weight off than people who lose weight faster. NIDDK notes that fast weight loss raises the risk of gallstones and that experts suggest starting with 5–10% of your weight over 6 months.
What is the lowest number of calories I should eat?
This calculator does not set a floor, so check the result. NIDDK defines a very low-calorie diet as about 800 kcal a day or less and describes it as supervised by a health care professional. If your target is close to that, pick a later date or talk to your doctor.
Can I lose weight by a date without counting calories?
Yes, but the date becomes a guess. The calculator gives a daily number you can check against food labels or an app for a few weeks, then adjust by feel once you know what that intake looks like.
Does exercise change the result?
Exercise raises your maintenance calories. Choose the activity level that matches your normal week. If you start exercising more, re-run the calculator with the higher level instead of eating back calories burned.
Is this the NIDDK Body Weight Planner?
No. The NIDDK planner uses Hall's full dynamic model with fat and lean compartments. This calculator uses the simpler linear version from Hall & Chow (2011) with average parameters, which the authors describe as accurate for weight changes of about 10%. For larger goals, use the NIDDK planner.
Sources & method
- Hall & Chow (2011), Am J Clin Nutr — linearized energy balance model (ρ = 9,100 kcal/kg, ε = 22 kcal/kg/day)
- Hall et al. (2011), The Lancet — quantification of the effect of energy imbalance on bodyweight
- Hall (2008), Int J Obes — what is the required energy deficit per unit weight loss?
- Thomas et al. (2013), Int J Obes — the 3,500 kcal rule overestimates weight loss
- NIDDK — Body Weight Planner
- NIDDK — Dieting and gallstones
- NIDDK — Definition: very low-calorie diet
- CDC — Losing weight
- National Academies (2023) — Dietary Reference Intakes for Energy (EER equations)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.