Why the scale jumps 1–2 kg in a day — and it isn't fat
A 1–2 kg difference between morning weigh-ins on different days worries a lot of people tracking their weight, even though it's physically impossible to gain or lose that much actual fat mass in 24 hours — that would require a surplus or deficit of 7,000–15,000 calories in a single day. The real cause of the swing is water, not fat.
A lot of unrelated factors affect water retention: how many carbs and how much salt were eaten the day before (both cause temporary water retention), a woman's menstrual cycle phase, stress levels, how well someone slept the night before, and even how hard yesterday's workout was — muscle micro-damage from a hard session temporarily causes local fluid retention as part of repair.
Practical takeaway: daily weigh-ins can be a useful tool, but what's worth interpreting isn't a single number, it's the 1–2 week trend averaged across several weigh-ins — a sharp jump on any single day almost never means something went wrong with fat loss or muscle gain, it's just ordinary data noise that smooths out over a longer view.