BMI Calculator

BMI introduction

Body mass index (BMI) compares weight with height to give a single number that screens for weight categories that may raise health risks. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet and later adopted by the World Health Organization and national health agencies because it is quick, free and needs only a scale and a tape measure.

BMI does not measure body fat directly, and it says nothing about where fat is carried or how fit someone is. It is best used as a first check: a reading outside the normal range is a prompt to look more closely, ideally with a health professional, rather than a diagnosis on its own.

BMI table for adults

For adults 20 and older, the World Health Organization groups BMI into the categories below. Men and women use the same cut-offs. Some countries, particularly in Asia, use lower thresholds for overweight and obesity because health risks appear at lower BMIs in those populations.

BMI chart for adults

The chart shows the same categories as bands of weight for each height. Find your height along the bottom and read up to your weight to see which band you fall in. The green band is the normal range: for someone 5 feet 10 inches tall it runs from about 129 to 174 pounds.

BMI table for children and teens, age 2-20

Children's body composition changes as they grow and differs between boys and girls, so a fixed BMI cut-off would be misleading. Instead, the US Centers for Disease Control and Prevention (CDC) compares a child's BMI with a reference group of the same age and sex and reports a percentile. A BMI at the 70th percentile, for example, is higher than that of 70% of children the same age and sex in the reference data.

BMI chart for children and teens, age 2-20

The CDC publishes separate BMI-for-age growth charts for boys and girls. Each chart plots age along the bottom and BMI up the side, with curved lines marking the 5th, 85th and 95th percentiles. Plotting a child's BMI at their exact age shows which band they fall in. Because the curves rise and fall through childhood, the same BMI can be healthy at one age and high at another. For children with severe obesity, the CDC also provides extended charts that express BMI as a percentage of the 95th percentile.

Risks associated with being overweight

Carrying excess weight, especially around the abdomen, is linked to a higher risk of many conditions. These include high blood pressure, high LDL cholesterol and low HDL cholesterol, type 2 diabetes, coronary heart disease and stroke, gallbladder disease, osteoarthritis, sleep apnea and other breathing problems, and several cancers, including endometrial, breast, colon, kidney and liver cancer. Excess weight is also associated with lower quality of life, depression and anxiety, chronic pain and higher overall mortality.

Risk rises with BMI and with waist size, so a waist measurement adds useful information: above about 40 inches (102 cm) for men and 35 inches (88 cm) for women is considered high. Losing even 5% to 10% of body weight can measurably improve blood pressure, blood sugar and cholesterol.

Risks associated with being underweight

Being underweight carries its own risks. It can signal or cause malnutrition and vitamin deficiencies, anemia, a weakened immune system and slower healing, and loss of bone density leading to osteoporosis and fractures. Women may experience irregular or missed periods and fertility problems, and underweight during pregnancy raises the risk of preterm birth. In children and teens it can slow growth and development.

A low BMI can also be a sign of an underlying problem, such as thyroid disease, digestive disorders, cancer or an eating disorder. Unexplained weight loss is worth discussing with a doctor.

Limitations of BMI

Because BMI uses only height and weight, it cannot tell muscle from fat. Athletes and people with a lot of muscle can register as overweight while having little body fat, and older adults who have lost muscle can have a normal BMI with excess fat. BMI also ignores where fat is stored; fat around the abdomen is more closely tied to heart disease and diabetes than fat on the hips and thighs.

Body composition differs by sex, age and ethnicity, so the same BMI can mean different things for different people. BMI is not intended for pregnant women, and for children it must be read as an age- and sex-specific percentile. For a fuller picture, combine BMI with waist circumference, body fat percentage, blood pressure and blood tests.

BMI formula

BMI is weight divided by the square of height. In metric units, a person who weighs 65 kg and is 1.80 m tall has a BMI of 65 ÷ 1.80² = 20.1. In US units, multiply by 703: someone 5 feet 10 inches (70 inches) tall who weighs 160 pounds has a BMI of 703 × 160 ÷ 70² = 23.0.

BMI = 703 × weight (lb) ÷ height (in)² · BMI = weight (kg) ÷ height (m)²

Both versions give the same number; 703 converts pounds and inches to kilograms and meters.

BMI Prime

BMI Prime is your BMI divided by the upper limit of the normal range, 25. A value below 1 means you are under the overweight threshold; above 1 means you are over it, and the decimal shows by how much. A BMI Prime of 1.2 is 20% above the limit. Because it is a ratio, it can be compared across countries that use different upper limits, such as 23 in some Asian guidelines, by dividing by that limit instead.

In BMI Prime terms, values under 0.74 indicate underweight, 0.74 to 0.99 normal, 1.00 to 1.19 overweight, and 1.20 or higher obesity.

Ponderal Index

The Ponderal Index (PI), also called Rohrer's index, divides weight by height cubed instead of squared: PI = weight (kg) ÷ height (m)³. Because body mass scales more closely with height cubed, PI gives more consistent results than BMI for very tall and very short people. A typical adult value is about 11 to 15 kg/m³. PI is widely used for newborns, where it helps identify babies who are small or large for their length.

Frequently asked questions

Sources

• CDC – About Adult BMI
• CDC – Child and Teen BMI Categories
• WHO – Obesity and overweight