Last updated: 2026-04-03

Weight-Loss Timeline

A weight-loss timeline is the number of days required to move from a current body weight to a target weight at a given daily calorie deficit. The estimate follows from energy balance: body fat is stored chemical energy, so a sustained shortfall between calories consumed and calories burned draws that store down at a predictable rate. This calculator computes the timeline and plots the projected weight as a daily trajectory.

Worked example

Consider an 80 kg starting weight, a 70 kg target, and a 500-kcal daily deficit. The total fat to lose is 10 kg, which holds about 77,160 kcal. Dividing by the 500-kcal daily deficit gives roughly 154 days — about five months — under the constant-rate assumption. Consider a 176 lb starting weight, a 154 lb target, and a 500-kcal daily deficit. The total fat to lose is 22 lb, which holds about 77,000 kcal. Dividing by the 500-kcal daily deficit gives roughly 154 days — about five months — under the constant-rate assumption.

The actual figure is longer, because the constant-rate assumption is the part of the rule that does not hold over a full diet.

Why the trajectory curves

Basal metabolic rate (BMR) scales with body mass: a smaller body burns fewer calories at rest. As weight falls, BMR and therefore TDEE fall with it, so a fixed daily intake produces a steadily shrinking deficit. A plan that begins as a 700-kcal deficit may be a 400-kcal deficit by the time the goal is in reach. The simulation recomputes BMR at each step, which is why the projected weight follows a decelerating curve rather than a straight line. The Wishnofsky figure is an average that, by holding metabolism constant, overstates long-term loss.

Several other dynamics push the real trajectory away from the linear estimate:

• Water and glycogen swings. The first 1–2 weeks usually show 1–2 kg of loss, most of it water.The first 1–2 weeks usually show 2–4 lb of loss, most of it water. Fat loss becomes visible only after the second week.
• Lean-mass loss at steep deficits. At very large deficits (−1,000 kcal/day or more), 20–30% of the weight lost can come from muscle rather than fat. A protein intake of 1.6–2.2 g per kg of body weight0.7–1.0 g per pound of body weight preserves most of that muscle.
• Metabolic adaptation. Beyond the mass-driven drop in BMR, the body reduces energy expenditure further during a deficit; after about three months, TDEE is typically 100–300 kcal below the formula's prediction.

Limits of the model

The linear energy-balance model is a planning estimate, not a clinical prediction, and it is least accurate in a few cases:

• Very-low-calorie diets. Below about 1,200 kcal/day, lean-mass loss and metabolic adaptation become severe and the model substantially overestimates progress. These diets warrant medical supervision.
• Endocrine conditions. PCOS and thyroid disorders can make actual TDEE diverge sharply from the formula. When standard deficits do not produce standard results, testing is more useful than cutting calories further.
• Body recomposition. A beginner gaining muscle while losing fat may see little scale movement even as body fat drops. Tape measurements and progress photos track this better than weight alone.
• Older adults. Above age 60, BMR estimation is less accurate and protein needs rise (2.0+ g/kg0.9+ g/lb to preserve lean mass).

Setting and adjusting a realistic plan

Run in reverse, the model turns a target date into a required deficit. A goal of 10 kg in two months22 lb in two months implies a daily deficit of around 1,300 kcal — more than double the sustainable range. A sustainable rate is 0.5–1 kg per week, or a 500–1,000 kcal/day deficit1–2 lb per week, or a 500–1,000 kcal/day deficit; the timeline then follows from that deficit rather than the other way around.

A deficit can come from eating less or from raising TDEE through activity. Hall et al. (2011) found that a roughly 70% diet, 30% exercise split tends to have the best long-term adherence; the daily-intake and activity-level inputs allow both contributions to be compared.

Most diets reach a plateau between weeks 4 and 8. This is metabolic adaptation rather than failure. A diet break at maintenance for 1–2 weeks can ease the adaptive response; re-entering the current weight updates TDEE so the deficit reflects the lower body mass; and because a given workout burns fewer calories at a lower weight, holding the same training effect calls for more duration or intensity. Across the whole process, the projection is best read as a starting hypothesis, with the measured two-week trend treated as the more reliable signal.