The scale measures everything: water, digestive content, and glycogen. Not just fat.
Quick answer
Because the scale measures everything: water, digestive content, glycogen, and hormones. Fat cannot fluctuate by two kilos in twenty-four hours—it is physically impossible.
TL;DR
"I've gained a kilo and a half since yesterday, even though I haven't done anything different."
This statement triggers absurd decisions: skipping meals, adding a compensatory workout, or tightening a diet plan that was working perfectly well.
They all stem from a misinterpretation, and this error is easy to correct.
The math makes it obvious.
A kilo of body fat represents about 7,000 calories. To gain one in a single day, you would need to consume 7,000 calories more than your requirements—that is, two to three times a full day's worth of food, on top of what you are already eating.
You didn't do that. So, what the scale is showing you is something else.
What the scale really measures.
It measures everything inside your body at that moment, without distinction.
Water, first of all. The amount varies based on salt intake, hydration, heat, activity levels, and, for women, the hormonal cycle. A meal saltier than usual can cause you to retain several hundred grams of water for twenty-four to forty-eight hours.
Digestive content, next. What you have eaten is still in your system, and the weight of this content can reach one or two kilograms.
Glycogen, finally. This is the form in which your body stores sugar in the muscles and liver. Each gram of glycogen retains about three grams of water. A carbohydrate-rich meal after a lighter period can therefore cause your weight to go up by a kilo or more, without a single gram of fat being stored.
This is also why very low-carb diets lead to quick weight loss at the start. The drop in the first few days is largely water bound to glycogen. It is real on the scale, but it is not fat.
A stock price can fluctuate by several percent in a single day without the company's actual value changing.
No serious investor sells their shares just because the stock dipped on a Tuesday morning. You look at the trend over months, ignore the daily noise, and understand that volatility is a normal part of how the market works.
Your weight is a stock price. Your body composition is the actual value. The two do not move at the same pace.
People who weigh themselves every day without understanding this phenomenon make poor decisions.
They see an increase, conclude that something isn't working, and overcompensate: they skip a meal, add extra cardio, or cut back even more. Then the weight drops back down two days later—for reasons that have nothing to do with what they did—and they draw the opposite conclusion.
They end up reacting to noise, tightening their routine for no reason.
Conversely, weighing yourself every day is very useful if you look at the weekly average rather than the daily figure. The more data points you have, the more reliable the trend. The problem is never the frequency of weighing; it is how you interpret the results.
An isolated number means nothing. A series of numbers tells a story.
Don't draw any conclusions from a single weigh-in. Neither good, nor bad.
Compare averages, not individual data points. This week's average versus the average from three weeks ago: that is the only meaningful comparison.
Weigh yourself under the same conditions. First thing in the morning, on an empty stomach, after using the bathroom. This won't eliminate variations, but it does reduce some of the noise.
Wait three to four weeks before making any adjustments. Anything less, and you won't be able to distinguish a trend from a fluctuation.
If the scale causes you anxiety, stop weighing yourself. Body measurements, how your clothes fit, and your performance are perfectly valid indicators—and often more useful ones.
You didn't gain a kilo of fat since yesterday. You gained a kilo of something, and that something will go away.
Literature on glycogen storage and associated water retention Research on daily body weight variability and tracking via moving averages