BMI Calculator

How do you calculate BMI?

Divide weight in kilograms by height in meters squared: BMI = kg ÷ m². With pounds and inches, use BMI = 703 × lb ÷ in². For adults, the CDC calls a BMI under 18.5 underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, and 30 or more obesity, on WHO cut-offs.

Adult BMI categories

The CDC and the WHO draw the adult lines at the same numbers and name the bands differently. Both split obesity into three classes, and the CDC calls class 3 severe obesity and applies the categories from age 20.

Below 18.5 the WHO also grades thinness: mild from 17 to below 18.5, moderate from 16 to below 17, and severe below 16. The calculator adds the grade to any underweight result.

The category is read from the unrounded BMI. 68 kg at 165 cm gives 24.98, inside the healthy range, but at one decimal place it rounds to 25.0, which would sit beside the wrong category. So a BMI just under a cut-off is shown rounded down, here as 24.9, and the working prints as many figures as it needs to stay on the right side of the line.

Where the 703 comes from

A pound is exactly 0.45359237 kg and an inch exactly 0.0254 m, so one pound per square inch is 0.45359237 ÷ 0.0254², or 703.07 kg/m². Multiplying pounds per square inch by that factor converts them to the metric BMI, and the CDC rounds it to 703, which leaves its form 0.01 percent low. For 180 lb at 5 ft 10 in that is 25.83 by kilograms and meters against 25.82 by the CDC form, and 25.8 either way at one decimal place.

The calculator always converts to kilograms and meters before it divides, so the answer does not depend on the units entered. When the height and weight are both in US units, its working also prints the CDC form, so the figure can be checked against a hand calculation done the American way.

BMI prime

BMI prime is BMI divided by 25, the top of the healthy range, proposed by Gadzik in 2006. At 1.00 a person is on the overweight cut-off, and the distance from 1.00 reads as a percentage: 1.10 is 10 percent above the cut-off and 0.90 is 10 percent below it. The underweight cut-off is 0.74, and obesity begins at 1.20, 20 percent above the top of the healthy range. As a ratio of two BMIs it has no units.

Healthy weight range for your height

The range is every weight that gives a BMI from 18.5 up to, but not including, 25: 18.5 × height² to 25 × height², in kilograms and meters. The calculator rounds both ends inward, to a tenth of a kilogram or a whole pound, so every weight it lists gives a BMI inside the range. The chart below is worked the same way.

A range this wide is a population screening band, not a target for any one person, and it says nothing about where in it a particular adult is best placed.

What BMI does not measure

BMI is weight for height. It does not measure body fat, and the CDC describes it as a screening measure, not a diagnostic one: a high BMI is a reason to look further, not a diagnosis.

• A scale cannot tell muscle from fat. A muscular athlete can have a BMI in the overweight range with little body fat, and an older adult who has lost muscle can carry excess fat at a BMI in the healthy range. At the same BMI, women on average carry more body fat than men.
• Where fat sits matters. Fat around the waist carries more cardiometabolic risk than fat elsewhere, which is why the NHLBI’s 1998 adult obesity guidelines pair BMI with waist circumference and flag more than 40 in (102 cm) in men and more than 35 in (88 cm) in women.
• Pregnancy. A BMI measured during pregnancy is not read against these categories, and weight gain guidance is set from the BMI before pregnancy.

Different cut-offs for Asian populations

A WHO expert consultation, published in The Lancet in 2004, found that a substantial share of Asian people are at high risk of type 2 diabetes and cardiovascular disease at BMIs below 25. The BMI at which risk rose ranged from 22 to 25 across the populations studied, and from 26 to 31 for high risk, so the consultation set no single Asian cut-off. It kept the international categories and added public health action points at 23, 27.5, 32.5 and 37.5, leaving each country to decide how to use them.

In the United States, the American Diabetes Association’s Standards of Care lower the BMI at which diabetes testing is considered for Asian American adults with other risk factors from 25 to 23. This calculator reports the international categories for everyone.

Children and teens

For ages 2 through 19 the CDC does not use the adult categories. It plots BMI on sex-specific BMI-for-age growth charts and reads it as a percentile: below the 5th percentile is underweight, the 5th to below the 85th healthy weight, the 85th to below the 95th overweight, and the 95th or above obesity. A child’s BMI normally changes as they grow, so the same number can mean different things a year apart. This calculator takes no age or sex and does not compute percentiles, so its result should not be read for anyone under 20.

BMI beside other body measures

Several clinical numbers are built from height and weight, and each answers a different question. The ideal body weight calculator gives the Devine and ARDSNet weights that drug doses and ventilator settings are worked from, and the body surface area calculator gives the surface area used for chemotherapy doses and for indexing cardiac output. The BMR and TDEE Calculator estimates the energy the body uses at rest. None of them measures body fat either.

Common mistakes

• Typing pounds as kilograms. The calculator’s opening adult, 160 lb at 5 ft 10 in, has a BMI of 22.96. Entered as 160 kg, the same number gives 50.6, deep in obesity class 3. Check the unit beside the box.
• Squaring the height in centimeters. The same adult worked as 72.57 ÷ 177.8² gives 0.002296, ten thousand times too small. Divide centimeters by 100 first.
• Rounding before classifying. A BMI of 24.98 is healthy weight, and rounded to 25.0 first it would be called overweight.
• Using the adult categories for a child. Under 20, BMI is read as a percentile for age and sex, as above.