BMI compares your height and weight using a simple math formula: Multiply your weight in pounds by 703, then divide by your height in inches squared.

For example, if you weigh 150 pounds and are 65 inches tall, your BMI would be:

(150 × 703) ÷ (65 × 65) = 25

You can also use a BMI calculator to help determine your BMI.

This number is then compared to a standard BMI chart to see if it falls into a healthy weight range.

BMI Ranges for Adults

Adult BMI is divided into categories:

• Underweight: Below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese: 30 and above

These ranges apply to all adults 20 and older.

BMI for Children and Teens

For children and teens, BMI is interpreted differently. Since they are still growing, their BMI is compared to other children and teens using percentiles:

• Underweight: Below the 5th percentile
• Healthy weight: 5th to 85th percentile
• Overweight: 85th to 94th percentile
• Obese: 95th percentile and above

Physicians use these percentiles to track growth and spot health risks early.

“BMI is a barometer for your child’s growth,” says Anita Chandra-Puri, MD, a pediatrician at Northwestern Medicine. “But BMI is only one tracking mechanism, like a vital sign.”

When addressing BMI in children, it’s important to look at the full picture.

“I talk with kids who have an elevated BMI about self-esteem and confidence, and how they’re feeling in general,” says Dr. Chandra-Puri. “Also, sometimes parents don’t realize family habits affect their child’s habits. Establish healthy habits together as a family.”

Instead of focusing on a specific diet or losing weight, Dr. Chandra-Puri talks with children about heart-healthy foods and habits that promote activity, like getting more exercise outside of gym class.

“Model healthy behaviors early on so your child can develop healthy habits that will last a lifetime,” says Dr. Chandra-Puri.

As always, talk to your child’s pediatrician if you have concerns about their growth and development.

What BMI Can Tell You About Your Health

BMI can show if someone is overweight or obese. It can also indicate a risk for health problems related to body fat.

A high BMI can be linked to:

• Type 2 diabetes
• Heart disease
• High blood pressure
• Sleep apnea
• Certain cancers
• Joint conditions like osteoarthritis

What BMI Doesn’t Tell You

BMI doesn’t measure body composition — the amount of body fat vs. muscle. Someone with a lot of muscle mass, like an athlete, might have a high BMI but little body fat. On the other hand, an adult might have a normal BMI but still carry excess fat around their stomach.

BMI also doesn’t show where fat is stored. Belly fat, for example, is linked to higher health risks than fat stored in other areas.

“Your body stores excess calories as fat. How much fat you have and where it is stored can make a big difference when it comes to your health,” says Jyothi Gogineni, MD, an endocrinologist at Northwestern Medicine. “Subcutaneous fat is just under the skin. Visceral fat surrounds the organs in the belly and contributes to health issues. Your lifestyle and your genetics will determine where it accumulates.”

Because body fat distribution can vary, physicians often use other tools along with BMI.

When BMI Is Not a Good Indicator

BMI may not be accurate for many groups of people, including:

• Athletes with high muscle mass
• People who are pregnant
• Anyone older than 65 due to age-related muscle loss

Other Ways to Measure Health

To get a better picture of overall health, medical professionals often consider:

• Waist circumference: Fat around the waist can raise health risks.
• Body composition tests: These measure fat, muscle and bone.
• Blood pressure, cholesterol and blood sugar levels: These can indicate if you are at risk for certain medical conditions.
• Lifestyle habits: Diet, exercise, sleep and stress can help gauge overall health.

Can You Be Overweight and Still Be Healthy?

Some people with a BMI in the overweight range may have normal lab results, be physically active and feel well.

“Obesity does not affect everyone in the same way,” says Dr. Gogineni. “Some people manage to temporarily avoid the usual health concerns. This is called ‘metabolically healthy obesity.’ But these individuals are still at a higher risk for health problems in the future.”

When to Talk to Your Physician

If BMI is too high or low, the next step is to talk to your physician. They may suggest:

• Lifestyle changes: Adjust eating, physical activity and sleep habits.
• Lab tests: Check for conditions like diabetes, high cholesterol and other medical conditions.
• Referrals: See a dietitian, physical therapist or counselor.
• Regular monitoring: Track progress and adjust plans over time.

Health is about more than body weight — it’s about how you feel, move, eat and live. BMI is useful, but it shouldn’t be the only thing you focus on. It’s one piece of your health puzzle.