Weight loss calculator: what a sustained deficit implies

Weight change a sustained 500 kcal deficit allows over 12 weeksworked example

2.6 kg – 5.4 kg

Straight-line figure (3,500 kcal per pound, 1958): 5.4 kgThe number most calculators show on its own
Eventual change this deficit implies (Hall 2011): 22.7 kgApproached gradually, not reached on a date
Share of that eventual change reached in 12 weeks: 12%

Two published methods, two answers. The high end is the 1958 arithmetic that treats every kilogram as costing the same and energy needs as fixed; the low end applies the finding by Hall and colleagues that a sustained change delivers about half of its eventual effect in the first year. The honest answer is the span, and it is wide.

Realistic range

2.6 kg to 5.4 kg

The 3,500 kcal per pound rule assumes two things that are not true: that every kilogram lost carries the same energy, and that the body needs as much energy at a lighter weight as it did at a heavier one. Hall (2008) showed the first depends on how much body fat a person starts with, and Hall and colleagues (2011) found that a sustained change in intake produces only about half of its eventual weight change in the first year. A straight-line figure is therefore a ceiling on what the arithmetic allows, not a forecast of what happens.

Daily calorie deficit (kcal/day)The gap between what you use and what you eat. The deficit calculator works it out from an intake figure.Over how longA duration, not a target date. The published anchors stop at three years, and so does this tool.

Current weight (optional)Only used to express the range as a weight rather than a change. Nothing is stored or sent.

What both ends of this range assume

The same deficit holds every single day for the whole period, weekends and holidays included.
The deficit is the real one. Self-reported eating and exercise are both commonly out by a few hundred kcal a day, in opposite directions.
Nothing else changes - no illness, no medication change, no large shift in daily movement.
Some of any early change is water and glycogen rather than fat, and it returns when eating does.
The low end applies the Hall 2011 rule of thumb over this period; the high end is the straight-line 3,500 kcal per pound figure, capped at the eventual change that rule implies.

A sustained 500 kcal deficit across every period the published anchors cover
Period	Range	Straight line alone	Share of eventual change
4 weeks	0.9 kg – 1.8 kg	1.8 kg	4%
12 weeks	2.6 kg – 5.4 kg	5.4 kg	12%
6 months	5.7 kg – 11.8 kg	11.8 kg	25%
1 year	11.3 kg – 22.7 kg	23.7 kg	50%
2 years	16.4 kg – 22.7 kg	47.3 kg	73%
3 years	21.5 kg – 22.7 kg	71 kg	95%

Straight line from Wishnofsky 1958. Range low end from the Hall 2011 anchors (about half the eventual change in a year, about 95% by three years), interpolated linearly and held at 95% thereafter; high end is the straight line capped at that eventual change. Notice how far apart the two columns drift with time — that gap is the reputation the 3,500 rule has.

To work out the deficit itself from an intake figure, use the calorie deficit calculator. The maintenance figure it works against comes from the TDEE calculator, and it carries a ±10% band of its own.

This projects weight change from a daily calorie deficit as a range, not a single figure and not a date. The high end is the straight-line 3,500 kcal per pound arithmetic; the low end applies the 2011 finding by Hall and colleagues that about half of an eventual change arrives in the first year.

The 3,500 kcal per pound rule (Wishnofsky) — 1 lb of body weight = 3,500 kcal, or about 7,716 kcal per kg. Verified 2026-07-30 against Wishnofsky, Am J Clin Nutr 1958 (Rests on the tissue lost being mostly fat of constant composition, and on energy needs staying the same at a lighter weight. Hall 2008 and Hall 2011 show both assumptions fail, so a straight-line figure is a ceiling rather than a forecast.)
Required energy deficit per unit weight loss (Hall) — the energy density of lost weight is not constant: more initial body fat means a larger cumulative deficit per kg lost. Verified 2026-07-30 against Hall, Int J Obes 2008 (A modelling result checked against published weight-loss data in lean and obese subjects. The 3,500 kcal per pound rule approximately matched only above roughly 30 kg of initial body fat, and overestimated the deficit required below that.)
Eventual weight change per sustained change in intake (Hall) — about 1 kg of eventual change per 100 kJ/day, or 10 kcal/day per pound; about half of it within 1 year and about 95% within 3 years. Verified 2026-07-30 against Hall et al., The Lancet 2011 (A rule of thumb for an average overweight adult, not a personal trajectory. This site interpolates linearly between the published 1-year and 3-year anchors and holds at 95% beyond three years rather than extrapolating past what was published.)
Resting energy expenditure (Mifflin-St Jeor) — BMR (kcal/day) = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) + 5 (male) or - 161 (female). Verified 2026-07-30 against Mifflin et al., Am J Clin Nutr 1990 (Derived from 498 healthy adults aged 19 to 78. It landed within 10% of measured resting metabolic rate for more people than any other equation in the Frankenfield 2005 review, which also means it missed by more than 10% for some of them.)
Activity multipliers for total daily energy expenditureconvention — TDEE = BMR x activity multiplier (1.2, 1.375, 1.55, 1.725 or 1.9). Verified 2026-07-30 against FAO/WHO/UNU, Human energy requirements (2004) - the documented PAL framework this ladder approximates (No primary source publishes this ladder and no error term was ever attached to it. Choosing one rung rather than its neighbour moves the answer by roughly 200 to 300 kcal/day, which makes it the largest single source of disagreement between two calculators given the same body.)

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Frequently asked questions

Why does this show a range instead of a number?

Because the two published methods disagree, and by a factor of about two at one year. The 3,500 kcal per pound rule from 1958 assumes a fixed energy cost per kilogram and unchanging energy needs; Hall showed both assumptions fail. Showing one figure would mean picking a side quietly.

Why is there no target date?

A date reads as a promise, and nothing in the arithmetic supports one. The published trajectory is gradual and asymptotic rather than linear, individual variation is wide, and a calendar date would convert an estimate with a factor-of-two spread into something that looks like a commitment.

Is the 3,500 calories per pound rule wrong?

It is a rule of thumb whose limits are documented. Hall found in 2008 that the required deficit per unit of weight lost depends on how much body fat someone starts with: the rule roughly matched above about 30 kg of initial body fat and overestimated the required deficit below that.

Why does the projection flatten out?

Because energy needs fall as body mass falls, so a fixed intake becomes a smaller and smaller gap. The Hall rule of thumb puts the eventual change at about 1 kg per 100 kJ a day of sustained change, with half of it in the first year, which is why the high end here is capped rather than extended.

What would make my own result differ from both ends of this range?

Every assumption listed under the result: whether the deficit was really held every day, whether intake and activity were measured or estimated, illness, medication changes, and the water and glycogen shifts in the first weeks that are not fat and come back.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Calorie deficit calculatorWork backwards: see what a given intake actually leaves as a gap against your maintenance estimate.TDEE calculatorThe projection is only as good as the maintenance figure under it, and that figure carries a 10 percent band.

Related calculators

Calorie deficit calculatorEnergy balance TDEE calculatorMetabolism Protein calculatorNutrition targets

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/weight-loss-calculator