Why does my weight go up and down every day?

Mostly water, and food still on its way through, not fat. Gaining or losing a pound of fat takes a gap of about 3,500 calories, which a single day rarely makes. So judge by the average of seven mornings, compared week to week, and let single days go.

A two-pound jump overnight feels like a disaster, and a two-pound drop feels like a win. Neither is likely to be fat. The scale weighs everything: water, the food in your gut, and fat, all in one number.

The simple version

Your weight is a stock price. It jumps around every day, and only the trend line tells you anything.

Why one day misleads

Stored carbohydrate (glycogen) is held with 3 to 4 times its weight in water. Eat more carbs and you store more of both. Eat fewer and you lose both. That’s why the first week of a diet looks spectacular and a pasta night looks like a setback. Both are mostly water.

The weekly rhythm

• In a 2014 study of 80 adults who weighed in daily, weight was highest on Sunday and Monday and fell through the week. The pattern was strongest in people who lost or held their weight.
• In a 2020 study of 1,421 people in a European weight program, the swing within a week averaged 0.35% of body weight, about two thirds of a pound at 180 pounds. Christmas added 1.35%, and the months after didn’t fully take it back.

What to do

• Weigh in every morning, after the bathroom and before eating or drinking.
• Compare this week’s average with last week’s. A Withings or Apple Health scale syncs to Gyroscope on its own, or photograph any scale’s display, and your weight trend gives a calorie-burn estimate.
• Give any change four weeks before you judge it.