Why a single date is always wrong
The Mifflin-St Jeor equation predicts resting metabolism within about ten percent for most adults, and the activity multiplier adds its own error on top. A maintenance estimate of 2,600 kcal could be 2,400 or 2,800 in reality. On a 500-calorie plan that is the difference between losing 0.45 and 0.65 kg a week, which over a twelve-kilo diet is a two-month spread in the finish date.
So the calculator runs the whole simulation three times, with true maintenance at 92, 100 and 108 percent of the estimate, and reports the earliest and latest finish. The chart shows the central line inside a band. If the slow run never reaches the target on a fixed intake, the range is shown as open-ended and the tool says why.
Maintenance falls as you lose, then falls a little more
Each week the calculator recomputes resting metabolism from the new bodyweight. A 90 kg man burns roughly 100 kcal a day less at rest by the time he reaches 80 kg, before any adaptation. On top of that, the body reduces energy expenditure beyond what the weight loss alone predicts. Rosenbaum and Leibel's work at Columbia found expenditure 10 to 15 percent below predicted after a 10 percent loss. The model applies a 6 percent reduction per 10 percent of starting weight lost, capped at 12 percent.
This is why a fixed intake that produced a kilo a week in month one produces half that by month four. In calorie mode the calculator shows the line flattening and, if the intake reaches maintenance before the target, tells you the week it stalls and the weight it stalls at. In rate mode it lowers the intake as you go and tells you the week it would have to drop below your resting metabolic rate, where it floors the number and says so.
Where the weight comes from: fat, lean, and the cost per kilo
The 7,700 kcal per kilo figure assumes every kilo lost is fat. It is not. Kevin Hall's work at the NIH, building on Forbes, shows that the share of each kilo that comes from lean tissue depends on how much fat you carry: the fraction is roughly 10.4 divided by 10.4 plus your fat mass in kilos. A person with 40 kg of fat loses about 20 percent lean; a person with 8 kg of fat loses over half of each kilo as lean tissue unless something stops it. Lean tissue costs about 1,800 kcal per kilo to lose and fat about 9,400, so a lean person actually loses weight faster per calorie of deficit, but more of it is muscle.
Resistance training with adequate protein reduces the lean share of weight loss in the trials that measured it; the tool halves the Forbes fraction when the training toggle is on, a model assumption that stands in for that effect. The calculator reports the projected split between fat and lean lost and warns when the lean share is high. Week one also shows a water and glycogen drop of about one percent of bodyweight on the scale line, labelled as not tissue, because that is the number that makes people think the first week's rate is sustainable.
Diet breaks
With diet breaks on, the calculator schedules a week at maintenance every seventh week while you are still more than a few kilos from the target. The MATADOR trial in 2018 found that men who alternated two weeks of dieting with two weeks at maintenance lost more fat over the study and kept more of it off than the continuous dieters, with a smaller drop in resting expenditure. The model credits a break with recovering a fifth of the adaptation. A break adds a week to the calendar and is drawn on the chart, so the plan is honest about the trade.
Rate mode versus calorie mode
Rate mode is a target: a percentage of bodyweight per week, and the tool works backwards to the daily deficit it needs, lowering the intake as bodyweight falls. Half a percent to three quarters of a percent keeps most of the muscle. Above about one percent the lean cost rises in the trial that measured it: in Garthe 2011, elite athletes losing 0.7 percent a week gained 2.1 percent lean mass over the diet while the faster group, at about 1 percent a week achieved, gained none. The Forbes model used here depends on fat mass rather than speed, so the calculator flags its lean figure as a best case at fast rates rather than inventing a penalty it cannot source. Calorie mode is a plan: a fixed daily number, and the tool shows what that number actually produces as maintenance drops to meet it. Calorie mode is the one that teaches you why the last five kilos take longer than the first ten.
An intake at or within 100 kcal of maintenance is refused, because that is inside the error of the estimate and there is nothing to project. A deficit that needs an intake below resting metabolic rate is floored and flagged. At a low activity level the room between maintenance and that floor is small, and the tool says that adding movement opens more room than cutting calories does.
The abs date and the essential-fat floor
If you enter body fat, the calculator projects body fat each week and reports the first week it crosses about 12 percent for men or 20 for women, where abs generally show. It also refuses a target that would take you below essential fat, about 5 percent for men and 13 for women, and tells you the lowest weight your projected lean mass supports. A target body fat below essential fat is refused outright rather than planned.
How to use the projection
Treat the central date as the plan and the range as the promise. Weigh daily, average the week, and compare the weekly average with the table. If you are ahead of the slow line for three weeks running, maintenance is higher than the estimate; if you are behind the fast line, it is lower. Re-run the calculator from the new weight every few weeks rather than adjusting on the fly. The calorie calculator sets the macros for the intake this tool projects, and the body fat calculator gives it the input that unlocks the lean-mass model.