Why does my weight go up and down every day?

Short answer: day-to-day swings of 1 to 2 kg are normal, and they’re almost never fat. They’re water, salt, carbs, and the food and drink still in your gut. To gain one kilo of actual fat in a day, you’d have to eat about 7,700 calories more than you burn. That’s not what happened.

What really moves the scale

More than half of your body is water. The amount you carry changes all the time, and the scale can’t tell water from fat.

Carbs

Your body stores carbs in your muscles and liver, and each gram is stored with about three to four grams of water (Kreitzman et al., 1992). A pasta dinner, a pizza night or a day of birthday cake fills those stores back up, and the water comes with it. That can be a kilo or more by the next morning.

It works the other way too. It’s why low-carb diets look so fast in the first week: much of that early drop is water leaving with the stored carbs.

Salt

Your body keeps the salt in your blood at a steady level. Eat a salty meal (takeaway, restaurant food, chips) and it holds on to extra water to dilute it. You’ll often see it the next morning, and it’s gone again a day or two later.

Food and drink still inside you

Whatever you ate and drank yesterday weighs something until it’s digested and out. A big dinner late at night shows up on the scale in the morning. So does a day with less fibre or less time in the bathroom.

Your cycle

Many women hold more water in the days before and at the start of their period. Hormones in the second half of the cycle make the body keep more water and salt. In one study that followed women for a year (White et al., 2011), fluid retention peaked on the first day of the period. The scale can be up by a kilo or more that week without any change in fat.

Training

After a hard or new workout, your muscles hold extra water while they repair. Start lifting or running and your weight can sit still or tick up for a week or two, even while you’re losing fat.

Weekends

Weight follows the week. Studies tracking people’s daily weight (Orsama et al., 2014; Turicchi et al., 2020) find it goes up over the weekend and comes back down during the week, a swing of about 0.35% of body weight. For a 70 kg person, that’s about 250 g. It’s normal, as long as Monday’s bump is gone by Friday.

Holidays are the exception. In the same research, weight rose over Christmas and wasn’t fully lost again in the months after. Here’s how to handle the big meals.

How to weigh yourself so it makes sense

Weighing often is fine. A review of the research (Zheng et al., 2015) found that regular weighing was linked to more weight loss, and not to more anxiety or low mood. The trick is to read it right:

1. Same conditions every time: in the morning, after the bathroom, before eating or drinking, in the same clothes (or none), on the same scale.
2. Look at the weekly average, not single days. One high morning means nothing. Three weeks of averages tell the truth.
3. Women: compare the same point in your cycle, month to month, rather than week to week.
4. Expect the jumps. After a salty dinner, a carb-heavy day or a hard workout, a higher number is normal.

When a gain isn’t just water

Normal water swings come and go within days. See a doctor if your weight goes up quickly and stays up along with swelling in your ankles, legs or belly, shortness of breath, or if your ankles leave a dent when you press them. That kind of water retention can be a sign of a medical problem.

Seeing the trend, not the noise

One number on one morning tells you very little. The picture you want is the trend, next to what you ate and how you slept.

That’s what Fabee keeps for you. Log your meals by just saying them, and ask “why am I up this morning?” It looks at what you actually ate the day before. If the trend itself has stopped moving for weeks, read why you might not be losing weight, and check your calorie target.