Worked example: 90 kg at 24% body fat, aiming at 15%
| Step | Value | Working |
|---|---|---|
| Lean body mass, held constant | 68.4 kg | 90 × (1 − 0.24) |
| Fat mass now | 21.6 kg | 90 − 68.4 |
| Goal weight at 15% | 80.5 kg | 68.4 ÷ (1 − 0.15) |
| Fat mass at goal | 12.1 kg | 80.5 − 68.4 |
| Fat to lose | 9.5 kg | 21.6 − 12.1 |
| Time at 300 kcal a day | 34.9 weeks | 9.5 × 7,700 ÷ (300 × 7) |
The number most people find surprising is the goal weight. Going from 24% to 15% body fat sounds like it should cost far more than 9.5 kg, and the mental arithmetic that says “nine percentage points of 90 kg is 8 kg” happens to land close by accident. The correct calculation divides rather than subtracts, because the denominator moves too: as fat comes off, the same lean mass is a larger share of a smaller body.
Why this is the only honest version of the question
“What will I weigh at 15% body fat?” has exactly one defensible answer: whatever weight puts your current lean mass at 85% of the total. That is the calculation on this page, and it deliberately assumes you neither gain nor lose lean tissue.
That assumption is not quite true, and it is wrong in a helpful direction. In a moderate deficit with enough protein and real resistance training, most people hold nearly all of their lean mass and some — particularly beginners, people returning after a break, and people carrying more fat — add a little. If you do, the real goal weight is slightly higher than the one printed here and you will look considerably better at it.
We do not model that muscle gain, on purpose. Adding a plausible-looking “+0.25 kg a month” term would shift the headline number by more than the measurement error of the body-fat input that feeds the whole thing. It would be a guess wearing the costume of a calculation.
What recomposition actually is
Losing fat and gaining muscle at the same time. It is real, it is measurable, and it is slow. It happens most readily in three groups: people new to resistance training, people returning after a long layoff, and people with a substantial amount of fat to lose. Outside those groups the two processes compete for the same energy and doing both at once is much less efficient than doing them in sequence.
The practical version, for anyone in one of those three groups: eat at maintenance or in a small deficit — 200 to 400 kcal, not 800 — keep protein high, and train with progressive loading. The scale will barely move for months, which is why measuring recomposition by scale weight alone is the fastest way to conclude nothing is happening while something is.
Methodology
Lean mass = weight × (1 − body fat ÷ 100). Goal weight = lean mass ÷ (1 − target body fat ÷ 100). Fat to lose is the difference between current and goal fat mass. Time converts at 7,700 kcal per kilogram of fat — the familiar 3,500 kcal per pound — divided by the daily deficit times seven. Every step is printed above so you can check it rather than take it.
Limitations
Everything here is downstream of one measurement you probably estimated. The body-fat input drives the entire calculation. A tape measurement carries roughly ±3 percentage points against hydrostatic weighing; a bathroom scale with hand grips is worse and drifts with hydration. At 90 kg, being three points out moves the goal weight by about 3 kg and the timeline by ten weeks. Use the tape method rather than a guess, and re-measure the same way each time so at least the error is consistent.
The timeline assumes a constant deficit against a constant maintenance. Neither holds: maintenance falls as you get lighter, so a fixed calorie target becomes a smaller deficit over time and the real timeline runs longer than the arithmetic. Recalculate every 4 to 5 kg.
Weekly weight change is also noisy in a way that swamps the signal over short windows. Water shifts from carbohydrate intake, sodium, and hormonal cycles routinely mask two weeks of genuine fat loss. Judge progress on a fortnightly average of daily weights, not on any single morning.
Target body-fat percentages below roughly 8% for men and 15% for women are competition-preparation territory, not a normal state, and are not a maintainable target for most people. The calculator will happily compute them; that does not make them a good idea.
This is a fitness estimate, not medical advice, and not a substitute for a professional who can actually see you. If reaching a target weight matters to you medically, or you have any history of disordered eating, that conversation belongs with a doctor or a dietitian rather than with a web page.