A calorie deficit means eating fewer Calories than you burn, so the body draws on stored energy. The classic rule of thumb is that about 3,500 Calories equals one pound of body fat (about 7,700 per kilogram), so a 500-Calorie daily deficit equals roughly 1 lb a week at first. With 2,400 maintenance Calories and a 1 lb/week goal, the daily target is about 1,900 Calories, and losing 20 lb would take about 20 weeks by that rule.
That rule is a starting point, not a timeline you can count on. NIH researchers (Hall et al., The Lancet, 2011) showed that as weight falls, energy needs fall too, so the same intake produces slower loss over time. The NIDDK Body Weight Planner, built on that model, gives more realistic projections and sets a floor of 1,000 Calories a day.
How to use the calorie deficit calculator
- Enter your maintenance calories from the tdee calculator.
- Choose pounds or kilograms and enter your current and goal weight.
- Choose a weekly loss rate. Slower rates are easier to sustain.
- Read your daily calorie target and rough timeline. Recalculate TDEE every 5–10 lb (2–5 kg), because it falls as you lose weight.
Formula
The 3,500 and 7,700 figures are the traditional energy-per-unit-weight rules of thumb. The timeline treats the deficit as constant, which Hall et al. showed overestimates long-term loss.
Worked example
Maintenance 2,400 Calories, 200 lb now, goal 180 lb, 1 lb a week.
- Deficit: 1 × 3,500 ÷ 7 = 500 Calories/day.
- Daily target: 2,400 − 500 = 1,900 Calories.
- Timeline by the simple rule: 20 ÷ 1 = 20 weeks. Expect it to take longer as maintenance needs fall with your weight.
- Rate check: 1 lb is 0.5% of 200 lb per week.
Why weight loss slows
A smaller body needs less energy to maintain, and metabolism adapts. Hall and colleagues estimated that each pound of eventual weight change corresponds to roughly 10 Calories a day of sustained change in intake, with about half the change happening in the first year. In practice, re-estimate TDEE as you go, and focus on habits you can keep. For individual targets, especially with a medical condition, a doctor or registered dietitian can help.
To split your calorie target into protein, carbohydrate and fat, use the macro calculator. To check whether your goal weight is within the healthy BMI range for your height, use the bmi calculator.
Calories, kcal and kilojoules
A food "Calorie" is a kilocalorie (kcal). Labels in many countries use kilojoules: 1 kcal = 4.184 kJ, so 2,000 kcal = 8,368 kJ.
Caveats and assumptions
- The 3,500 Cal/lb rule overstates long-term weight loss; treat the timeline as optimistic.
- Do not go below 1,000 Calories a day without medical supervision (NIDDK planner floor).
- Not for people under 18, during pregnancy or breastfeeding, or with eating disorders. Not medical advice.
Frequently asked questions
How many calories should I eat to lose 1 pound a week?
About 500 fewer than your maintenance calories each day by the 3,500-Calorie rule. With 2,400 maintenance, that is 1,900.
Is 3,500 calories really one pound?
It is a rough rule of thumb for the first weeks. NIH research shows it overestimates long-term loss because energy needs fall as weight falls.
What is a safe calorie deficit?
That depends on the person. The NIDDK Body Weight Planner will not plan intakes below 1,000 Calories a day. Ask a health professional for a personal target.
Why did my weight loss stall?
Lower body weight lowers maintenance calories, so the original deficit shrinks. Re-estimate your TDEE and adjust.
Sources
- NIDDK — Body Weight Planner — model-based planning and the 1,000 Cal/day minimum
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet 2011 — why the 3,500 Cal rule overestimates loss (abstract confirmed through search results)
- Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990 (PubMed) — resting energy equation behind TDEE
This calculator gives general estimates for healthy adults and is not medical advice, diagnosis or treatment. Equations are averages and individuals differ. Talk to a qualified health professional before changing your diet, exercise or medication, and during pregnancy. See our disclaimer and how we build and check calculators.