To lose body fat you need to take in less energy than you use. The gap between the two is the calorie deficit, and the question most people have is how large it should be and how long it will take to reach a goal weight. This calculator estimates your maintenance calories, subtracts the deficit you choose, and then divides the weight you want to lose by the expected weekly loss to give a timeline.
The timeline is deliberately labelled an estimate. Real weight loss slows over time, so use it as a planning aid and check your progress against the scale and tape measure.
How to use the calorie deficit calculator
- Enter your sex, age, height and current weight.
- Enter the goal weight you are aiming for. It must be below your current weight.
- Choose an activity level that reflects an ordinary week.
- Select a daily deficit. 250 to 500 kcal suits most people; larger deficits need more care.
- Read your target calories, weekly loss and estimated weeks. If the safety note warns you, pick a smaller deficit.
Calorie deficit formula
Calorie target = maintenance − daily deficit
Weekly loss (kg) = daily deficit × 7 ÷ 7,700
Weeks to goal = (current weight − goal weight) ÷ weekly loss
BMR comes from the Mifflin-St Jeor equation (Am J Clin Nutr 1990;51:241–247). The 7,700 kcal per kilogram (about 3,500 kcal per pound) figure goes back to Wishnofsky (1958) and treats weight loss as if it were all fat. It is a simplification; see the section below on why progress slows.
| Activity level (IOM 2005) | PAL range | Multiplier used here | Example: 1,755 kcal BMR |
|---|---|---|---|
| Sedentary | 1.0 – 1.39 | 1.2 | 2,106 kcal |
| Low active | 1.4 – 1.59 | 1.5 | 2,632 kcal |
| Active | 1.6 – 1.89 | 1.75 | 3,071 kcal |
| Very active | 1.9 – 2.5 | 2.2 | 3,861 kcal |
The multipliers are the midpoints of the Physical Activity Level (PAL) categories in the US Institute of Medicine’s Dietary Reference Intakes for Energy (2005): sedentary 1.0–1.39, low active 1.4–1.59, active 1.6–1.89 and very active 1.9–2.5. They are rounded to 1.2, 1.5, 1.75 and 2.2. Your own PAL may sit anywhere inside a band, which is one reason the result is an estimate.
Worked example
A 35-year-old woman, 168 cm, 82 kg, sedentary, wants to reach 72 kg with a 500 kcal deficit.
- BMR = 820 + 1,050 − 175 − 161 = 1,534 kcal
- Maintenance = 1,534 × 1.2 = 1,841 kcal; target = 1,341 kcal
- Weekly loss = 500 × 7 ÷ 7,700 = 0.45 kg
- Time to lose 10 kg = 10 ÷ 0.45 ≈ 22 weeks
How long different deficits take
Using the static model, which is a best-case timeline.
| Daily deficit | Weekly loss (static model) | To lose 5 kg | To lose 10 kg | To lose 20 kg |
|---|---|---|---|---|
| 250 kcal | 0.23 kg (0.50 lb) | 22 weeks | 44 weeks | 88 weeks |
| 500 kcal | 0.45 kg (1.00 lb) | 11 weeks | 22 weeks | 44 weeks |
| 750 kcal | 0.68 kg (1.50 lb) | 7 weeks | 15 weeks | 29 weeks |
| 1000 kcal | 0.91 kg (2.00 lb) | 6 weeks | 11 weeks | 22 weeks |
Why real loss slows down
In 2011 Hall and colleagues published a dynamic model of energy balance (Lancet 2011;378:826–837). It shows that the 3,500 kcal-per-pound rule describes only the first weeks. As you become lighter your maintenance calories fall, and the body adapts to lower intake, so a fixed deficit produces less loss each month. A person who keeps the same target for a year will lose notably less than the table above suggests. Recalculate every 4 to 5 kg and adjust.
Keeping a deficit healthy
- Aim for gradual loss. Public-health guidance usually describes 0.5 to 1 kg a week as a sensible maximum.
- Keep protein adequate and include resistance training to protect muscle; see the Protein Intake Calculator.
- Do not drop below supervised-diet levels without medical advice. The NHLBI guide gives 1,000–1,200 kcal for women and 1,200–1,600 kcal for men as the usual low range.
- Take diet breaks or maintenance weeks if you feel run down.
Start with the Calorie Calculator for maintenance figures, see the BMR Calculator for resting energy, or use the Weight Loss Calculator for a different view of the same goal.
This page is for information only and is not medical or dietary advice. Speak to a doctor or registered dietitian before making large changes to your diet or exercise.
Sources
- Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990;51:241–247.
- Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr 1958;6:542–546.
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet 2011;378:826–837.
- NHLBI. The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. NIH Publication 00-4084, 2000.
- Institute of Medicine. Dietary Reference Intakes for Energy, 2005 (Physical Activity Level categories).
Frequently asked questions
What is a calorie deficit?
A calorie deficit means eating fewer calories than your body uses over a period. When intake is consistently below expenditure the body draws on stored energy, mostly fat, and weight falls. The size of the daily gap is the deficit.
How big a calorie deficit is safe?
Most guidance for adults favours a moderate deficit of about 500 kcal a day, or 250 for smaller or older people. The NHLBI guide treats 1,000–1,200 kcal for women and 1,200–1,600 kcal for men as the usual lowest range of supervised diets. If your target falls below that, choose a smaller deficit or ask a clinician.
Why will I probably lose weight more slowly than the calculator says?
The calculator uses the static 7,700 kcal per kg (3,500 kcal per lb) rule. In practice your energy needs fall as you get lighter and the body adapts, so the loss per week usually tapers. Hall and colleagues showed in 2011 that the rule overestimates long-term loss; treat the weeks figure as a best case.
Should I eat back exercise calories?
Not if you chose an activity level that already includes your exercise. If you set “sedentary” and then add workouts, you are creating a larger deficit than planned. Pick the level that matches your typical week and do not add exercise on top.
Who should not use a calorie deficit?
People who are pregnant or breastfeeding, under 18, underweight, or who have a history of disordered eating or a medical condition affecting nutrition should get personal advice rather than rely on a calculator.