Why does your weight swing 1-2 kg from day to day?

Short answer

It's mostly water. 1 kg of fat takes about 7,700 kcal, so it can't appear overnight; daily swings of 1-2 kg come from water bound to carbs (at least 3 g of water per 1 g of stored sugar), sweat and the menstrual cycle.

Why does your weight swing 1-2 kg from day to day?
'Up 1 kg since yesterday = 1 kg of fat gained' — does this formula hold?
When you're dieting and the morning scale reads 1 kg more than yesterday, it's easy to think you've gained fat. We checked whether an overnight change is really fat, and if not, what it is.
Morning weight moves every day
It went up and down by 0.51 kg a day on average
65 people weighed themselves every morning for 4-15 days. Even with all sweat losses replaced, their weight moved by 0.51 kg a day on average. Weight records from 1,421 people showed a weekly rhythm of 0.35%: up on weekends, down on weekdays.
Fat does not pile on 1 kg in a day
1 kg of body weight is usually counted as about 7,700 kcal
The common rule says changing body weight by 1 kg takes a surplus or deficit of about 7,700 kcal. To gain 1 kg of fat in one day, you would have to eat nearly 4 times a 2,000 kcal day on top of your normal food. An overnight change of 1-2 kg is mostly not fat.
So what is it? Carbs pull water in
Each 1 g of stored muscle sugar holds at least 3 g of water
Your body stores carbs as sugar (glycogen) in muscle and liver, and holds 3-4 times that weight in water with it. When 9 people ate carbs after exercise, each 1 g of muscle sugar was stored with at least 3 g of water. That is why the scale jumps the day after a big carb meal.
Flip side: cut carbs and water leaves first
In 10 low-carb days, 61% of the weight lost was water
6 people ate the same 800 kcal for 10 days on each diet. The low-carb diet dropped 467 g a day and the regular diet 278 g. But on low-carb, 61% of the weight lost was water and only 35% was fat. On the regular diet, fat made up 60%. The fast early drop was mostly water.
Sweat and the menstrual cycle move the scale too
8-1.5 L of sweat in 1 hour of exercise
Across 1,303 people, sweat losses ran 0.83-1.51 L per hour of exercise depending on the sport. Weight drops by that much right away and comes back when you drink. In a study following 42 menstrual cycles, weight was 0.45 kg higher during menstruation, and the extra was water outside the cells.
Bottom line: a 1-2 kg daily swing is water
Only long stretches of overeating stuck
Daily swings of 1-2 kg come from water bound to carbs, sweat and the menstrual cycle. The 0.35% gained on weekends came off on weekdays, but the 1.35% gained over Christmas had not fully come off months later. Watching the trend of averages over several weeks, not the daily number, is enough.