Why your weight goes up and down from day to day

29 October 2026

The scale can show a kilo more than yesterday without a gram of fat having changed. Most of what moves from one morning to the next is water, and the weight of food still on its way through.

How much does weight normally fluctuate from day to day?

More than most people expect. One man weighed himself every morning, under the same conditions, for 9,521 days. From one day to the next his weight moved by a typical 0.5%, around 0.4 kg at 80 kg, and that stayed true across the whole record.

In practice, half a kilo to a kilo between two mornings is ordinary. Over a few days, when what you eat changes a lot, more carbohydrate, more salt, a big meal out, it can reach two kilos. None of that is fat: a kilo of body fat in a day would take something like 7,700 kcal on top of what you burn.

Why can you weigh a kilo more overnight?

Food and drink still on its way through

A litre of water weighs a kilo, and a meal weighs what it weighs until it has been digested and has left the body. A late dinner, or a big glass of water before bed, is still on the scale in the morning.

Glycogen and the water it holds

Your muscles and liver store carbohydrate as glycogen: about 600 g in an adult, most of it in muscle. Each gram is stored with at least 3 g of water. Eat more carbohydrate than usual and the store fills, water and all; eat less and it empties. That can move the scale by a kilo or two within days, and it is why the first week of eating differently often shows a large change that then slows down.

Salt

With more salt, the kidneys hold on to more water to keep the body's salt and water in balance. How much of that shows on the scale varies. In one long study, extra salt made the body conserve water; in a tightly controlled ward study, even very high salt intakes changed neither total body water nor body weight. A salty meal can be part of a heavier morning, but less reliably than its reputation suggests.

Your cycle

Many women hold more water around their period. In a study that weighed 42 women twice a week through their cycle, weight was on average about 0.45 kg higher during menstruation than in the first week of the cycle, almost all of it water outside the cells. The average hides a lot of difference between one woman and the next.

The weekend

In 80 adults who weighed themselves daily, weight tended to be highest on Sunday and Monday and to come down through the week. A heavier Monday is a pattern, not a verdict.

Why does a trend line say more than one weigh-in?

Because the noise averages out and the real change does not. Water goes up and down around your actual weight; a change in body tissue is slow and goes one way. An average over many weigh-ins cancels most of the first and keeps the second.

Here is a made-up week in which nothing real changes, as the scale and a trend line see it:

The scale moved 1.2 kg between its highest and lowest morning; the trend moved 0.1 kg. Tuesday's jump, perhaps a salty dinner, nudges the line instead of rewriting it. The price is patience: when something really changes, the trend shows it over a week or two, not overnight.

When should you weigh yourself?

Under the same conditions each time: in the morning, after the toilet, before eating or drinking, in the same clothes or none. Daily weigh-ins give a trend the most to work with, but weekly works too; the line just takes longer to form. If weighing every day makes you anxious, weigh less often.

How does Unpound draw the trend line?

On Unpound's Weight screen, the dots are what the scale said and the line is the trend. Each weigh-in pulls the line a tenth of the way towards itself for every day since the one before: an exponentially smoothed moving average. Weigh in every morning and a heavy morning moves the line by a tenth of the difference; weigh in after a week away and the new weigh-in pulls it about halfway.

You can weigh in when you like, or let Apple Health bring your weigh-ins across. The same line is what your measured maintenance reads its change from, so a kilo of water on one morning does not move that figure by hundreds of kilocalories. Unpound makes no projections of where your weight is heading.

Sources

1. Schneditz D, Hofmann P, Krenn S, et al. Day-to-day variability in euvolemic body mass. Ren Fail 2023;45(2):2273421. doi:10.1080/0886022X.2023.2273421 · PubMed
2. Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev 2018;76(4):243–259. doi:10.1093/nutrit/nuy001 · PubMed
3. Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. Eur J Appl Physiol 2015;115(9):1919–1926. doi:10.1007/s00421-015-3175-z · PubMed
4. Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr 1992;56(1 Suppl):292S–293S. doi:10.1093/ajcn/56.1.292S · PubMed
5. Rakova N, Kitada K, Lerchl K, et al. Increased salt consumption induces body water conservation and decreases fluid intake. J Clin Invest 2017;127(5):1932–1943. doi:10.1172/JCI88530 · PubMed
6. Heer M, Baisch F, Kropp J, Gerzer R, Drummer C. High dietary sodium chloride consumption may not induce body fluid retention in humans. Am J Physiol Renal Physiol 2000;278(4):F585–F595. doi:10.1152/ajprenal.2000.278.4.F585 · PubMed
7. Kanellakis S, Skoufas E, Simitsopoulou E, et al. Changes in body weight and body composition during the menstrual cycle. Am J Hum Biol 2023;35(11):e23951. doi:10.1002/ajhb.23951 · PubMed
8. Orsama AL, Mattila E, Ermes M, et al. Weight rhythms: weight increases during weekends and decreases during weekdays. Obes Facts 2014;7(1):36–47. doi:10.1159/000356147 · PubMed
9. Walker J. Signal and noise. In: The Hacker's Diet. On the exponentially smoothed moving average Unpound's trend follows. fourmilab.ch

More guides

• Maintenance calories: a formula, or measured

  What a TDEE formula can tell you, and how to measure your own.

• Counting calories, and keeping it up

  When to log, what to repeat, and what to do about a missed day.

Unpound is a calorie diary for iPhone that draws this trend line through your weigh-ins.