How Long Does It Take to Lose 20 Pounds?

20 weeks — and the model that says so is known to run fast

20 pounds at a 500-calorie daily deficit:

140 days. 20 weeks. About 4.6 months.

That's the 3,500-calorie rule, and it's the arithmetic behind nearly every calculator that answers this question.

It's also documented as wrong, in one direction: it overestimates. This one tells you so, which is a strange thing for a calculator to do and the most useful thing on the page.

The arithmetic, and where "500 a day" comes from

One pound of body fat is treated as 3,500 calories.

Days = pounds × 3,500 ÷ daily deficit

500 × 7 = 3,500 = one pound a week.

That's the entire derivation of the most-repeated number in dieting. It comes from an arithmetic convenience, not from anything about human physiology.

Daily deficit	Per week	20 lb takes
250	0.5 lb	40 weeks
500	1 lb	20 weeks
1,000	2 lb	10 weeks

Why it overestimates

Because it's a static model applied to a system that moves.

The rule assumes your deficit stays the same size for the whole period. It doesn't. As you lose weight your body gets smaller, and a smaller body burns fewer calories at rest and in motion — so the same food intake produces a progressively smaller deficit.

The target moves as you approach it.

Kevin Hall and colleagues published this directly in the International Journal of Obesity, and the criticism is mainstream in obesity research rather than a fringe objection. The accepted alternative is a dynamic model that simulates the changing state instead of assuming a fixed one.

Adherence drifts too, and some early loss is water rather than fat. But the shrinking-deficit problem is the structural one.

The error is small at first and grows

This is the part worth internalising:

• Over a few weeks, the straight line is roughly right and the difference hardly matters
• Over six months, it isn't — real loss flattens well before this arithmetic does

And here's why that matters more than the numbers: the bend arrives at almost exactly the point where people conclude the diet has stopped working.

It hasn't. The model was optimistic. Expecting the plateau in advance is the difference between adjusting and quitting — and that's a bigger determinant of the outcome than any deficit you choose.

So what is it good for?

Comparing options, and getting the shape of an answer.

Whether a 250, 500 or 750 deficit is worth the difference to you is a genuinely useful question — and the ratio between those answers is far more reliable than any one of them. Halving the deficit really does roughly double the time, and that relationship holds even where the absolute numbers don't.

It's also good as a reality check running the other way. If a goal needs a deficit you'd never sustain, week one is a much better time to find that out than week fifteen.

What it's not good for is picking a date and putting it in a diary.

A more actionable framing

The tool shows the deficit split across three meals — 167 calories each, on the defaults.

That's often a more useful way to hold it than a daily total. "Roughly 170 fewer calories at each meal" is a thing you can act on at the moment of the decision. "500 a day" is a number you reconcile at bedtime, when it's too late to do anything about it.

Is a bigger deficit better?

Not straightforwardly — and this calculator can't answer it, because the answer isn't arithmetic.

Larger deficits are harder to sustain, and an aggressive deficit that lasts three weeks loses to a moderate one that lasts a year. There are also real considerations around muscle loss, nutrition and health that no formula sees.

A 1,000-calorie deficit is aggressive enough that it's worth discussing with someone qualified rather than picking from a dropdown.

Print the tracking sheet — it logs predicted against actual each week, so the gap between the model and your body becomes visible instead of demoralising.

Sources: Hall et al. — Why is the 3500 kcal per pound weight loss rule wrong? · Response, Int. Journal of Obesity

This is arithmetic, not medical advice. It knows nothing about your health, medication or history. Talk to a doctor or a registered dietitian.