Calorie Deficit Math: Setting a Target and Estimating a Timeline
A calorie deficit means eating fewer calories than you burn. The arithmetic that links a deficit to weight loss is simple, which is why it is so widely used, and also why it is easy to over-trust. A plan built on it needs an estimate of what you burn, a target intake, and some realism about how the numbers drift as weight changes.
This guide works through the calculation with an example, shows how the timeline stretches as your body gets smaller, and covers the limits you should respect. It is general information, not a personalised plan.
The energy in a kilogram of fat
Body fat tissue is mostly fat, with some water and protein mixed in. Its energy content works out to roughly 7,700 kcal per kilogram, or about 3,500 kcal per pound. That is where the familiar "3,500 calories equals a pound" rule comes from.
Treat these as approximations. Not everything lost on the scale is fat tissue, and the exact figure depends on what is being lost.
weekly weight change (kg) = daily deficit × 7 ÷ 7,700For pounds, use 3,500 in place of 7,700.
The basic calculation
- Estimate your maintenance calories (TDEE). The guide to activity multipliers explains how.
- Choose a daily deficit and subtract it to get your target intake.
- Divide the weight you want to lose, multiplied by 7,700, by the daily deficit to estimate the number of days.
Worked example: TDEE is 2,720 kcal and the deficit is 500 kcal a day, so the target is 2,220 kcal. A 500 kcal deficit repeated for 7 days is 3,500 kcal, and 3,500 ÷ 7,700 = 0.45 kg a week (about 1 lb). To lose 5 kg, the time is 5 × 7,700 ÷ 500 = 77 days, or 11 weeks.
The weight loss calculator and TDEE calculator automate these steps.
Why the real timeline is longer
A smaller body burns fewer calories, so the deficit shrinks even if you keep eating the same amount. In the Mifflin-St Jeor equation each kilogram of weight is worth 10 kcal of BMR. With an activity factor of 1.55, that is 10 × 1.55 = 15.5 kcal of TDEE per kilogram.
After losing 5 kg on a fixed intake of 2,220 kcal, TDEE has fallen by 5 × 15.5 = 77.5 kcal, and the deficit has dropped from 500 to about 422 kcal. Recalculating day by day gives the following comparison.
| Weight to lose | Simple estimate | Allowing for falling TDEE |
|---|---|---|
| 5 kg | 77 days (11 weeks) | about 84 days (12 weeks) |
| 10 kg | 154 days (22 weeks) | about 184 days (26 weeks) |
| 15 kg | 231 days (33 weeks) | about 311 days (44 weeks) |
The bigger the goal, the larger the gap. This is a simplified model. Real bodies also respond in other ways: daily movement tends to drop, water weight moves up and down, and progress rarely follows a straight line. Plan on the simple figure as the earliest likely date, not the expected one.
Choosing a sensible size
Many health agencies describe about 0.5 to 1 kg (1 to 2 lb) per week as a steady rate of loss. By the arithmetic above that corresponds to a deficit of roughly 550 to 1,100 kcal a day, since 0.5 × 7,700 ÷ 7 = 550 and 1 × 7,700 ÷ 7 = 1,100.
A bigger deficit is not automatically better or even possible. For someone whose TDEE is 1,800 kcal, taking 1,100 off leaves 700 kcal, which is far too little to meet nutritional needs. A deficit is better judged as a share of TDEE: 500 out of 2,720 is about 18%.
- Very low intakes make it harder to get enough protein, vitamins and minerals, and can cost muscle as well as fat.
- Children, teenagers, and people who are pregnant, breastfeeding or recovering from illness should not follow a calorie target without medical guidance.
- Anyone with a history of disordered eating, or a condition such as diabetes, should plan changes with a health professional.
Tracking progress without overreacting
Daily weight swings of a kilogram or more are normal and reflect water and food in the gut, not fat. Compare weekly averages instead, and give any plan two to three weeks before deciding it is too fast or too slow. If your average weight is not moving at all, your real TDEE is probably lower than the estimate, and the target intake should be adjusted slightly.
If you only want a starting calorie number, the calorie calculator gives maintenance calories from your details.
This article is general education and not medical advice. Speak with a doctor or registered dietitian before starting a weight-loss plan, particularly if you have a health condition, take medication, or are under 18.
- Weight Loss Calculator: Enter your measurements, your current and future activity level, a target weight and the time you want to take. The calculator returns the daily calories to eat, what you can eat once you arrive, and how your body composition changes along the way.
- TDEE Calculator: Enter your age, sex, height, weight and how active you are. The calculator estimates your basal metabolic rate, your total daily energy expenditure, and Calorie targets for losing or gaining weight.
- Calorie Calculator: Enter your age, sex, height, weight and activity level to see the daily calories that hold your weight steady, plus targets for losing or gaining about half a kilogram to a kilogram a week.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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