Free food maths tool
Calories to Fat Calculator
Enter a total deficit or surplus and the days it covers. The calculator turns the calories into kilograms and pounds of body fat by the well-known rule of thumb, and tells you where the rule stops being reliable.
Worked example
A 15,400 kcal deficit over 28 days (about 550 kcal a day) is 2 kg (4.4 lb) of body fat lost by the 7,700 kcal per kg rule, a rough guide that tends to overstate the change over weeks and months.
Convert calories to body fat
The result below starts from the example's numbers. Change any field and it updates as you type.
Track this target every day
Use the number above as your daily goal in CalorTracker, then log meals by photo, voice or text and see where your day stands against it. Free to start, no card needed to explore.
Results are estimates from published formulas, not medical advice. Your own body can sit above or below them.
Where the formulas come from, and how we check them: how we calculate.
The rule and the formula
- Kilograms of fat = calories ÷ 7,700.
- Pounds of fat = calories ÷ 3,500.
The rule goes back to Wishnofsky (1958), who put the energy in a pound of body weight lost or gained at about 3,500 kcal, which is about 7,700 kcal per kilogram. The calculator uses 7,700 kcal per kilogram (the figure the rest of the site uses) and shows the 3,500 kcal per pound reading alongside it. Because 1 kg is 2.2 lb, the two rules differ by under 1%.
Why it is an estimate, and where it fails
The rule treats body fat as a fixed store of energy and ignores everything else that moves. In 2011 Hall and colleagues showed in The Lancet that it overstates the weight change over weeks and months, because a lighter body burns fewer calories, so a fixed daily deficit produces less and less change. Weight lost is also not all fat: it includes water, glycogen and some lean tissue, particularly early on and in a large deficit. The weight on the scale moves by the day with water and food in the gut, so a short period will not match the arithmetic.
So read the result as the early, upper estimate of fat change for a given deficit, best for a week or two, and use your own weight trend over several weeks as the real answer. The calculator adds a note when the period is four weeks or longer.
Safety and scope
The calculator does not recommend a deficit or a surplus, and it adds a note when a deficit averages more than 1,000 kcal a day, the largest the calorie calculators on this site offer. Large deficits are for a doctor or dietitian to supervise. This page is for adults and is not for children, pregnancy or anyone with a medical condition.
Calories as body fat by the two rules
The same calories read two ways: 7,700 kcal per kilogram, and 3,500 kcal per pound.
| Calories | kg of fat | lb of fat |
|---|---|---|
| 500 | 0.06 | 0.14 |
| 1,000 | 0.13 | 0.29 |
| 3,500 | 0.45 | 1 |
| 7,700 | 1 | 2.2 |
| 15,400 | 2 | 4.4 |
| 30,800 | 4 | 8.8 |