Weight change timeline calculator

Two answers to the same question

Every weight plan online starts from the same sentence: a pound of fat is about 3,500 calories, so a daily deficit of 500 calories removes a pound a week. It is simple, and it is a fair guide for the first weeks. Over months it runs ahead of real life, because the body you are shrinking burns less energy at every size. This calculator shows both timelines side by side so you can see how far apart they get for your own numbers, and a weekly table so you can check progress against either one.

How long will it take to lose 20 pounds?

It depends on the gap between what you eat and what you burn. The old rule says 3,500 calories per pound, so a 500-calorie daily deficit loses 1 pound a week and 20 pounds takes 20 weeks. The adaptive model on this page gives 23 weeks for the example person, because a lighter body burns less and the deficit shrinks as the weight comes off.

Is the 3,500-calorie rule true?

It is a rough, static rule of thumb: one pound of body fat holds about 3,500 calories, and the rule assumes your deficit stays the same for as long as it takes. The Lancet paper by Hall and colleagues (2011) shows why that fails over time: weight responds slowly to a change in intake, with half times of about a year, because expenditure itself changes with body weight. The rule is a reasonable guide for the first few weeks and a poor one for months.

What is a safe rate of weight loss?

The CDC says people who lose weight at a gradual, steady pace, about 1 to 2 pounds a week, are more likely to keep the weight off. The calculator compares your starting pace with that range. It does not say what is safe for you: talk with a doctor or dietitian before a large deficit, and if you have a medical condition, are pregnant, or are under 18.

Why does my weight loss slow down?

Several reasons, and this page models only one: a smaller body needs fewer calories, so the same intake leaves a smaller deficit. Water shifts, changes in muscle, in how much you move and in how carefully you track also matter and are not in the model. A plateau is not proof the math has failed, but if the scale stalls for many weeks, ask your clinician rather than cutting calories further.

What should I enter for maintenance calories?

The calories you eat in a typical day while your weight stays steady, or an estimate from the calorie calculator, which uses the Dietary Guidelines tables by age, sex and activity. The adaptive model uses it to work out your activity multiplier, your maintenance calories divided by the Mifflin–St Jeor resting figure for today's weight. A wrong maintenance number moves the whole result.

Does the calculator work for gaining weight?

Yes, with the same two models and a daily surplus. Gaining weight is not covered by the CDC pace we cite, so no pace comparison is shown. A gain is usually a mix of fat, lean mass and water, and this model treats all of it as the same 3,500 calories per pound.

Why does it say my goal is not reached?

The adaptive model finds the weight at which your new intake exactly matches what you burn. If your goal is past that weight, the intake is too close to maintenance to ever get there: for the example above with a 500-calorie deficit the model levels off near 109 pounds, and a goal of 100 pounds is never reached (109 pounds is the plateau). That is a warning that a constant intake is not a plan for a large change.

How the two models work

Old rule. Weeks = pounds to change × 3,500 ÷ (daily gap × 7). The gap is assumed to stay the same until the goal is reached, so the weight line is straight.

Adaptive model (our derivation, not a published model). Your intake is held constant: maintenance minus the gap for a loss, plus the gap for a gain. Your expenditure is your maintenance calories, scaled as your weight changes using the Mifflin–St Jeor equation, in which each kilogram of body weight adds 10 calories a day to resting energy. We divide your maintenance calories by the equation's resting figure for today's weight to get an activity multiplier, and assume it stays constant, so expenditure changes by multiplier × 10 calories per kilogram (about 7 per pound in the example). The daily weight change is the difference between intake and expenditure, divided by the energy value of a kilogram of tissue taken from the old rule (7,716 calories). That equation has a closed solution: your weight approaches a floor exponentially, with a time constant of 71 weeks in the example, and the time to a goal is that constant times the natural logarithm of how far the goal is from the floor. The Lancet model of Hall and colleagues is far more detailed: it tracks fat and lean tissue, body water and the adaptation of expenditure to intake separately. In their paper the half time of the weight response is about a year; the half time of our simplified curve is 49 weeks, the same order of magnitude, which is as much as a simplification can claim.

Worked example

A 40-year-old man, 5 ft 7 in (170 cm), weighs 180 lb and wants to reach 160 lb. He maintains his weight on 2,600 calories and eats 2,100, a 500-calorie deficit. The old rule gives 20 pounds × 3,500 ÷ 3,500 = 20 weeks. His resting energy by Mifflin–St Jeor is 1,684 calories, so his activity multiplier is 1.54. As he loses weight, his expenditure falls, and the adaptive model reaches 160 lb after 23 weeks, 3 weeks later than the rule. At week 12 the old rule says 168 lb and the adaptive model 169 lb. His starting pace of 1 lb a week is inside the CDC's 1 to 2 pounds a week. A heavier example, 250 lb to 180 lb at 3,200 maintenance calories with the same 500-calorie deficit, takes 70 weeks by the old rule and 565 by the adaptive model: the longer the road, the more the two diverge. Doubling the example deficit to 1,000 calories brings the two answers to 10 and 11 weeks, but a pace of 2 pounds a week is at the top of the CDC range and needs a clinician's advice.

Limits of this method

• One constant for every pound. Both models treat each pound of weight change as 3,500 calories. Real weight change is a mix of fat, lean tissue and water whose energy content differs, and early loss is often more water. The Lancet model tracks these separately.
• The multiplier is held constant. If you move less when you eat less, or more when you train, the real expenditure differs. Our model also ignores any extra drop in expenditure beyond what the lower body weight explains.
• Your maintenance number carries the whole result. An error of 200 calories in maintenance changes the gap by 200 calories. Use measured intake and weight history if you have them.
• A constant intake is a simplification. People rarely eat the same every week. For a research-grade plan with changing intake and activity, use the NIDDK Body Weight Planner, the web simulator that goes with the Hall model.
• Pace. The CDC's 1 to 2 pounds a week describes a gradual and steady pace for people who keep the weight off. It is not a medical clearance for any individual.

Sources and related calculators

Sources, read October 28, 2026: K. D. Hall et al., "Quantification of the effect of energy imbalance on bodyweight", The Lancet 2011;378(9793):826–837, pubmed.ncbi.nlm.nih.gov/21872751 (abstract read; the full text and model equations were not); CDC, "Losing Weight", cdc.gov/healthy-weight-growth/losing-weight (1 to 2 pounds a week); NIDDK, Body Weight Planner; resting energy: M. D. Mifflin et al., Am J Clin Nutr 1990;51:241–247. The 3,500-calorie figure is the traditional rule of thumb; we did not find a primary source we could open for its origin. This page is not endorsed by or affiliated with the NIH or the CDC. Related: calorie calculator, BMR calculator, ideal weight calculator and macro calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.