How Long Does It Really Take to Lose 5, 10 or 20kg?
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It is one of the most-searched questions online, and one of the worst served by the usual arithmetic: dividing a kilo target by a supposedly fixed weekly rate. The model published by Hall and colleagues in 2011 offers a different kind of answer — not a guaranteed date, which no population model can ever provide, but a well-documented shape of trajectory that explains why “how long” never really has one single answer.
Why there is no single answer in days
Three things set the pace of any given trajectory: the size of the deficit sustained, the reported activity level, and, to a smaller degree, starting body composition. Our article on calorie deficits covers why that number already varies substantially from person to person before duration even enters the picture. A figure in days, without pinning down those three things, does not really mean anything.
The shape of the curve: it always slows down
What the published model establishes with confidence, regardless of the exact deficit chosen, is the shape of the trajectory: at a constant sustained intake, “about half” of the eventual weight change shows up within one year, and “about 95%” of it after three years — the study authors’ own words. Our article on the weight-loss plateau covers this continuous slowdown in more detail.
That curve shape has a direct consequence for “how long”: the question only really makes sense paired with a tolerance (“how close to my goal”), not a clean finish line. The final stretch — often the most eagerly awaited part — is also, structurally, the slowest to cover.
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What reported activity level changes
The table below illustrates, for the same 500 kcal/day deficit sustained by a hypothetical profile (a 100 kg, 40-year-old man, 1.80 m tall), how reported activity level shifts both the final equilibrium point and the speed to reach it:
| Activity level | Estimated equilibrium weight | Time constant (τ) |
|---|---|---|
| Sedentary (×1.2) | ≈ 67.5 kg | ≈ 780 days |
| Lightly active (×1.375) | ≈ 73.3 kg | ≈ 592 days |
| Moderately active (×1.5) | ≈ 76.5 kg | ≈ 505 days |
| Active (×1.725) | ≈ 80.8 kg | ≈ 399 days |
| Very active (×1.9) | ≈ 83.2 kg | ≈ 343 days |
The result is counter-intuitive at first glance: the most active profile settles fastest, but at a higher weight than the sedentary profile, which takes considerably longer to stabilise but ends up lower. Activity level is not just an accelerator here — it also redefines the destination.
Two worked examples: reaching a goal, or never quite getting there
The published model also illustrates a commonly overlooked point: a numeric goal is not always reachable at a given deficit, no matter how much time passes. Two hypothetical scenarios, built from the same starting profile (a 100 kg, 40-year-old man, 1.80 m, moderate activity level), make the point clearly:
| Sustained deficit | Target | Model outcome |
|---|---|---|
| −750 kcal/day | 85 kg | Target reached around day 244 (≈ 8 months); the equilibrium point sits even lower, around 66.5 kg. |
| −250 kcal/day | 70 kg | Target never reached: the trajectory settles around 87.7 kg after roughly 3.8 years, regardless of how much longer is waited. |
The second scenario is the more instructive one: this is not a matter of patience. A deficit too modest relative to the stated goal leads to a stable equilibrium above the target, and nothing changes that fact beyond a certain number of years — increasing the deficit, not waiting longer, is what would bring the target within reach.
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Why losing 20kg does not simply take “twice as long” as losing 10kg
Because pace keeps slowing continuously, a total goal twice as large does not translate into a simply doubled duration — the final stretch, always the slowest, weighs proportionally more for a large total than a small one. It is the same logic, in reverse, that explains why the 3,500-calorie-per-pound rule overstates real weight loss by more and more as the time horizon stretches out.
What is reasonable to take from this
Without claiming a guaranteed individual date — a population model can never offer one, by construction — these benchmarks help calibrate realistic expectations: most of the journey within a year, nearly all of it within three, at a pace that depends heavily on the deficit actually sustained and on real, not just reported, activity level. Our own calorie calculator gives a concrete starting point for estimating your own deficit; a simulator that projects a full trajectory from this same published model is in development on this site: weight-loss timeline tool.
In short: “how long” depends on too many individual factors for one universal answer, but the shape of the journey is well documented — and it always slows down toward the end, never the other way around.
Frequently asked questions about how long weight loss takes
How long does it actually take to lose 10kg?
There is no single answer in days: it depends on the deficit maintained, the reported activity level, and starting weight. What the published model does offer is the shape of the trajectory — a pace that keeps slowing continuously, never a straight line — rather than a single guaranteed date.
Does doubling the calorie deficit cut the time in half?
No. Because energy expenditure itself falls as weight is lost, the relationship between deficit size and duration is not proportional. A bigger deficit does speed things up, but by less than a straight-line calculation would suggest — and it also moves the new equilibrium point, which changes the picture as much as speed does.
Can a goal simply never be reached, no matter how long you wait?
Yes, and this is a commonly overlooked point. If the sustained deficit is too small relative to the target, the trajectory settles at a higher weight than the goal, no matter how much time passes — waiting longer does not change that plateau, because it is an equilibrium point, not a temporary pause.
Does losing 20kg take exactly twice as long as losing 10kg?
Usually longer than double. Because pace keeps slowing continuously along the curve, a total change twice as large pushes the point where 95% of it is complete out by more than twice as far — most of the slowdown is concentrated at the end of the journey, not the start.
Does reported activity level really change the timeline?
Yes, substantially. At an identical deficit, the model predicts a different equilibrium point — reached at a different speed — depending on reported activity level. A very active profile settles faster than a sedentary one, but the sedentary profile ends up, over time, at a lower stabilised weight.
Why do the first two weeks go faster than the rest?
Because a water-and-glycogen compartment, separate from body fat, empties quickly at the start of a deficit and is not represented in the pacing described here. That fast start does not set the pace for the rest of the journey, which slows down once that compartment has settled.
What is the point of a target date, if nothing is guaranteed?
To set realistic expectations rather than a promise. The published model gives solid benchmarks on the general shape of the journey — most of it within a year, nearly all of it within three — useful for calibrating expectations, even though it can never replace tracking your own actual trajectory.
This tool is informational and does not constitute a diagnosis or medical advice. Consult a healthcare professional.