Why does my weight fluctuate from day to day?

A kilo overnight isn't fat

To gain a kilo of fat you'd need to eat about 7,700 kcal more than you burn⁶. For most people that's three or four days' worth of food on top of what they normally eat. So when the scales show an extra kilo after a big Friday night, it's almost never fat. It's water and food that hasn't made its way through yet.

Body weight responds slowly to how much you eat. Researchers have calculated that it takes around a year for the body to get halfway to a new balance after a lasting change in diet⁷. What you see from day to day is mostly noise.

Carbs hold water

Your body stores carbohydrate as glycogen in your muscles and liver. And glycogen isn't stored dry. Each gram is stored with at least 3 g of water¹.

Eat more pasta, bread or cake than usual and your body tops up its stores, with the water coming along for the ride. It's also why the first few days of a low carb diet show a big drop on the scales. It's mostly water leaving as the stores empty, and it comes back when you eat carbs again.

Salt moves water around

Plenty of people find their weight goes up after a salty meal. It's more complicated than it sounds, though. In a controlled study, extra salt shifted fluid into the blood without raising the total amount of water in the body⁵. So salt can change where water sits, but the research doesn't agree on how much it moves the scales.

Hormones and your period

For women, weight also moves with the cycle. A study of 42 women weighed them twice a week and found weight was on average 0.45 kg higher during their period than in the first week of the cycle. Almost all of that difference was water outside the cells².

Another study followed 62 women for a year. The feeling of retaining fluid was strongest on the first day of the period and lowest in the middle of the first half of the cycle³. So it's worth comparing with the same point in your last cycle, rather than with last week.

Days of the week, and the loo

Many people are heaviest on Sunday and Monday. A Finnish study of 80 adults and more than 4,600 weigh ins found weight typically rises from Saturday and falls again from Tuesday⁴. The researchers concluded that these swings between weekend and weekdays should be seen as normal rather than as signs of weight gain⁴.

Then there's the obvious bit. A litre of water weighs a kilo. Food and drink you've had weigh something until they've passed through. So whether you've been to the loo before stepping on the scales makes a difference.

Why the trend is what counts

Take an average over several days and the ups and downs cancel each other out. A salty dinner, a pasta heavy day or a heavy Monday matter much less once they're averaged with the other days. What's left is the direction.

• Weigh yourself under the same conditions: in the morning, after the loo, before you eat or drink.
• Look at the average over a week, not at any single day.
• Compare week with week. If the average is lower than two weeks ago, you're heading the right way.
• Don't let one jump knock you off course. Look at the trend again in three days.

Wondering how often to weigh yourself? Read should I weigh myself every day?.