Fat loss · 6 min read
How Long Will It Take to Lose the Weight?
Everyone wants the date. The arithmetic that produces it is simple, and it is also systematically optimistic: the standard 7,700 calories per kilogram rule assumes a body that never adapts. Here is the honest calculation, with the correction applied.
The basic calculation
A kilogram of body fat stores roughly 7,700 calories (about 3,500 per pound). So:
Worked example. Losing 10 kg on a 500 kcal daily deficit: (10 × 7,700) ÷ (500 × 7) = 77,000 ÷ 3,500 = 22 weeks, about five months. On a 750 kcal deficit it drops to 15 weeks; on 250 it stretches to 44.
Why that answer is too optimistic
The 7,700 rule assumes your calorie expenditure stays constant while you shrink. It does not. Three things change as you lose weight:
- You get smaller. A lighter body burns fewer calories at rest and costs less to move. A 10 kg loss typically drops TDEE by 150 to 250 kcal per day on that basis alone.
- You move less without noticing. Non-exercise activity falls in a deficit: fewer spontaneous steps, less fidgeting, more sitting. This is often the largest single part of the shortfall; see the NEAT guide.
- Adaptive thermogenesis. A modest additional reduction in energy expenditure beyond what the smaller body predicts.
Researchers have criticised the fixed 3,500-calorie rule explicitly for this reason, showing that it overpredicts long-term weight loss because it ignores the dynamic fall in expenditure.1,2 Dynamic models that account for the change, such as the validated simulation behind the NIH Body Weight Planner, predict considerably slower loss over long horizons.3
Practical correction: add roughly 20 to 30% to whatever the simple formula gives you for goals beyond about 5 kg. That is what the second bar in the chart above represents.
Timeline table
| Goal | At 0.25 kg/week | At 0.5 kg/week | At 0.75 kg/week |
|---|---|---|---|
| 5 kg | ~24 weeks | ~12 weeks | ~8 weeks |
| 10 kg | ~50 weeks | ~25 weeks | ~17 weeks |
| 15 kg | ~75 weeks | ~37 weeks | ~25 weeks |
| 20 kg | ~100 weeks | ~50 weeks | ~33 weeks |
These figures already include the adjustment above. They also assume you hit your deficit most days, which almost nobody does perfectly, so treat them as the optimistic end of realistic.
How fast should you go?
The sensible rate is expressed as a percentage of body weight, not a fixed number of kilograms, because losing 0.5 kg a week is trivial at 120 kg and aggressive at 55 kg.
| Rate | Who it suits | Trade-off |
|---|---|---|
| 0.25–0.5% of body weight per week | Already lean, or prioritising muscle | Slow, but very little lean mass lost |
| 0.5–1.0% per week | Most people, most of the time | The usual sweet spot |
| Above 1.0% per week | Higher starting body fat, short phases only | Rising risk of lean mass loss and poor adherence |
Rate matters for composition, not just speed. In elite athletes, a slower rate (about 0.7% of body weight per week) preserved lean mass and improved strength outcomes compared with a faster rate producing the same total loss.4 Losing weight faster does not only feel worse; it changes what you lose. The deficit sizing guide covers how to convert these rates into a calorie number.
The parts of the timeline nobody puts in the calculation
Weeks that do not go to plan. Holidays, illness, birthdays, work trips. Over a 25-week diet you will lose 3 to 5 weeks of progress to real life. Plan for that rather than treating each one as a failure.
Plateaus. Expect at least one stretch of two to three weeks where the weekly average does not move even though you are compliant. That is usually water masking fat loss, not fat loss stopping. Read why weight fluctuates daily before you react to one.
Diet breaks. Planned maintenance weeks add calendar time but improve adherence and may reduce the drop in resting expenditure. Building in a maintenance week every 8 to 12 weeks is time well spent on any diet longer than about three months.
Putting it together. An 85 kg man wants to reach 75 kg. At 0.6% of body weight per week (about 0.5 kg), the simple maths gives 20 weeks. With the dynamic correction that becomes roughly 25 weeks. Add two planned maintenance weeks and an allowance for three disrupted weeks: call it 30 weeks, about seven months. That is a realistic answer, and it is the one that will actually happen.
The timeline starts with your maintenance calories. Get that number, subtract your deficit, and the arithmetic below does the rest.
Calculate your TDEE and deficit →Frequently asked questions
How long does it take to lose 10 kg?
At a 500 kcal daily deficit, roughly 22 weeks by the textbook calculation and about 25 to 28 weeks in practice once the fall in energy expenditure is accounted for. At a 750 kcal deficit, expect around 17 to 19 weeks.
Is the 3,500 calories per pound rule wrong?
It is a reasonable approximation for the first few kilograms and increasingly wrong after that. It assumes energy expenditure stays constant, but expenditure falls as you get lighter and move less, so the rule overpredicts long-term loss. Adding 20 to 30% to its estimate gets you much closer.
What is a safe rate of weight loss?
0.5 to 1.0% of body weight per week suits most people. Faster is possible at higher starting body fat but raises the risk of losing lean mass and of the diet becoming unsustainable. Slower is the better choice for anyone already fairly lean.
Why did my weight loss stop after two months?
Usually a combination of a smaller body needing fewer calories, unnoticed reductions in daily movement, and portion sizes creeping up. Recalculate your maintenance at your current weight and log intake accurately for a week before assuming your metabolism is at fault.
References
- Hall KD. What is the required energy deficit per unit weight loss? Int J Obes. 2008;32(3):573-576. doi:10.1038/sj.ijo.0803720
- Thomas DM, et al. Can a weight loss of one pound per week be achieved with a 3500-kcal deficit? Commentary on a commonly accepted rule. Int J Obes. 2013;37(12):1611-1613. doi:10.1038/ijo.2013.51
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. doi:10.1016/S0140-6736(11)60812-X
- Garthe I, et al. Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes. Int J Sport Nutr Exerc Metab. 2011;21(2):97-104. doi:10.1123/ijsnem.21.2.97