How long it takes to lose 20 pounds on a 500-calorie deficit

Twenty weeks, if the arithmetic held — and it does not quite. Here is the table for every deficit from 250 to 1,000 kcal, why the last few pounds take longer than the first, and what 500 calories below maintenance actually means on a plate.

Sep 19, 2026 · By Calcelate Team

The textbook answer is twenty weeks, and it comes from two numbers. A pound of body fat is conventionally treated as 3,500 kcal, so a 500 kcal daily deficit is 3,500 kcal a week, which is one pound a week, which is twenty weeks for twenty pounds.

That table is the honest arithmetic, and it is where every calculator on the internet starts, including the calorie deficit calculator here. It is also, in a specific and predictable way, optimistic. Understanding how it goes wrong is more useful than the table itself, because it tells you what to do when the scale stops moving in month three.

Why the last five pounds take longer than the first five

The 3,500 kcal figure assumes you burn the same number of calories in week 20 as in week 1. You do not.

A smaller body costs less to run. It takes less energy to maintain, less to heat, and less to carry up a flight of stairs. So as the weight comes off, maintenance falls, and a deficit that was 500 kcal against your starting maintenance is perhaps 400 kcal against your current one. The gap narrows every week. Kevin Hall and colleagues quantified this in The Lancet in 2011 and showed that the simple rule substantially overstates long-term loss — the longer the diet runs, the further the prediction drifts from reality. The US National Institutes of Health built the Body Weight Planner on that more realistic model.

That straight line is the prediction, not the outcome. Draw the real thing on top of it and the two overlap for the first month or two, then the real one bends upward and away as maintenance falls with the weight.

The practical version: expect the first weeks to go roughly as the table says and the later ones to run slower. Twenty weeks becomes twenty-three or twenty-five. Nothing has gone wrong when that happens, and it is not a reason to cut calories further. It is the expected shape of the thing.

There is also a second, noisier effect that has nothing to do with fat. Body weight swings by two or three pounds from water alone — glycogen, salt, hydration, where you are in your cycle, whether you slept. A weekly loss of one pound is smaller than the daily noise, which is why a single weigh-in tells you almost nothing and a fortnight of daily weigh-ins averaged together tells you almost everything.

Five hundred below what?

The table above is about a deficit. The number you actually have to live with is a calorie target, and that depends entirely on what you burn — which varies far more between people than most diet advice admits.

Same 500 kcal deficit, four completely different days of eating. The woman in the first row has to plan carefully to be well fed at 1,204 kcal; the man in the last row has room for four proper meals at 2,614 kcal and may find it hard to eat little enough by accident. Any article that tells you to eat 1,500 calories without asking your size, sex, age and activity is guessing.

Maintenance here comes from the Mifflin-St Jeor equation (Am J Clin Nutr, 1990) with an activity multiplier — the same method the TDEE calculator uses. It is the best-validated general-purpose estimate available, and it can still be off by 200 kcal in either direction for any individual, because metabolic rate varies between people of identical size. Treat the output as a starting hypothesis. Eat at it for two weeks, weigh yourself daily, take the average of week two against the average of week one, and adjust from what happened rather than from what was predicted.

Choosing a pace

The US Centers for Disease Control recommends one to two pounds a week, noting that “people who lose weight at a gradual, steady pace are more likely to keep the weight off than people who lose weight quicker.” On the table above, that is the 500 and 1,000 kcal rows, and the gap between them is the whole decision.

The 250 kcal row deserves more respect than it gets. Forty weeks sounds interminable, but it is a deficit small enough that you may not consistently notice it, which means you may actually complete it. The 1,000 kcal row looks efficient and mostly is not: at that depth most people are hungry enough to break the diet, and a larger share of what is lost comes from muscle rather than fat. The CDC also points out that the benefits start early — a 5% reduction, which is ten pounds for a 200-pound person, is already enough to move blood pressure and blood sugar.

If the goal has a real deadline behind it, put it in the calorie deficit calculator as a date rather than a deficit and let it tell you what that deadline demands. It is a fast way to discover that a goal is unreasonable while there is still time to move it.

Two things that make the same deficit easier

Eat enough protein. In a deficit the body will take some of the shortfall from muscle unless you give it a reason not to. Adequate protein plus some resistance training is what keeps most of the loss coming from fat, and it has the side effect of being the most filling macronutrient per calorie — the same 1,700 kcal built around protein leaves you considerably less hungry than the same 1,700 kcal of anything else. The protein calculator gives a target for your weight and goal.

Do not eat back your exercise. The activity multiplier in the maintenance figure already includes your training. Adding a workout and then eating a post-workout snack to “replace” it is double counting, and fitness trackers are famously generous about calories burned. If you want exercise to accelerate the loss, let it deepen the deficit rather than fund a reward.

When the number is not the point

A calorie target is a planning tool, not a prescription, and there are situations where it is the wrong tool entirely. If you are pregnant or breastfeeding, under 18, over 65, managing diabetes or another condition, taking medication that affects appetite or weight, or have any history of disordered eating, the right next step is a conversation with a doctor or dietitian, not a calculator. The same goes for anything below roughly 1,500 kcal for men or 1,200 for women — that is the floor these formulas are generally considered safe above, and going under it belongs under supervision.

For everyone else, the plan is boringly simple. Get a maintenance estimate, take 500 off it, hold it for a fortnight, and then believe the scale over the arithmetic. Twenty weeks is a good guess. Twenty-four is a good outcome.