Body mass index (BMI), is a useful tool for determining healthful weight. However, experts have expressed uncertainty about its reliability and concerns about its limitations.

A pregnant athletic woman who would have a high BMIShare on Pinterest
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BMI alone cannot show whether a person’s weight is healthful, but using it in combination with other indicators can provide a more complete picture.

For adults ages 20 years and older, BMI incorporates weight and height, but it does not take age or sex into account.

A woman tends to have more body fat than a man with the same BMI. Likewise, an older person tends to have more body fat than a younger person with an equal BMI.

For these reasons, BMI may not give the detail necessary to determine whether a person’s weight is healthful.

BMI does not reflect the location or amount of body fat, and these factors can impact health.

For example, studies have indicated that people who have fat around the waist and surrounding the abdominal organs may be more at risk of health problems than those with fat in other areas.

A 5-year investigation of 1,964 people, published in Scientific Reports in 2017, was one study that confirmed these findings.

If a person has a high BMI, they are likely to have a high proportion of body fat, especially if their BMI falls in the obesity category.

However, it is possible to be “overweight” according to BMI, but have healthful levels of fat.

For extremely muscular people, such as athletes and bodybuilders, height and weight measurements alone may not accurately indicate health, because muscle weighs more than fat.

A healthy, muscular person may have a BMI in a very high range. Meanwhile, a frail, inactive person may have a low BMI, but more body fat and less lean tissue than is healthful.

Regular BMI cannot accurately indicate the state of a person’s health at some stages in life.

These include:

  • pregnancy and breastfeeding
  • childhood and adolescence, while a person is still growing

For this reason, BMI calculations are different for children and teens. These measurements take age and sex into account.

Norm can also vary among people of certain races and ethnicities.

The National Heart, Lung, and Blood Institute (NHLBI) suggest that an assessment of weight and health risks should incorporate three key measures:

  • BMI
  • waist circumference
  • risk factors for diseases and conditions associated with obesity

Waist circumference

If fat accumulates around the waist rather than the hips, a person may have a higher risk of heart disease and type 2 diabetes.

This risk increases with a waist size greater than 35 inches for non-pregnant women or greater than 40 inches for men, according to the Centers for Disease Control and Prevention (CDC).

To measure their waist, a person should:

  1. Place a tape measure around their middle, just above the hip bones and the naval.
  2. Take the measurement just after breathing out.

Weight, obesity, and health risks

The following information, adapted from the NHLBI, may help indicate the risks associated with BMI and waist circumference.

The chart shows weight categories according to BMI, and the effects of higher waist circumference on the risks of type 2 diabetes, hypertension, and cardiovascular disease.

ClassificationBMI (kg/m2)Obesity class



Men 40 inches or less

Women 35 inches or less

Men: 40 inches or more

Women: 35 inches or more

Underweight18.4 or less
Healthful weight18.5–24.9
Overweight25.0–29.9Increased riskHigh risk
Obesity30.0–34.9IHigh riskVery high risk
35.0–39.9IIVery high riskVery high risk
Extreme obesity40.0+IIIExtremely high riskExtremely high risk

A doctor may also measure body fat composition.

Risk factors for obesity-related conditions

Being overweight or having obesity can increase the risk to the heart.

The following issues can also increase the risk of developing heart disease, for example.

  • high blood pressure (hypertension)
  • high levels of low-density lipoprotein (“bad”) cholesterol
  • low levels of high-density lipoprotein (“good”) cholesterol
  • high levels of triglycerides
  • high blood sugar levels
  • a family history of early heart disease
  • physical inactivity
  • cigarette smoking
  • a high consumption of alcohol

A doctor will recommend that a person consider losing weight if they:

  • have a BMI of 30 or greater
  • have a BMI of 25–29.9 plus two or more risk factors

If a person has obesity or excess weight plus two or more risk factors, they may be at risk of a number of obesity-related health problems in the future.

Losing 5–10 percent of their current weight can reduce the risk of developing these health problems.

Some people are overweight but have no other risk factors, such as high cholesterol or high blood pressure. They should follow a healthful and varied diet, and get regular exercise to prevent additional weight gain.