Calorie Deficit Calculator

A calorie deficit is the gap that opens when you eat less than you burn. The tool starts from your maintenance calories, then applies a small daily cut. After that, it works out how many weeks the deficit could take to reach your goal weight. It shows a range because two methods disagree.

Formulas

How it works, step by step

1. First, the calculator estimates your maintenance and sets your daily target from the deficit you chose.
2. Then it turns your body fat percentage into fat mass.
3. Next, it finds how much of your loss comes from fat and how much from lean tissue.
4. After that, it works out the energy in each kilogram lost.
5. Finally, it divides the total energy by your daily deficit. It repeats the step with the 3,500-calorie rule to get the other end of the range.

Worked example

Take a 35-year-old man who is 180 cm tall, weighs 90 kg and has 28% body fat. He wants to reach 80 kg. His maintenance is about 2,550 calories with light exercise, so a 500-calorie deficit gives a target of 2,050. The model puts the energy cost near 6,970 calories per kilogram. Losing 10 kg then takes 69,700 calories, or about 20 weeks. The 3,500-calorie rule gives about 77,200 calories, or 22 weeks.

Accuracy and limits

Hall's 2008 paper models how much of lost weight is fat and how much is lean tissue. It finds that people with more initial body fat need a larger cumulative energy deficit per unit of weight lost.

The paper also compares the 3,500-calorie rule with that model. The rule roughly matches the energy density for people with obesity. However, it overestimates the deficit needed for people with lower body fat.

The result is a range because it comes from a model and a rule of thumb. The model uses Hall's energy densities: 39.5 MJ per kg for fat and 7.6 MJ per kg for lean tissue. It also uses the Forbes constant of 10.4 kg. For the paper's first worked example, a 5 kg loss at 20 kg of body fat, our version gives 28.0 MJ per kg. That matches the paper's 28. The paper's second example is 24.7 MJ per kg for a 15 kg loss at 20 kg of body fat. The same equation gives 26.5 for those inputs, so our version differs from the paper as printed. A 7 kg loss at 14 kg of body fat gives 24.7, so the printed inputs may hold a typo.

Neither method includes metabolic adaptation. Hall notes that weight loss usually slows on a constant diet. That suggests that expenditure falls, that people relax their diet, or both. A 2011 model also found that body weight responds slowly to a change in intake. Its half-times were about a year.

• The timeline assumes a steady deficit, so it is a best case. Real time is usually longer.
• Because the model starts from your body fat percentage, any error in that number carries through.
• The tool will not project a goal below a BMI of 18.5, a loss larger than your estimated fat mass, or a body fat below 10% for men or 18% for women. Those limits and its calorie floor are its own conservative rules.
• If you are pregnant or breastfeeding, have a medical condition, or have a history of disordered eating, talk to a clinician first.

Sources

1. Hall KD. What is the required energy deficit per unit weight loss? Int J Obes. 2008;32(3):573-576. Used for: The energy densities of fat and lean tissue, the modified Forbes relation and how the 3,500-calorie rule compares.
2. Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. Used for: Why body weight responds slowly to a change in calories.
3. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. Used for: The Mifflin-St Jeor equation and the 498 adults it came from.
4. Ramirez-Marrero FA, Edens KL, Joyner MJ, Curry TB. Predicted vs. actual resting energy expenditure and activity coefficients: post-gastric bypass, lean and obese women. Obes Control Ther. 2014;1(2):1-7. Used for: The 1.2 to 1.9 activity multipliers and how self-reported activity can overshoot.

Sources and math last reviewed: November 4, 2026.