Deficit & Goal-Date Planner
A dynamic model that accounts for metabolic slowing, so the projection flattens the way real weight loss does.
- 01Enter your current details and a target weight.
- 02Choose whether to plan by target date (we solve the calories) or by daily calories (we solve the date).
- 03Watch the chart: the solid line is the realistic projection, the dashed line is the old linear rule.
What the terms mean
- TDEE
- Total daily energy expenditure — the calories you burn in a day.
- Adaptive thermogenesis
- The extra fall in expenditure during a diet, beyond what weight loss alone explains.
- Dynamic model
- A projection that recalculates expenditure as your weight changes.
- 3,500 kcal rule
- The old assumption that a pound of fat equals 3,500 calories, applied linearly.
Changes what a safe intake looks like — or whether we plan at all.
We'll work out the daily calories required.
To reach 80 kg in 24 weeks — a 741 kcal daily deficit.
The gap between the two lines is the error in the static rule. It predicts 9 kg more loss than is realistic over this period.
When you reach your goal, maintenance will be about 2,679 kcal — roughly 154 kcal below what it is today. Planning for that number is what stops the weight coming back.
Method & sources
This is a simplified dynamic simulation in the spirit of Hall et al. (Lancet, 2011), the model behind the NIH Body Weight Planner. It recomputes your expenditure daily against your current weight, applies adaptive thermogenesis, and splits weight change between fat and lean tissue using the Forbes relationship. It is not the full NIH model, which solves a coupled system of differential equations.
Known limitations: the NIH model itself tracked closely against the two-year CALERIE trial (mean bias about −0.5 kg) but has been shown to overestimate weight for people on very-low-calorie diets, most pronounced in men. Treat any projection beyond about six months as a direction of travel rather than a forecast.
Hall KD et al., Lancet 2011 · Forbes GB 1987 · Trexler ET et al., J Int Soc Sports Nutr 2014 · Guo J, Brager DC, Hall KD, Am J Clin Nutr 2018
What these numbers
actually tell you.
Why the line bends
A lighter body costs less to maintain and less to move, so the same intake becomes a smaller deficit every week. Adaptive thermogenesis lowers expenditure further. Real weight loss therefore decelerates, and a straight-line prediction overshoots — often by several kilograms over six months.
The most important number is at the end
Maintenance calories at goal weight will be meaningfully lower than maintenance today. Not knowing that figure is one of the most common routes back to the starting weight. Plan the exit before you start the diet.
If it says your target is unreachable
That is the model working. At a fixed intake you eventually plateau, because falling expenditure meets your intake and the deficit disappears. Lower the intake, extend the timeline, or raise activity — but do not simply wait longer at the same calories.
Safety limits
Losing more than roughly 1% of bodyweight per week risks lean mass, and intakes below about 1,500 kcal (men) or 1,200 (women) are generally not appropriate without supervision. The tool flags both. Treat any projection beyond six months as a direction of travel rather than a forecast.
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