On Friday the scale said 85.9 kg (189.4 lb), on Monday 87.2 kg (192.2 lb). You grilled over the weekend, ate bread and drank a few beers, and now you wonder whether that is fat already.
The short answer: water moves in days, fat moves in weeks. A gain of more than a kilo overnight or over a weekend is almost always water, because fat only builds up through a lasting calorie surplus, and one weekend does not deliver that. Your trend follows the slow part. It shows you whether your body fat is actually changing.
What moves water on the scale
Two mechanisms explain a large share of the fast swings: salt and glycogen.
Salt makes the body hold on to water. In a study by Rakova et al. 2017, the body excreted less free water when salt intake was higher. A salty dinner therefore shows up as a gain the next morning and fades again after a few days.
Glycogen is the storage form of carbohydrate in muscle and liver. It is stored together with water, roughly three to four grams of water per gram of glycogen (Kreitzman et al. 1992). If you eat a large plate of pasta after a low carb week, the stores refill and the water comes with them. When you cut carbohydrates, the same process runs backwards and your weight drops within a few days. Kreitzman and colleagues describe glycogen as a driver of short-term weight swings for that reason.
Gut content, your cycle and unfamiliar training add to this. They are covered in Why your weight fluctuates every day.
What fat needs to show up on the scale
One kilo of body fat corresponds to roughly 7,700 kilocalories (Hall 2008). The figure is a rule of thumb and varies with body composition, but it is good enough for a plausibility check.
| Water | Fat | |
|---|---|---|
| Time scale | Hours to a few days | Weeks |
| Typical triggers | salty food, carbohydrates, cycle, unfamiliar training | a lasting surplus or a lasting deficit |
| Course in the raw value | jumps and returns | stays and shifts the line |
| Course in the trend | small dent or notch | a slope over several weeks |
Two examples with numbers
The weekend. Friday 85.9 kg, Monday 87.2 kg: plus 1.3 kg (2.9 lb). As fat, that would be about 10,000 kcal of surplus in three days, roughly 3,300 kcal above your expenditure, every day. Hardly anyone eats that much, not even at a long barbecue with beer and bread. On Thursday the scale shows 86.1 kg and the value sits close to your line again. Your trend weight moved from 85.8 to 85.9 and back to 85.8 kg in that time.
The carbohydrate stop. You drop bread and pasta. Monday 86.0 kg (189.6 lb), Friday 84.6 kg (186.5 lb): minus 1.4 kg (3.1 lb) in four days. As fat, that would be about 10,800 kcal of deficit, around 2,700 kcal a day. Almost nobody sustains that. What emptied here are the glycogen stores and the water bound to them. In the week after, the value falls by only 0.3 kg (0.7 lb), for example. That equals a deficit of about 330 kcal a day, and that is the order of magnitude in which fat changes. Once you allow carbohydrates again, part of the water returns and your raw value rises without any fat gain.
How to read the question in your trend
The daily value cannot answer the question, because water and fat sit in the same number and you cannot separate them on the display. A body fat reading from a bathroom scale helps little as well: it estimates from electrical resistance in the body and reacts to water content itself.
Trend weight separates them by time. It smooths short swings and follows what stays. How the smoothing works is explained in Trend weight and the moving average.
Trendcurve shows the 7-day change of your trend weight and the weekly rate, both in the free version. A water spike appears as a dot beside the line, and the line itself barely moves. Fat loss moves the line, week after week. If you see a decline of about 0.3 to 0.5 kg (0.7 to 1.1 lb) per week over several weeks, that is fat, even when single days show the opposite.
A marker records what was different on a day. Under "Settings & more" → "Experiments and markers" you create a marker of the type nutrition, for example "Barbecue night", and it appears as a line in the chart. Creating markers is free. The before and after analysis of a marker is Pro. If one reading clearly falls out of line, you can ignore the measurement: it stays visible in the history but no longer counts toward the trend.
Plateau detection is Pro as well. It reports a plateau only when the rate stays under 0.1 kg per week for several days, not after one flat day or a weekend with extra water.
If swelling persists, appears in one spot only, or you gain quickly for no clear reason, talk to a doctor. Water retention can also come from illness or medication. Trendcurve is not a medical device.
How to tell water from fat in daily life
These four questions help when the raw value jumps:
- How fast? More than a kilo in one or two days is water. Check it against 7,700 kcal per kilo: that much fat would need a surplus you did not eat.
- Was there a trigger? Salty food, a carbohydrate surge, unfamiliar training or a trip usually explain a swing completely.
- Does the value come back? If it sits near the line again after a few days, it was water. If it stays up, look at the trend.
- Did the line move? Fat shows only when the trend runs in the same direction for two to three weeks. How fast it moves in a deficit is covered in How fast should you lose weight.
Keep weighing every morning under the same conditions, log the value, and judge no earlier than after two weeks whether the line has shifted.