You eat well all week, step on the scale, and it says a kilo more than yesterday. Nothing is wrong. Body weight is not just fat: it is also water, the food still travelling through your gut, and stored carbohydrate, and all of those move around from day to day. Public health sources describe a typical adult's weight shifting by roughly 1–2 kg over a few days, with no change in body fat at all.
This chart shows 28 days for an imaginary person. The grey line is their real weight, which you control. The dots are what the scale shows each morning. Turn up the wobble and watch how the dots scatter, then switch on the 7-day average.
One reading is a data point, not a verdict. Look at the average over a week or more.
NHS dietetic services suggest weighing at the same time of day on the same scale, ideally first thing in the morning after using the toilet and before eating or drinking. They also say once or twice a week is usually enough, and to look for change over several weeks rather than from one day to the next. If you do weigh daily, compare weekly averages, not single days.
It also helps to remember that weight is only one thing to track. Energy, sleep, how clothes fit, how far you can walk and how you feel about food all tell you something the scale cannot. For some people, especially anyone who has had a difficult relationship with food or body weight, it is fine and sensible to skip the scale altogether.
This guide is general information, not medical advice. Day-to-day swings are normal, but see a doctor if your weight changes a lot without you trying, if you notice sudden swelling in your legs, face or belly, or if you are pregnant, take medication that affects fluid or blood sugar, or have or have had an eating disorder. Do not cut your food below recommended minimums because a number went up; a dietitian can help you plan safely.