Body Mass Index (BMI)
What is the body mass index (BMI)?
The body mass index (BMI) is a measure that uses your height and weight to work out if your weight is healthy.
Body Mass Index (BMI) is a person’s weight in kilograms (or pounds) divided by the square of height in meters (or feet). A high BMI can indicate high body fatness. BMI screens for weight categories that may lead to health problems, but it does not diagnose the body fatness or health of an individual.
The BMI calculation divides an adult's weight in kilograms by their height in metres squared. For example, A BMI of 25 means 25kg/m2.
BMI ranges
For most adults, an ideal BMI is in the 18.5 to 24.9 range.
For children and young people aged 2 to 18, the BMI calculation takes into account age and gender as well as height and weight.
If your BMI is:
- below 18.5 – you're in the underweight range
- between 18.5 and 24.9 – you're in the healthy weight range
- between 25 and 29.9 – you're in the overweight range
- between 30 and 39.9 – you're in the obese range
How is BMI calculated?
BMI is calculated the same way for both adults and children. The calculation is based on the following formulas:
| Measurement Units | Formula and Calculation |
|---|---|
| Kilograms and meters (or centimeters) |
Formula: weight (kg) / [height (m)]2 With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Because height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters. Example: Weight = 68 kg, Height = 165 cm (1.65 m) |
| Pounds and inches |
Formula: weight (lb) / [height (in)]2 x 703 Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703. Example: Weight = 150 lbs, Height = 5’5″ (65″) |
- About BMI for Adults
Body mass index (BMI) is a person’s weight in kilograms divided by the square of height in meters. BMI is an inexpensive and easy screening method for weight category—underweight, healthy weight, overweight, and obesity.
BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat. Furthermore, BMI appears to be as strongly correlated with various metabolic and disease outcome as are these more direct measures of body fatness.
How is BMI interpreted for adults?
For adults 20 years old and older, BMI is interpreted using standard weight status categories. These categories are the same for men and women of all body types and ages.
| BMI | Weight Status |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and Above | Obesity |
For example, here are the weight ranges, the corresponding BMI ranges, and the weight status categories for a person who is 5′ 9″.
| Height | Weight Range | BMI | Weight Status |
|---|---|---|---|
| 5′ 9″ | 124 lbs or less | Below 18.5 | Underweight |
| 125 lbs to 168 lbs | 18.5 to 24.9 | Healthy Weight | |
| 169 lbs to 202 lbs | 25.0 to 29.9 | Overweight | |
| 203 lbs or more | 30 or higher | Obesity |
Is BMI interpreted the same way for children and teens as it is for adults?
For children and teens, the interpretation of BMI depends upon age and sex. For adults, the interpretation of BMI does not depend on sex or age.
BMI is interpreted differently for children and teens, even though it is calculated using the same formula as adult BMI. Children and teen’s BMI need to be age and sex-specific because the amount of body fat changes with age and the amount of body fat differs between girls and boys. The CDC BMI-for-age growth charts take into account these differences and visually show BMI as a percentile ranking. These percentiles were determined using representative data of the US population of 2- to 19-year-olds that was collected in various surveys from 1963-65 to 1988-9411.
Obesity among 2- to 19-year-olds is defined as a BMI at or above the 95th percentile of children of the same age and sex in this 1963 to 1994 reference population. For example, a 10-year-old boy of average height (56 inches) who weighs 102 pounds would have a BMI of 22.9 kg/m2. This would place the boy in the 95th percentile for BMI – meaning that his BMI is greater than that of 95% of similarly aged boys in this reference population – and he would be considered to have obesity.
What are the health consequences of obesity for adults?
People who have obesity are at increased risk for many diseases and health conditions, including the following:
- All-causes of death (mortality)
- High blood pressure (hypertension)
- High LDL cholesterol, low HDL cholesterol, or high levels of triglycerides (dyslipidemia)
- Type 2 diabetes
- Coronary heart disease
- Stroke
- Gallbladder disease
- Osteoarthritis (a breakdown of cartilage and bone within a joint)
- Sleep apnea and breathing problems
- Chronic inflammation and increased oxidative stress
- Some cancers (endometrial, breast, colon, kidney, gallbladder, and liver)
- Low quality of life
- Mental illness such as clinical depression, anxiety, and other mental disorders
- Body pain and difficulty with physical functioning
- About BMI for Children and Teens
Body mass index (BMI) is a person’s weight in kilograms divided by the square of height in meters. It is an inexpensive and easy way to screen for weight categories that may lead to health problems. For children and teens, BMI is age- and sex-specific and is often referred to as BMI-for-age.
Regardless of the current BMI-for-age category, help your child or teen develop healthy weight habits and talk with your doctor or other healthcare provider as part of ongoing tracking of BMI-for-age. If your child has significant weight loss or gain, he or she should be referred to and guided by a doctor or other healthcare provider.
Tracking growth patterns over time can help you make sure your child is achieving or maintaining a healthy weight. A single BMI-for-age calculation is not enough to evaluate long-term weight status because your child’s height and weight will change as they grow. With individuals, health care providers should consider BMI along with other factors such as family history, blood pressure, blood sugar levels, and eating patterns and physical activity level.
What are the health consequences of obesity during childhood and what can adults do about it?
Obesity during childhood can harm the body in a variety of ways, now and in the future. Learn more about the health consequences of obesity for children.
To help reduce the health risks associated with having obesity during childhood, encourage children and teens to practice healthy habits by:
- Eating healthy food and drinking plenty of water.
- Making mealtimes a family affair. Have your children help with cooking and let them choose healthy foods as well.
- Helping children find a physical activity they enjoy and participating in physical activity on most (preferably all) days of the week.
- Getting adequate sleep.
- Limiting screen time.
- Taking time for self-care and stress reduction. Try strategies like breathing exercises, meditation, yoga, and journaling.
Accuracy of BMI
BMI takes into account natural variations in body shape, giving a healthy weight range for a particular height.
As well as measuring your BMI, healthcare professionals may take other factors into account when assessing if you're a healthy weight.
Muscle is much denser than fat, so very muscular people, such as heavyweight boxers, weight trainers and athletes, may be a healthy weight even though their BMI is classed as obese.
Your ethnic group can also affect your risk of some health conditions. For example, adults of South Asian origin may have a higher risk of some health problems, such as diabetes, with a BMI of 23, which is usually considered healthy.
You should not use BMI as a measure if you're pregnant. Get advice from your midwife or GP if you're concerned about your weight.
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