How much weight can you safely lose in a month or a week

0.5-1% of body weight a week

You can safely lose 0.5-1% of your body weight a week. At 70 kg that's 0.35-0.7 kg a week, or 1.5-3 kg a month, and at 100 kg it's 0.5-1 kg a week and 2-4 kg a month, because the more excess weight you carry, the faster it can come off without harm. Next to adverts promising "minus 10 kg in a month" these numbers look modest, but this is the pace at which you lose fat rather than muscle and water, and you can keep the diet up for months without breaking.

The first week: why the scale looks so good

For almost everyone the first week in a deficit looks like magic, minus 1.5-2.5 kg, and then the pace drops sharply and people are disappointed to be doing exactly the same things while the scale barely moves. Nothing is broken, the first kilos were simply mostly water. Your muscles and liver hold about 400-500 g of glycogen, a carbohydrate reserve for quick energy, and each gram of it binds about 3 g of water. When calories and carbs go down, the body spends glycogen first and the water leaves with it, so the scale shows minus 1.5-2 kg even though only 0.3-0.5 kg of fat went that week. As far back as 1992 a British team led by Stephen Kreitzman called this the "illusion of easy weight loss".

It's the same story with the extra 1.5 kg that appears after a single pizza night. No one can store that much fat in one evening, so what you're seeing is glycogen and water returning, and two or three days of normal eating will take them off again. Treat the first week as a bonus and judge your real pace by weeks two to six, using weekly averages rather than any single morning.

How to calculate your timeline from the deficit

The old rule of thumb puts about 7,700 kcal in a kilogram of fat, so losing one means eating 7,700 kcal less than you burn, whether that takes a week, a fortnight or a month.

If you want to lose 5 kg and keep a 500 kcal daily deficit, that's 5 × 7,700 / 500 = 77 days, or about 11 weeks. For other goals and deficits the timelines look like this:

The real timeline is usually 10-20% longer. The 7,700 kcal rule is an approximation: as you lose weight, your body burns less because there's less of it, and the same diet gives a smaller and smaller deficit. That's why it's worth recalculating your target every 4-5 kg. Researchers at the US National Institutes of Health (NIH) even built a mathematical model that accounts for this: by their estimate, every kilogram lost reduces the deficit on the same diet by about 24 kcal a day.

Which deficit to choose? 300-500 kcal is gentle and comfortable for most people. 700 kcal suits people with a higher weight and a high target: for a man with a 2,800 kcal target that's 25%, but for a woman with a 1,700 kcal target it's already 41%, which is too much. You can find your target in the calculator, and how to turn it into a weight-loss number is covered in How many calories a day do you need to lose weight.

Minus 10 kg: the real timeline is 3.5 months or more

"How to lose 10 kg in a month" is one of the most popular searches. The honest answer: if we're talking about fat, you can't.

Let's do the maths. 10 kg of fat is 77,000 kcal. To lose it in 30 days you'd need a deficit of over 2,500 kcal a day. For most women, total daily energy use is lower than that. So even a complete fast wouldn't remove 10 kg of fat in a month.

So where do the "minus 10 in a month" stories come from? Three sources:

• water and glycogen - 2-3 kg at the start of any strict diet;
• gut contents - with very little food that's another 1-2 kg;
• muscle - a lot of it is lost on a starvation diet, and that's the worst part of the "result".

A month after such a diet ends, water and glycogen come back, muscle doesn't, and metabolism is lower. Hence the famous "yo-yo effect".

A realistic timeline for losing 10 kg of fat is 3.5 to 6 months with a 500-700 kcal deficit, nearer 3.5-4 months if you weigh 100 kg or more and nearer six months at 70-75 kg. It's less dramatic, but six months from now you'll be 10 kg lighter and stay that way.

How much you can lose in a week

After the first "water" week you can go by these numbers:

Weight hardly ever drops evenly from one week to the next, minus 0.8, then plus 0.2, then minus 0.6, because water swings of 0.5-1.5 kg mask the real fat loss. It's fairer to judge your pace by averages over two or three weeks, which is what the Progress screen and weight tracker in MyFoodMate AI are for, and how to read them is explained in the guide Weekly averages.

When the pace is too fast

Fast isn't the same as good. If after the first week you keep losing more than 1% of your weight a week for several weeks, pay attention to how you feel. Constant hunger, thoughts of food and irritability, weakness and dizziness on the stairs, hair loss, brittle nails and dry skin that appear after two or three months, menstrual changes in women, weekend binges followed by the urge to "punish" yourself with an even bigger deficit - all of this means the deficit is too big. Gritting your teeth won't help; add 200-300 kcal a day, mostly from protein, and your pace will slow a little but you'll get to your goal. How to hold a deficit without breaking is covered in A calorie deficit without relapses.

Sources

1. Hall KD et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011.
2. Hall KD. What is the required energy deficit per unit weight loss? International Journal of Obesity, 2008.
3. Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. American Journal of Clinical Nutrition, 1992.
4. Wishnofsky M. Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition, 1958.

MyFoodMate editorial team

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