BMI and Waist Size: What These Measurements Can and Can't Tell You

Body mass index (BMI) is one of the most widely used measures in healthcare. You've probably seen it on a doctor's chart or calculated it online. But what does it actually mean — and what doesn't it tell you?

What is BMI?

BMI is a number calculated from your weight and height. It's used as a screening tool to help identify possible weight-related health risks. The CDC describes BMI as an inexpensive and easy screening method, but notes that it is not diagnostic of body fat or health.

How BMI is calculated

• Formula: weight in pounds ÷ (height in inches × height in inches) × 703
• Or: weight in kilograms ÷ (height in meters)²

The easiest way is to use the CDC's free online Adult BMI Calculator.

Adult BMI categories

For adults 20 and older, the CDC uses these categories:

The CDC further divides obesity into Class 1 (30 to under 35), Class 2 (35 to under 40), and Class 3 (40 or higher). Children and teens use different, age- and sex-specific BMI charts.

The limitations of BMI

BMI is useful for population-level screening, but it has important limitations for individuals:

• It doesn't distinguish fat from muscle. Very muscular people may have a high BMI without excess body fat.
• It doesn't show where fat is located. Fat around the abdomen is linked to higher health risks than fat elsewhere.
• It may not reflect differences related to age, sex, and race or ethnicity in the same way for everyone.
• It doesn't measure health directly. Blood pressure, blood sugar, cholesterol, fitness, and habits all matter.

That's why healthcare providers use BMI as a starting point, along with other measurements and your overall health picture.

Why waist size matters

Waist circumference can provide additional information about health risk. Excess fat around the abdomen is associated with higher risks of type 2 diabetes, heart disease, and high blood pressure.

According to the National Heart, Lung, and Blood Institute (NHLBI), health risks increase with a waist size greater than:

• 40 inches for men
• 35 inches for women who are not pregnant

How to measure your waist

1. Stand up straight and breathe out normally.
2. Place a tape measure around your middle, just above your hip bones.
3. Make sure the tape is snug but doesn't press into your skin, and is level all the way around.
4. Read the measurement after exhaling.

Looking at the bigger picture

BMI and waist size are only part of your health story. Other important factors include:

• Blood pressure — see understanding blood pressure
• Blood sugar and cholesterol levels
• Physical activity and fitness
• Eating patterns
• Sleep
• Smoking and alcohol use
• Family history
• Mental health

Many health benefits come from healthy habits, regardless of the number on the scale. Regular physical activity, a balanced diet, and enough sleep all support health at any size.

Talking with your healthcare provider

If you have questions about your BMI or waist size, ask your provider:

• What do my measurements mean for my health?
• Are there other tests I should have?
• Would changing my weight benefit my health? If so, what's a realistic goal?
• What support is available?

If weight loss is recommended, see our guide to healthy, sustainable weight loss.

The bottom line

BMI is a quick screening tool, and waist size adds helpful information about health risk. But neither tells the whole story. Use them as conversation starters with your healthcare provider, and focus on healthy habits that support your well-being.