UNDERSTAND THE METHOD
How to use this calculator
Enter an adult profile and, if available, a measured body-fat estimate. The calculation compares estimated energy needs with a theoretical transfer coefficient of 48.51 kcal per kilogram of fat per day (about 22 kcal/lb).
What the result can tell you
This is a theoretical calculation. It cannot guarantee fat-only loss or muscle preservation. The 1,500/1,200 kcal display floors are product limits, not evidence of a safe personal intake. The model is not for pregnancy or breastfeeding.
How much fat can you lose in a day? Limits by fat mass
In a 2005 paper, Alpert estimated that body fat can supply at most about 290 kJ of energy per kilogram of fat per day, using data from underfed people who kept moderately active. That is about 69.3 kcal per kg of fat, or about 31.4 kcal per pound. In his model, a daily energy gap larger than this causes an immediate loss of fat-free mass (muscle, organs, water and bone). This calculator uses a lower coefficient, 48.51 kcal per kg of fat (about 22.0 kcal per pound), which does not come from Alpert's paper.
Largest daily energy from body fat, by fat mass (kcal per day)
| Fat mass | Alpert 2005: 69.3 kcal/kg | This calculator: 48.51 kcal/kg |
|---|---|---|
| 5 kg (11 lb) | 347 | 243 |
| 10 kg (22 lb) | 693 | 485 |
| 15 kg (33 lb) | 1,040 | 728 |
| 20 kg (44 lb) | 1,386 | 970 |
| 25 kg (55 lb) | 1,733 | 1,213 |
| 30 kg (66 lb) | 2,079 | 1,455 |
| 40 kg (88 lb) | 2,772 | 1,940 |
| 50 kg (110 lb) | 3,465 | 2,426 |
Fat mass is body weight times body fat percentage. For example, 80 kg at 20% body fat is 16 kg of fat, so this calculator's limit is 48.51 × 16 = 776 kcal a day, and Alpert's estimate is about 1,109 kcal a day. People with more body fat have a higher limit.
The calculator then takes that limit away from your estimated daily energy use (Mifflin–St Jeor resting energy times your activity level) to get a model intake. The model intake never goes below 1,500 kcal a day for men or 1,200 kcal a day for women. When the floor applies, the model intake is marked (floor) and is higher than daily energy minus the fat limit. These floors are product limits, not a safe intake for any one person. The next table runs the calculator for four example adults aged 40 with an activity level of 1.6.
Worked examples, kcal per day
| Example | Estimated daily energy | Largest fat transfer | Model intake |
|---|---|---|---|
| Woman, 163 cm, 64 kg, 28% body fat | 2,080 | 869 | 1,211 |
| Woman, 163 cm, 85 kg, 40% body fat | 2,416 | 1,649 | 1,200 (floor) |
| Man, 178 cm, 80 kg, 20% body fat | 2,752 | 776 | 1,976 |
| Man, 178 cm, 105 kg, 32% body fat | 3,151 | 1,630 | 1,521 |
This is a theoretical limit from one model, not a target, and staying under it does not promise that all weight lost is fat. It does not apply during pregnancy or breastfeeding.
Common questions
What does the fat-store transfer coefficient represent?
The scenario compares estimated energy needs with a theoretical transfer coefficient of 48.51 kcal per kg of fat per day, or about 22 kcal per lb.
Does this result guarantee fat-only loss or muscle preservation?
No. It is a theoretical model. The displayed calorie floors are product limits, not evidence of a safe personal intake, and the model is not for pregnancy or breastfeeding.
Sources & limitations2 sources
Sources checked September 12, 2026. These notes identify methods and assumptions. They have not received an independent clinical review.
- Alpert model of energy transfer from fat stores.pubmed.ncbi.nlm.nih.gov
- Hall discusses energy balance and body composition.pmc.ncbi.nlm.nih.gov