BMI Calculator

What BMI measures — and what it misses

Body Mass Index is a single number that relates your weight to your height: weight in kilograms divided by height in meters squared. It was designed as a population statistic — a cheap way to sort large groups into weight categories — and it survives because it is simple, free, and roughly right for most people. A BMI far outside the healthy range is a reliable signal that weight is worth a closer look; a BMI inside the range is reassurance, not proof, that body composition is healthy.

What BMI cannot see is what the weight is made of. Muscle is denser than fat, bone mass varies, and fat stored around the waist carries more risk than the same fat stored on the hips. That is why this page pairs your BMI with a healthy weight range for your height rather than a single "ideal" weight, and why the body fat calculator is the natural next step when your BMI and your mirror disagree.

BMI table for adults

The World Health Organization's classification, used for men and women age 20 and over. This calculator switches to the adult table at age 21, when the CDC growth charts end.

BMI for children and teens, ages 2–20

Children are not small adults: healthy body fat changes with age and differs between boys and girls, so a child's BMI is converted into a BMI-for-age percentile using the CDC growth charts — the same child's BMI can be perfectly healthy at one age and a concern at another. The calculator also reports weight-for-age and height-for-age percentiles, the healthy BMI range for the child's age (the 5th to 85th percentile), and how much weight would bring them into it.

For BMIs above the 95th percentile, percentiles come from the CDC's 2022 extended BMI-for-age method, which models the upper tail instead of stretching the standard chart — that is how readings like the 99.9th percentile are produced.

Risks associated with being overweight

Carrying excess weight raises the risk of a long list of serious conditions. According to the CDC, these include:

• High blood pressure
• Unhealthy cholesterol — higher LDL and triglycerides, lower HDL
• Type II diabetes
• Coronary heart disease and stroke
• Gallbladder disease
• Osteoarthritis, as cartilage in weight-bearing joints breaks down
• Sleep apnea and other breathing problems
• Certain cancers, including endometrial, breast, colon, kidney, gallbladder and liver
• Depression, anxiety and reduced quality of life
• Body pain, difficulty with physical functions, and higher overall mortality

None of this is destiny — risk climbs with the degree and duration of excess weight, and it falls as weight comes back toward the healthy range. Aiming first for the top of your healthy range (a BMI of 25) is a concrete, achievable target: the result above tells you exactly how much weight that means.

Risks associated with being underweight

• Malnutrition, vitamin deficiencies and anemia
• Osteoporosis — weaker bones and a higher fracture risk
• A weakened immune system
• Growth and development problems in children and teenagers
• Reproductive problems in women, from hormonal disruption of the menstrual cycle to a higher risk of miscarriage in the first trimester
• Complications during and after surgery
• Higher overall mortality compared with the healthy BMI range

Being underweight is sometimes a symptom rather than a condition — of an illness, a thyroid problem, or an eating disorder such as anorexia nervosa. If your BMI is low and you cannot explain why, that is worth a doctor's visit, not just a bigger appetite.

Limitations of BMI

BMI estimates excess weight, not excess fat, and it knows nothing about how either is distributed. Three patterns from the CDC are worth remembering: at the same BMI, older adults generally carry more body fat than younger adults; women generally carry more than men; and muscular athletes can score as overweight while being lean. In children, height and the timing of puberty also move the number, and in very thin children a low BMI may reflect low fat-free mass rather than low fat. Used as one signal among several — waist size, body fat percentage, blood pressure, blood work — BMI earns its keep. Used alone, it can mislead in both directions.

The BMI formula

In metric units, BMI = mass (kg) ÷ height² (m). In US units, BMI = 703 × mass (lb) ÷ height² (in) — the 703 converts pounds and inches into the metric result. Worked example, a 5′10″, 160-pound adult: metric first — 160 lb = 72.57 kg and 70 in = 1.778 m, so BMI = 72.57 ÷ 1.778² = 23.0; directly in US units, 703 × 160 ÷ 70² = 23.0. Same person, same answer.

BMI Prime

BMI Prime is simply your BMI divided by 25, the upper limit of the healthy range. It expresses your BMI as a fraction of that limit, which makes comparisons immediate: a Prime of 0.8 means you are at 80% of the limit; 1.2 means 20% over it. The classification table mirrors the adult BMI table, scaled by the same division.

Ponderal Index

The Ponderal Index (also called Rohrer's index) is BMI's older cousin: mass (kg) ÷ height³ (m). Cubing height matches how mass actually scales with size, so the index treats very tall and very short people more fairly — populations where BMI is known to over- or under-state fatness. For the worked example above it gives 72.57 ÷ 1.778³ = 12.9 kg/m³. Everyday healthy values for adults usually fall around 11–14.

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