Worked example: 2,759 kcal maintenance, 80 kg
| Rate | Deficit | Daily calories | Loss per week | % of bodyweight |
|---|---|---|---|---|
| Conservative | 275 kcal | 2,484 | 0.28 kg | 0.35% |
| Moderate | 500 kcal | 2,259 | 0.5 kg | 0.63% |
| Aggressive | 800 kcal | 1,959 | 0.8 kg | 1.0% |
To lose 6 kg at the moderate rate takes about twelve weeks. That is the whole answer, and the twelve weeks is the part people find harder to accept than the 2,259.
Why the rate is capped as a percentage of bodyweight
A 500 kcal deficit means something completely different to a 55 kg person and a 110 kg person. As a share of maintenance it might be 25% for one and 12% for the other. Expressing the rate as percentage of bodyweight per week normalises that, and the commonly used ceiling is about 1% per week. Above that, an increasing share of what you lose is lean tissue rather than fat, and the practical failure rate rises steeply because very large deficits are difficult to hold.
This is why the calculator caps the aggressive option rather than letting you enter any number you like. A tool that will happily hand a 70 kg person a 1,200 kcal deficit is not being helpful.
The 3,500-calorie rule, and why it drifts
The arithmetic here uses the conventional figure of roughly 7,700 kcal per kilogram of body fat (3,500 per pound). It is a reasonable approximation over weeks and a poor one over months, because maintenance falls as you get lighter — a smaller body costs less to run, and people move less spontaneously when in a deficit. A cut planned entirely on week-one arithmetic will run behind schedule by the end. Recalculate maintenance every 4 to 6 kg.
Methodology
Cutting calories = maintenance − deficit. Weekly rate = deficit × 7 ÷ 7,700 kcal per kg. Maintenance comes from the maintenance calorie calculator using Mifflin-St Jeor, or Katch-McArdle if you have a measured body-fat percentage.
Limitations, stated plainly
Every source of error in the maintenance estimate carries straight through to this one, and maintenance is uncertain by roughly ±10% before you have gathered any of your own data. Weekly weight loss is also not smooth: water shifts from carbohydrate intake, salt and hormonal cycles routinely mask two weeks of real fat loss on the scale. Judge progress on a fortnightly average, not on any single morning.
Protein and training matter more than the exact deficit size for how much of the loss is fat rather than muscle. See the protein intake calculator.
The floor, and when to stop using this page
This calculator will not tell you to eat below roughly 1,500 kcal a day if you are a man, or roughly 1,200 if you are a woman, without saying that you should be speaking to someone. Those are the conventional lower bounds below which a general-population tool has no business sending anyone: below them it becomes difficult to meet micronutrient requirements from food, and the deficit stops being a diet and starts being a problem to manage with help.
If the arithmetic on this page produces a number under those figures, the correct response is not to eat it. It is to take a smaller deficit and accept a longer timeline, or to speak to a registered dietitian or your doctor, who can look at your actual situation rather than at three inputs. A calculator cannot see you, does not know your medical history, and has no way of telling the difference between someone with fat to lose and someone who should not be dieting at all.
This is a fitness estimate, not medical advice, and not a substitute for a professional who can actually see you. If you have any condition affected by what you eat, or any history of disordered eating, that conversation belongs with a doctor or dietitian, not with a web page.