People who regularly weigh themselves may notice that the number on the scale changes, depending on when they do it. In fact, a person’s body weight can change within a single day.

The National Institutes of Health (NIH) and the Centers for Disease Control and Prevention (CDC) state that consistent self-weighing is an effective weight management strategy.

Here, we discuss the factors that influence body weight, when a person should weigh themselves for the most accurate results, and other useful body measurements to keep in mind.

a woman standing on scales to check for Weight fluctuationShare on Pinterest
Energy balance and water retention are possible causes of weight fluctuation.

The average adult’s body weight fluctuates between 1–2 kilograms (kg) or 2.2–4.4 pounds (lb) over a few days.

Numerous factors influence a person’s body weight. Individuals have no control over some factors, such as their genetics, age, and gender.

However, people have control of many lifestyle factors that contribute to weight fluctuations.

Energy balance

Energy balance directly impacts a person’s body weight. It refers to the number of calories a person consumes compared with the number of calories they burn.

A person can lose weight if they use up more calories than they consume. Conversely, people can gain weight if they consume more calories without increasing their physical activity.

Water retention

Water intake can contribute to short-term weight fluctuations. On average, water makes up 60% of an adult’s body weight. Excess water can build up in the body, leading to bloating and weight changes.

Factors that can lead to water retention include:

  • Dehydration: As counterintuitive as it may seem, drinking more water may actually reduce water retention. When a person does not drink enough water, their body stores water to prevent dehydration.
  • Sodium intake: Sodium, or salt, is an essential compound that helps transport nutrients, regulate blood pressure, and balance water content in the body. However, too much salt can lead to water retention and weight gain.
  • Glycogen storage: The body turns carbohydrates from food into glycogen molecules, which provide energy for the cells. The body stores excess glycogen in the muscles, liver, and fat cells.
  • Urine and stool weight: Drinking a glass of water or eating a full meal can increase a person’s body weight. However, a person’s weight will go down again once their body eliminates the food and drink. The average adult passes 128 grams of stool per day.
  • Exercise: People who recently started exercising may experience weight fluctuations as their body builds muscle and adjusts its water output to accommodate the increased physical demand.

Other factors that can lead to weight fluctuation include:

  • genetics
  • hormonal changes, such as menstruation
  • alcohol consumption
  • medical conditions, such as hypothyroidism or diabetes
  • medications

Consistency is vital when it comes to monitoring body weight. People should try to weigh themselves at the same time of day.

Many people weigh themselves in the morning after they go to the bathroom and before they eat or drink anything.

This method may provide more accurate bodyweight measurements because most people are less likely to have excess weight from food or water first thing in the morning.

Body weight is not the only way to monitor a person’s health in relation to their mass. Other useful measurements include:

Body mass index

Body mass index (BMI) is a general estimation of a person’s body fat. They can calculate their BMI by dividing their weight in kilograms by their height in meters squared.

People can use the calculator below to determine their BMI.

According to the CDC, the range for normal BMIs is between 18.5–25. A BMI of 30 or higher indicates obesity.

Waist circumference

Waist circumference provides a general measurement of the amount of fat stored around the organs, also known as visceral fat.

Having too much visceral fat can increase a person’s risk for type 2 diabetes and cardiovascular disease.

The NIH recommend aiming for a waist size less than 40 inches (in) or 101.6 centimeters (cm) for adult males and less than 35 in or 88.9 cm for adult females.

Body fat percentage

The body stores fat in muscle tissue, around organs, and under the skin. Body fat percentage measures how much a person’s body weight is fat.

Although some fat is necessary for regulating essential bodily functions, having too much or too little fat can negatively impact a person’s health.

The American Council on Exercise (ACE) provide the following body fat percentage ranges:

Essential 10–13% 2–5%
Athletes 14–20% 6–13%
Fitness 21–24% 14–17%
Acceptable 25–31% 18–24%
Obesity >32% >25%

A few ways to measure body fat percentage include:

  • Skinfold calipers, which measure the thickness of subcutaneous fat located below the skin.
  • Hydrostatic weighing, whichmeasures a person’s body composition while submerged in water.
  • Bioelectrical impedance analysis (BIA), whichis a noninvasive test that uses handheld devices or specialized scales that emit weak electrical impulses through the body.

A person’s body weight naturally fluctuates throughout the day. Eating, drinking, and going to the bathroom can result in minor, temporary weight changes.

Water retention can increase a person’s weight and cause bloating. People can reduce their water weight by drinking more water and reducing their intake of sodium and carbohydrates.