The scale jumped and you're worried it's fat. This check works out the least energy it would take for all of the change to be fat tissue, and compares it with how much people have actually been measured eating. Then it lists what else weighs that much, with sizes from studies.

How the check works

Energy in fat tissue. Pure fat holds about 9440 kcal per kg (39.5 MJ/kg). Fat tissue also holds water and cells: in six men it was 54–85% fat. The check uses the leanest figure, 0.54 × 9440 ≈ 5100 kcal per kg. The popular rule of 7700 kcal per kg (3500 kcal per pound) would make every number bigger. The lower figure makes "it's not fat" harder to claim, so that's the one used.
Energy needed. Change in kg × 5100 is the least energy all of it would take, rounded to 10 kcal. Divided by the days, it's the energy needed each day.
Compared with what people have been measured eating. For a gain:
- more than 3113 kcal a day, the largest meal measured in a lab: you didn't gain that much fat;
- more than 954 kcal a day, the extra food volunteers ate every day of an 8-week overfeeding study: probably not that much fat;
- anything less: part of it could be fat, but water swings are this size too.
For a loss, the comparisons come from a hospital study of six men with obesity: their bodies used up about 2800 kcal a day with no food at all (more than that: you didn't lose that much fat), and about 2000 kcal a day on 800 kcal a day (more than that: probably not that much fat).
Your extra food, if you enter it. At most 95% of an energy surplus has been measured to end up stored. So the most fat tissue it can add is extra kcal × 0.95 ÷ 5100, and the rest of the change is water, glycogen and food.

Enter numbers without thousands separators. Both 1.5 and 1,5 work. Nothing you type leaves your browser.

Worked example: 2 kg overnight

You weigh yourself the morning after a salty dinner, and the scale says 2 kg more than yesterday.

2 kg × 5100 kcal per kg = 10200 kcal more than your body used, all in one day.
The largest meal measured in a lab was 3113 kcal: men ate pizza until they couldn't take another bite. Even that meal, on top of a normal day, could add at most 3113 × 0.95 ÷ 5100 ≈ 0.58 kg of fat tissue.
So the check reads: You didn't gain 2 kg of fat. With the popular 7700 kcal per kg rule, the same 2 kg would take 15400 kcal.

The list below shows what else could weigh that much, with the sizes measured in studies.

What else moves the scale

Stored carbohydrate. Each gram of glycogen is stored with 2.7–4 g of water. Refilling empty stores, about 500 g of glycogen, adds about 1.9–2.5 kg over several days, water included. What one high-carb day adds hasn't been measured.
Salt. After about 27 g of salt in a day, most of it given as a drip in a lab, salt-resistant people weighed 0.16 kg less and salt-sensitive people 0.82 kg more. A week of high salt weighed 0.45 kg more than a week of low salt. When men stayed on a fixed high salt intake, body water and weight did not rise at all.
Food and drink. They weigh what they weigh until they're digested. A whole cheese pizza (727 g cooked) plus a litre of beer (about 1 kg) is about 1.7 kg.
Menstrual cycle. Studies range from no change before a period to about 0.3 kg more in the days a period starts than in the second half of the cycle. The evidence is thin.
Creatine. A loading dose holds about 0.5–1 litre of extra fluid in the first days.
Weekends and holidays. Weight runs about 0.35% higher after weekends and drifts back during the week. Over the whole winter holiday season, adults gained 0.37 kg on average.
Early weight loss. In 10-day periods on 800 kcal a day, water was 37% of the weight lost on a mixed diet and 61% on a low-carbohydrate diet. Fat loss was nearly the same on both, about 165 g a day.

What the check can't tell you

It measures nothing about your body. It bounds fat by energy alone, with the assumptions that favour fat: the least energy per kg of fat tissue and the most of a surplus stored. The comparisons come from small studies, and your extra kcal are an estimate. It can say that a change is too big or too fast to be all fat. It can't say how much of a small change is fat. Morning weigh-ins averaged over a week show that better than any single day.

How many calories are in a kilo of fat?

Pure fat holds about 9440 kcal per kg. Fat tissue is fat plus water and cells, 54–85% fat in the six men measured, so a kilo of it holds roughly 5100 to 8000 kcal. The popular 7700 kcal per kg rule sits inside that range. In an 8-week overfeeding study, it took about 14500 extra kcal for each kilo of fat actually gained, because the body burns part of any surplus.

How long does water weight last after a salty meal?

In studies, the extra weight from a very large salt load showed up within a day, and a week of high salt left people 0.45 kg heavier than a week of low salt. How fast it goes after one salty dinner hasn't been measured, so your next few mornings are the best guide.

Why did I lose so much in my first week?

Much of it is water. In 10-day periods on 800 kcal a day, water made up 37% of the weight lost on a mixed diet and 61% on a low-carbohydrate diet, while fat loss was about the same on both. With no food at all, the same men lost 751 g a day, and 243 g of it was fat.

Sources

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To see what the next week of weigh-ins adds up to, use the weight trend calculator.