How fast can you safely lose weight?
What is a safe rate of weight loss?
About 0.5 kg per week for most people, and no more than 1 kg per week even when you are in a hurry. For weight gain the equivalents are 0.25 and 0.5 kg per week — muscle is built far more slowly than fat is lost.
Those are not arbitrary. They are the pace at which the deficit stays small enough to protect muscle and stay livable.
How a pace becomes a calorie number
A kilogram of body fat is treated as roughly 7,700 kcal, so:
0.5 kg/week works out to about 550 kcal per day below maintenance; 1 kg/week to about 1,100.
Why the app's finish date almost never arrives
Because that 7,700 figure is a static model. It assumes your energy needs stay fixed while you get lighter, and they do not — a smaller body burns less at rest, so the same intake produces a shrinking deficit as you go.
Measured loss runs roughly 27% behind what the fixed rule predicts [3]. The rule is fine for sizing a deficit. It is not fine for promising a date, and any tool that gives you a confident goal date is overselling the arithmetic.
How steep is too steep?
Two limits apply, and the higher one wins:
- Never more than 25% below maintenance. Current guidance is a 500–750 kcal/day deficit, or roughly 20–25% off maintenance, whichever is more conservative.
- Never below the intake where meeting your vitamins and minerals from food gets hard — about 1,500 kcal for men and 1,200 for women [2].
A percentage cap matters more than a flat floor because it scales with body size. A fixed minimum over-restricts a large man and under-restricts a small woman; 25% means the same thing to both.
One consequence people find surprising: if your maintenance is already at or below that floor — which happens to small, older, sedentary people — there is no safe room to cut at all. The honest advice then is more movement, not less food.
What actually goes wrong when you cut harder
Past roughly a 1,000 kcal daily deficit you start losing meaningfully more lean tissue alongside fat, and adherence collapses. Losing muscle is self-defeating: it lowers the resting burn you are relying on, so the same intake stops producing the same deficit.
Protein is the countermeasure — around 2 g per kg while dieting is what preserves muscle in a deficit [4].
What if the scale isn't moving at all?
Give any change two to three weeks before judging it. Day-to-day weight moves on water, salt, glycogen and digestion — swings of a kilogram or more mean nothing on their own. Compare weekly averages, not mornings.
If the weekly trend genuinely has not moved after three weeks at a target, the estimate was wrong for you. Every input to it — resting metabolism, activity, portion sizes — carries error, and the correct response is to adjust the target rather than to cut harder on principle.
Keep reading
How your daily calories are calculated
Mifflin-St Jeor, the activity ladder, and the 25% deficit cap.
How much protein do you actually need?
Grams per kilogram by goal, the ceilings, and what to multiply them by.
Sources
- Mifflin MD, et al. A new predictive equation for resting energy expenditure. Am J Clin Nutr 1990;51:241–247.
- National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, 1998.
- Thomas DM, et al. Time to correctly predict the amount of weight loss with dieting. J Acad Nutr Diet 2013 — measured loss falls ~27% short of the fixed 7,700 kcal/kg rule.
- Morton RW, et al. Protein supplementation and resistance-training gains. Br J Sports Med 2018;52:376–384.
- Institute of Medicine (NASEM). Dietary Reference Intakes for Energy, 2005.