How Long Does It Take to Lose 1 kg of Body Weight?
For most adults trying to lose weight gradually, about one to two weeks is a reasonable general estimate for losing 1 kg of body weight.
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Current NHS advice is to aim for about 0.5 to 1 kg per week. [1]
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NICE states that 0.5 to 1 kg per week is generally considered safe and sustainable, while emphasising that the target should be personalised. [2]
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Therefore:
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At 0.5 kg per week → around 2 weeks to lose 1 kg.
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At 1 kg per week → around 1 week to lose 1 kg.
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However, the number on the scales does not fall in a perfectly straight line. Someone may appear to lose 1 kg in two or three days because of water loss, or may take more than two weeks to show a 1 kg reduction on the scales even while losing body fat. [3,4]
Losing 1 kg on the scales is not the same as losing 1 kg of fat
Your total body weight includes much more than body fat. It includes:
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Body fat
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Muscle and other lean tissue
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Water
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Stored carbohydrate
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Food and drink in the digestive system
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Bowel contents [3–5]
Therefore, when the scales fall by 1 kg, this does not necessarily mean that 1 kg of body fat has been lost. During weight loss, the body normally loses a mixture of fat, water and some lean tissue, and the proportions change over time. [3,5]
This distinction is particularly important during the first few days or weeks of a change in diet. Early weight loss can be disproportionately affected by changes in stored carbohydrate and water. [3,4]
For example, carbohydrate is stored in the body as glycogen, and each gram of glycogen is stored with at least about 3 grams of water. When glycogen stores fall, some of that associated water is also lost. [6]
That is why someone may lose more than 1 kg in their first week without having lost 1 kg of body fat. [4,6]
As a person becomes lighter, their energy requirements fall, and the body also makes physiological adjustments to reduced food intake and weight loss. [3,9]
Water can completely hide short-term fat loss
A person could lose some body fat during a week but temporarily hold additional water, causing the scales to show:
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No weight change
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A very small loss
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Occasionally even a temporary increase [3,11]
This does not break the principle of energy balance. It simply means that the scales measure total body mass rather than body fat alone. [3]
Carbohydrate intake can change water weight
Changing carbohydrate intake alters glycogen stores, and glycogen is stored with substantial amounts of water. [6]
This is one reason some diets cause a striking reduction in scale weight at the beginning. Part of that early change may be water rather than fat. [4,6]
Menstrual-cycle changes can affect the scales
In a 2023 study, body weight was about 0.45 kg higher during menstruation compared with the first week of the cycle, with the difference largely explained by increased extracellular water. [11]
Constipation and bowel contents can affect scale weight
What is inside the digestive system contributes to total body weight. Average stool output is measurable and varies substantially between individuals and diets; constipation involves less frequent or incomplete emptying, so retained bowel contents can affect short-term scale readings without representing an increase in body fat. [13,14]
For this reason, a single weigh-in is much less useful than the trend across several weeks.
What determines how quickly someone loses 1 kg?
There is no universal answer because several factors alter either energy expenditure, food intake or the amount of water shown on the scales.
People beginning with greater body mass and greater body-fat stores can have a different weight-loss response to the same change in calorie intake. Hall and colleagues' validated models found that people with greater starting body fat generally lose more weight for the same sustained reduction in energy intake. [3]
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Age, sex and muscle mass
Resting energy requirements vary substantially between people. Body size and the amount of lean tissue are particularly important contributors, meaning that two people eating the same number of calories may not have the same calorie deficit. [15]
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Physical activity
Movement contributes to total energy expenditure. Changes in everyday activity as well as planned exercise can therefore change the size of an energy deficit. [3]
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Diet
The amount of energy eaten is fundamental, but changing the proportions of carbohydrate, fat and protein can also produce short-term differences in water, glycogen and therefore scale weight even when calorie intake is similar. [3]
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Sleep
Poor or insufficient sleep can influence eating behaviour. In a randomised trial of 80 adults with overweight who usually slept less than 6.5 hours, extending sleep reduced average energy intake by approximately 270 kcal per day during the short study period. This does not mean sleep automatically causes weight loss, but it illustrates how sleep can affect energy intake. [16]
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Medicines
Medicines can alter appetite, digestion, activity, fluid balance or metabolism, so they can alter the speed of scale-weight change. Weight-management medicines are one obvious example. [17,18]
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Medical conditions
Conditions that affect appetite, physical activity, digestion, fluid balance or energy expenditure may change the rate of weight loss. This is one reason weight-loss targets should be personalised rather than calculated from a fixed formula. NICE specifically recommends tailoring weight-management treatment to the individual. [2]
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Menstrual-cycle fluid changes, sodium intake and constipation
All can change what appears on the scales without representing equivalent changes in body fat. [11–14]
What about semaglutide and tirzepatide?
