Water weight: why your weight jumps, and how to lose it

You weigh two pounds more than you did yesterday. You didn’t eat 7,000 extra calories, which is about what two pounds of fat would take. So it isn’t fat. It’s water, plus the food still on its way through.

Water weight is the water your body holds on top of its usual amount. It moves with what you ate, how salty it was, how hard you trained and, for many women, where you are in your cycle. Most of it leaves on its own within a few days. The trick is not to read anything into one morning.

How much does weight fluctuate day to day?

More than most people expect. In a 1974 study, researchers weighed 140 young soldiers every day. About a third of the day-to-day changes were half a kilogram (about a pound) or more. One in 16 was a kilogram (2.2 lb) or more.

There’s a weekly rhythm too. In a 2020 study of more than 1,400 people in a weight-maintenance program, weight was highest on Monday and lowest on Friday, a swing of about 0.35%. For a 180 lb person, that’s about 0.6 lb that shows up over the weekend and is gone by Friday. Christmas added 1.35% on average, and some of it was still there in March.

Within a day, you weigh the least in the morning, after the bathroom and before you eat or drink. That’s why it’s the best time to weigh in.

What causes water weight?

Carbs

Your body stores carbs as glycogen in your muscles and liver, and it stores water along with it. In a 1970 study, 19 people did hard exercise and then ate only protein and fat for three days. Then they ate plenty of carbs for four days. They gained 2.4 kg (5.3 lb) in those four days, and 2.2 liters of that was water. The researchers worked out that each gram of glycogen holds 3 to 4 grams of water.

A review of glycogen puts a typical adult’s store at about 600 grams. Add its water, and that’s roughly 5 to 6½ lb (2.4 to 3 kg) that can come and go with your carbs. It’s why the scale jumps after a pasta dinner, and why it drops fast when you cut carbs.

Salt

A salty meal can make you hold water, but people vary more than most articles admit. In a 2018 study, 12 healthy men weighed about 2.5 kg (5½ lb) more after eight days on a high-salt diet than after eight days on a low-salt one. But in a 2000 study of 32 men on a metabolic ward, even a very high salt intake didn’t raise their body water or their weight.

Your kidneys adjust to a change in salt within a few days. In one study, they matched a new, higher salt intake within three days. So water from a salty weekend usually clears by midweek.

Food in your gut

Not all of it is water. Yesterday’s meals are still on their way through, and they weigh what they weigh. A big dinner adds to the morning scale before any water is involved.

Hard or new training

A hard workout, especially one your muscles aren’t used to, makes them swell while they repair. In a 1998 study, the arm muscles of 11 untrained women swelled after an unfamiliar workout. The swelling peaked on day four and was gone by days 7 to 11. That study measured the size of the arm, not the scale, but it shows how long the swelling lasts. If your weight goes up in the first week of a new program, this is a big part of why.

Your cycle

Many women hold more water around their period. In a 2011 study that followed 62 women for a year, fluid retention peaked on the first day of the period and was lowest about a week later. So compare your weight with the same point in your last cycle, not just with last week.

Creatine

How much creatine shows on the scale depends on how you take it. A review of the research says a loading phase of 20 grams a day for five to seven days typically adds 1 to 3 kg (2 to 6½ lb), mostly water.

Most people skip loading and take 3 to 5 grams a day. That fills your muscles more slowly, over about four weeks, and any water comes on slowly too. In a 2011 study, about 2.3 grams a day for six weeks didn’t change body weight or body water at all. Either way, it’s not fat, and it’s not a reason to stop.

How to lose water weight

Most of the time, you don’t need to do anything. Water weight leaves on its own once whatever caused it has passed. A few things help, and a few only look like they do.

• Eat a steady amount of salt. Swinging between bland days and salty ones keeps the scale bouncing. After a salty weekend, give it a few days.
• Eat fruit and vegetables. They bring potassium, which helps your kidneys clear sodium. Most of that research is about blood pressure, not the scale, so don’t expect an overnight drop.
• Cut carbs if you want a fast drop, and know what you’re losing. In a 1976 study, people on an 800-calorie low-carb diet lost about a pound a day, against about 0.6 lb a day on the same calories with carbs. They lost the same amount of fat on both. Most of the difference was water, and it started coming back when they moved on to a 1,200-calorie diet with carbs. A 2015 study found the same pattern. Cutting carbs lost more weight, and cutting fat lost more fat.
• Keep drinking normally. You’ll read that extra water flushes water weight out. I couldn’t find a good study that shows it.
• Don’t sweat it off. A sauna or a sweat suit pulls water out for a few hours, and it comes back as soon as you drink. A sauna may be worth it for other reasons. In a Finnish study that followed 2,315 middle-aged men for about 20 years, those who used one four to seven times a week were less likely to die of heart disease than those who went once a week. It’s an observational study, so it can’t prove the sauna did it. Just don’t use one to move the scale.
• Leave water pills to your doctor. Diuretics are a prescription drug for a reason.

Water weight or fat?

Speed tells you. A pound of body fat holds about 3,500 calories, so gaining one means eating about 3,500 calories more than you burn. A two-pound jump overnight can’t be fat. Neither can a two-pound drop.

Fat changes slowly, and it shows up as a trend over weeks. If your weight is still up a week or two later, after the salt and the carbs and the hard workout have passed, that change is real.

When water weight isn’t normal

Some fluid retention is a medical problem, not a salty dinner. MedlinePlus, from the US National Library of Medicine, says to contact your doctor right away if swelling in your feet, ankles or legs

• is new or worse in one leg only
• is red or warm to the touch
• comes with a fever
• gets worse and you have heart or kidney disease, or you have liver disease and new swelling
• comes on suddenly, or is more than mild, while you’re pregnant

Call 911 if the swelling comes with shortness of breath or chest pain.

People with heart failure are often told to call their doctor if their weight goes up more than 2 to 3 lb in a day, or 5 lb in a week. If that’s you, follow your own doctor’s numbers.

How Fit Shapes handles water weight

Fit Shapes doesn’t react to any single weigh-in. It turns them into a Trend Weight that moves 10% of the way toward each day’s reading. A two-pound jump after a salty dinner moves your trend about a fifth of a pound. If the water is gone the next day, the trend barely noticed. If the weight stays, the trend follows it.

Your weekly check-in measures what you burn over about the last six weeks, from your log and that trend. A few days of water barely move it.

So weigh in most mornings, the same way each time, and let the trend do the rest. The best smart scale is the one that syncs covers how to weigh in, and which scales send each weigh-in to Fit Shapes for you.

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