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How to calculate BMI by hand

In metric units, divide weight in kilograms by height in metres, squared. Someone who weighs 82 kg and is 1.80 m tall has a BMI of 82 ÷ (1.80 × 1.80) = 82 ÷ 3.24 = 25.3.

In pounds and inches, multiply weight by 703 and divide by height in inches, squared. The 703 is a unit conversion: one pound per square inch of height equals 0.45359237 ÷ 0.0254² = 703.07 kg/m². For 140 lb at 5 ft 6 in (66 in), BMI is 703 × 140 ÷ 4,356 = 22.6.

Three habits keep hand calculations honest:

• Square the height, not the weight. Forgetting the square gives a number around 45 for most adults.
• Convert feet and inches to inches before squaring: 5 ft 6 in is 66 in, not 5.6 ft.
• Round only at the end. The calculator keeps 50 digits and shows one decimal, rounded half-up.

BMI categories for adults

WHO uses the same cut-offs for men and women aged 20 and over. The CDC adult categories match them.

A result that displays as 25.0 can come from a BMI of 24.96, which is still in the healthy band. The calculator places you in a band using the unrounded value, so the category shown is the one the full-precision number belongs to.

Healthy weight for your height

Multiplying 18.5 and 25 by your height in metres, squared, gives the band of weights that keeps BMI in the normal range. The table below is computed that way. Heights in feet and inches are rounded to the nearest inch.

The band is wide on purpose: at 175 cm it spans 20 kg. BMI says nothing about where in that band a given person is healthiest. For a single target figure, the ideal weight calculator compares the Devine, Robinson, Miller and Hamwi formulas.

What BMI does and does not tell you

BMI tracks body fat reasonably well across large groups, which is why it is used in national health surveys and clinical screening. Across 239 prospective studies covering 10.6 million adults, the Global BMI Mortality Collaboration (Lancet, 2016) found all-cause mortality was lowest at a BMI of 20 to 25 and rose steadily above it.

For one person, BMI misses three things:

• Composition. Muscle is denser than fat, so a rugby player at 100 kg and 1.85 m has a BMI of 29.2 with little excess fat. A body fat calculator using the US Navy tape method separates the two.
• Distribution. Fat around the waist carries more metabolic risk than fat on the hips. NICE advises adults to keep their waist measurement below half their height; a waist-to-height ratio of 0.5 or more signals increased risk even at a normal BMI.
• Age and sex. Women carry more body fat than men at the same BMI, and older adults lose muscle, so the same number can mean different things.

BMI is not used for children and teenagers against these fixed cut-offs, and it is not a measure of health during pregnancy.

BMI prime and the ponderal index

The calculator also shows two related indices.

BMI prime is your BMI divided by 25, the upper limit of the normal range. A value of 1.00 sits exactly on that limit; 0.92 is 8% below it and 1.20 is 20% above it. It turns the result into a fraction of the threshold, which is easier to track over time than the raw number.

The ponderal index divides weight by height cubed instead of squared (kg/m³). Because body volume grows roughly with the cube of height, it reads less high for very tall people and less low for very short ones than BMI does. It is mostly used for newborns and in research, and has no widely adopted adult cut-offs.

Where BMI comes from

The Belgian statistician Adolphe Quetelet noticed in the 1830s that adult weight rises roughly with the square of height, and proposed weight ÷ height² as a way to describe the "average man". Ancel Keys and colleagues compared several weight-for-height indices in 1972 and found this one correlated best with measured body fat. They named it the body mass index. WHO adopted the current cut-offs in its 1995 and 2000 technical reports on obesity.

Next steps after checking your BMI

• To estimate how many calories your body uses each day, use the BMR and TDEE calculator.
• To set a daily calorie and protein target for weight change, use the calorie and macro planner.
• To check the result against a measurement of fat rather than weight, use the body fat calculator.

A BMI outside the normal range is a reason to talk to a doctor, who can weigh it against blood pressure, blood tests, waist size and family history.