You stuck to your plan for a week. On Monday the scale says 84.1 kg, on Tuesday 85.6 kg. One and a half kilos overnight, without you doing anything differently.
The reassuring part first: one kilo of body fat holds roughly 7000 kilocalories. To gain one and a half kilos of fat overnight you would have had to eat about 10,500 kilocalories above your expenditure the day before. That does not happen. What changed is almost always water.
The four big causes
1. Salt and water
Sodium binds water in the body. A salty dinner, a restaurant meal, ready made food or even a single bag of crisps shifts your water balance by several hundred grams to more than a kilo. The effect appears within hours and disappears on its own over one to three days, as soon as your kidneys have excreted the surplus.
The same works in the other direction: on a day with little salt and plenty of movement you lose water and appear to be "doing better" the next morning, without anything having changed about your fat mass.
2. Glycogen
Carbohydrates are stored as glycogen in muscles and liver, and every gram of glycogen binds about three grams of water. Anyone cutting carbohydrates therefore loses one to two kilos very quickly in the first few days, consisting almost entirely of glycogen and the water attached to it. That is the reason for the spectacular start of every low carb diet, and for the equally spectacular jump upwards when you eat normally again.
This is where the most common frustration comes from: a single pasta evening after two weeks of restriction can push the number up by more than a kilo the next morning. Nothing has been lost.
3. Gut content
For most people, 24 to 72 hours pass between a meal and its excretion. The content of your digestive tract weighs along during that time, varies with the amount of fibre and is not the same as body mass. Irregular digestion while travelling reliably produces swings on the scale too.
4. Hormones and cycle
In women, cycle related water retention of one to two kilos in the second half of the cycle is common, peaking shortly before menstruation. Looking at several months of history reveals a regular wave pattern layered on top of the actual trend. Stress, poor sleep and unusually hard training also raise the water share short term, via cortisol and muscle repair.
How much is "normal"?
As a rule of thumb, a daily swing of about 1 to 2 kilograms, more for larger bodies and less for smaller ones. Anyone weighing in only once a week cannot tell that spread apart from real change: a measurement that happens to land on a high day, compared with one on a low day, can make a week of real loss look like a week of gain.
That is exactly why frequent weighing plus smoothing beats infrequent weighing: more measurements mean more noise that cancels itself out.
What you can do about it
Always weigh under the same conditions. In the morning, after getting up, after the first trip to the bathroom, before breakfast, without clothes. That does not eliminate the fluctuation, but it keeps the conditions constant so the rest actually becomes comparable.
Measure daily rather than weekly. It sounds like more pressure but is the opposite: seeing the number every day gets you used to the spread and stops you from judging each individual value. Research on self monitoring tends to find better rather than worse outcomes for daily weighing, as long as the focus stays on the trajectory.
Do not judge the daily value, judge the trend. A moving average over 14 days turns the zigzag into a calm line. How exactly that works is in Trend weight instead of the daily value.
Do not delete outliers. The day after the restaurant belongs in the series just as much as any other. Logging only the "good" values gives you a beautiful curve and a wrong one.
The right question
Instead of "why am I heavier today than yesterday?" the useful question is: "where does my average this week sit compared to last week?"
A trend weight of 84.2 kg this week against 84.6 kg last week is real progress of 400 grams, regardless of whether the scale showed 83.4 or 85.3 this morning. That number can be interpreted. The daily value cannot.
Trendcurve works out exactly that: every entry flows into the smoothed trend, and what you see is the 7-day change of that value. The bad morning stays visible, but it no longer decides anything.