How the calorie deficit calculator works
First the calculator estimates your basal metabolic rate (BMR) with the Mifflin-St Jeor equation (1990), which performs most accurately in comparison studies:
- Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Multiplied by your activity factor (PAL), this gives your total daily energy expenditure, the number of calories at which your weight stays stable on paper. Your target range for losing weight sits 300 to 500 kcal below it. A safety floor is built in: if the deficit would push your target below your basal metabolic rate, the calculator raises the lower bound to your BMR.
What is a calorie deficit?
A calorie deficit means you give your body less energy than it uses. It draws the missing energy from its reserves, mainly from body fat. As a rough conversion: about 7,000 kcal correspond to roughly 1 kg of body fat. A daily deficit of 500 kcal adds up to 3,500 kcal over a week, so about half a kilogram on paper. How well this rule of thumb really holds up is covered in the research section further down.
Why moderate instead of drastic?
A larger deficit sounds like faster results, but it has side effects. The tighter the energy budget, the harder it becomes to cover your needs for protein, vitamins, and minerals. On very strict diets the body also breaks down more muscle alongside fat, which lowers your expenditure even further. And whatever you cannot sustain in daily life often ends in the familiar yo-yo loop of quitting and regaining. A moderate deficit of 300 to 500 kcal largely avoids these traps.
Two levers make the deficit easier to live with: protein and movement. Adequate protein intake protects your muscle mass and keeps you full, the protein calculator shows you your personal amount. Movement, especially strength training, raises your total energy expenditure, so you have to cut less strictly from your diet. What individual activities contribute is shown by the calories burned calculator.
Moderate deficit: examples
| Person | Total expenditure | Target range (kcal/day) | Effect (kg/week) |
|---|---|---|---|
| Woman, 35 years, 1.68 m, 68 kg, lightly active (PAL 1.375) | 1,917 kcal | 1,417 to 1,617 | 0.3 to 0.5 |
| Man, 42 years, 1.82 m, 88 kg, moderately active (PAL 1.55) | 2,809 kcal | 2,309 to 2,509 | 0.3 to 0.5 |
| Woman, 58 years, 1.62 m, 62 kg, sedentary (PAL 1.2) | 1,418 kcal | ≈ 1,182 (raised to BMR) | ≈ 0.2 |
The third example shows the safety logic: with a low total expenditure, a deficit of 300 to 500 kcal would drop below the basal metabolic rate. The calculator therefore caps the target at the BMR, and the projected pace of weight loss shrinks accordingly.
What research says about the 7,000 kcal rule
The conversion of 7,000 kcal per kilogram of body fat goes back to a paper by Max Wishnofsky from 1958. It is a usable approximation for the first weeks, but not a permanent law. In 2011, metabolism researcher Kevin Hall and his team showed in the journal The Lancet that the body adapts its energy expenditure to reduced intake and falling weight. The NIH Body Weight Planner of the US National Institutes of Health is also based on this dynamic model.
In practical terms: weight loss slows down over months compared to the simple calculation, without you doing anything wrong. This is exactly the point where a professional nutrition coach pays off, adjusting your approach to your real progress instead of sticking rigidly to the initial number.
Related calculators for your next step
If you are only interested in your maintenance needs without a deficit, use the calorie calculator. Whether your starting weight is in a range where weight loss makes sense at all is something the BMI calculator can put into context. And for planning protein in a deficit there is the protein calculator.