Medicines such as Semaglutide and Tirzepatide can increase the amount of weight people lose over time when used appropriately alongside dietary and physical-activity measures. NICE recommends both medicines for eligible adults under defined circumstances. [17,18]
However, they do not create a predictable “number of days per kilogram”.
For context:
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In the STEP 1 trial, people receiving semaglutide 2.4 mg lost an average 14.9% of starting body weight over 68 weeks, compared with 2.4% with placebo. [19]
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In SURMOUNT-1, tirzepatide produced substantial weight loss over 72 weeks, with average reductions of around 19.5% with 10 mg and 20.9% with 15 mg. [20]
Those figures are averages over large groups and long periods. They should not be divided by the number of weeks to predict how quickly an individual will lose their first or next kilogram. Weight loss with these medicines changes over time, doses are increased gradually, people start at different weights, and individual responses differ considerably. [19,20]
So someone taking semaglutide or tirzepatide may lose 1 kg faster or slower than another person taking the same medicine. The appropriate measure is the person's overall weight trend and clinical response, not a fixed number of days per kilogram. [17–20]
What the evidence means in practice
The commonly used 0.5–1 kg per week target remains supported by current UK public-health guidance: NHS advice gives this as a target, and NICE describes this range as generally safe and sustainable while recommending that it is tailored to the person. [1,2]
References
[1] NHS. Tips to help you lose weight. Current NHS guidance advises aiming to lose 1–2 lb, or 0.5–1 kg, per week.
[2] National Institute for Health and Care Excellence (NICE). A guide for prescribing medicines to manage overweight and obesity. 2025. NICE states that weight loss of 0.5–1 kg per week is generally considered safe and sustainable and should be tailored to the individual.
[3] Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet. 2011;378:826–837. The paper develops a dynamic model of weight change and explains why static calorie-to-weight rules overpredict long-term weight loss.
[4] Thomas DM, Martin CK, Lettieri S, et al. Time to correctly predict the amount of weight loss with dieting. Journal of the Academy of Nutrition and Dietetics. 2014. Reviews the limitations of the static 3,500-kcal rule and the rapid early phase of weight loss.
[5] Heymsfield SB, Gonzalez MCC, Shen W, Redman L, Thomas D. Weight loss composition is one-fourth fat-free mass: a critical review and critique of this widely cited rule. Obesity Reviews. 2014.
[6] Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutrition Reviews. 2018;76:243–259. Glycogen is stored with substantial amounts of associated water.
[7] Wishnofsky M. Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition. 1958;6(5):542–546. This is the historical source of the approximately 3,500 kcal per pound rule.
[8] Hall KD, Chow CC. Why is the 3500 kcal per pound weight loss rule wrong? International Journal of Obesity. 2013;37:1614.
[9] Most J, Redman LM. Impact of calorie restriction on energy metabolism in humans. Experimental Gerontology. Review of the metabolic changes accompanying calorie restriction and weight loss.
[10] NICE. Overweight and obesity management, NG246: Physical activity and diet. Published January 2025. NICE recommends low-energy diets of 800–1,200 kcal/day only as part of an appropriate multicomponent weight-management strategy.
[11] Kanellakis S, et al. Changes in body weight and body composition during the menstrual cycle. American Journal of Human Biology. 2023. The study found approximately 0.45 kg higher body weight during menstruation, primarily associated with extracellular fluid.
[12] Rakova N, et al. Increased salt consumption induces body water conservation and decreases fluid intake. Journal of Clinical Investigation. 2017.
[13] Cummings JH, Bingham SA, Heaton KW, Eastwood MA. Fecal weight, colon cancer risk, and dietary intake of nonstarch polysaccharides. Gastroenterology. 1992;103:1783–1789.
[14] Bharucha AE, Lacy BE. Mechanisms, evaluation, and management of chronic constipation. Gastroenterology. 2020.
[15] McMurray RG, Soares J, Caspersen CJ, McCurdy T. Examining variations of resting metabolic rate of adults. Medicine & Science in Sports & Exercise. 2014.
[16] Tasali E, Wroblewski K, Kahn E, Kilkus J, Schoeller DA. Effect of sleep extension on objectively assessed energy intake among adults with overweight in real-life settings: a randomized clinical trial. JAMA Internal Medicine. 2022;182:365–374.
[17] NICE. Semaglutide for managing overweight and obesity, TA875. NICE recommends semaglutide alongside a reduced-calorie diet and increased physical activity in eligible adults.
[18] NICE. Tirzepatide for managing overweight and obesity, TA1026. Published December 2024. NICE recommends tirzepatide alongside a reduced-calorie diet and increased physical activity in eligible adults.
[19] Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine. 2021;384:989–1002.
[20] Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity. New England Journal of Medicine. 2022;387:205–216.
Disclaimer:
This content was prepared with the assistance of AI and has been reviewed, edited and approved by Hassan Bhatti (Pharmacist Independent Prescriber), before publication. It is intended for general information only and does not replace individual medical advice
Reviewed : 31.08.2026
