BMI Calculator — Body Mass Index (Metric & Imperial)

What is BMI?

Body mass index (BMI) is a number calculated from a person's height and weight. Doctors and researchers use it as a quick screening measure for whether a person's weight falls in a range linked to good health for their height. This calculator works out BMI from height and weight, in metric or imperial units, and shows the weight category and the healthy weight range for that height.

BMI is a screening tool, not a diagnosis. The standard cutoffs below apply to adults aged 20 and over. They do not apply to children, teenagers, pregnant women, or highly muscular athletes (see What BMI does not measure).

BMI formula

In metric units, BMI equals weight in kilograms divided by height in metres squared:

$BMI = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

In imperial units, weight in pounds is multiplied by 703, then divided by height in inches squared:

$BMI = 703 \times \frac{\text{weight (lb)}}{\text{height (in)}^2}$

The number 703 converts pounds and inches into the same result the metric formula gives. It comes from the exact conversion factors 1 kg ≈ 2.2046 lb and 1 in = 2.54 cm; 703 is a rounded version of that conversion, so the imperial formula can give a result a few hundredths off from a metric calculation of the same person. This calculator converts pounds and inches to kilograms and metres directly, so its output is more precise than the 703 shortcut.

How to calculate BMI

Metric example. A person is 170 cm (1.70 m) tall and weighs 70 kg.

$BMI = \frac{70}{1.70^2} = \frac{70}{2.89} \approx 24.2$

A BMI of 24.2 falls in the normal weight category.

Imperial example. A person is 5 feet 9 inches (69 inches) tall and weighs 160 lb.

$BMI = 703 \times \frac{160}{69^2} = \frac{112{,}480}{4{,}761} \approx 23.6$

This also falls in the normal category.

For a height of 170 cm, a normal BMI corresponds to a weight of about 53.5 to 72.2 kg. The calculator shows this healthy weight range alongside the BMI result.

BMI categories

The World Health Organization (WHO) defines four main adult categories, based on the unrounded BMI value:

The "18.5–24.9" range is a display convention. The real cutoff is an exact BMI of 25, so a BMI of 24.98 still counts as normal weight even though it rounds to 25.0 on screen. This calculator classifies weight using the unrounded value, not the rounded one, so results stay consistent right at the category edges.

Obesity is often split further into Class 1 (30 to 34.9), Class 2 (35 to 39.9), and Class 3, also called severe obesity (40 and above). Many health guidelines use lower cutoffs for people of Asian descent, since health risks tend to rise at a lower BMI in these groups: overweight starts at 23 and obesity at 25.

Two related measures come up in discussions of BMI. BMI Prime is BMI divided by 25, the upper edge of the normal range; a value above 1.0 means BMI is above that range. The Ponderal Index divides weight by height cubed instead of height squared, which some researchers argue works better for people who are unusually short or tall.

What BMI does not measure

BMI only compares weight with height. It cannot tell what that weight is made of, which limits what it can show:

• It cannot separate muscle from fat. A heavily muscled athlete can score as overweight or obese while carrying little body fat.
• It does not show where fat sits on the body. Fat around the waist carries more health risk than fat on the hips or thighs, but BMI treats them the same.
• It does not account for age or sex on its own. Older adults tend to have less muscle at a given BMI, and women tend to carry more body fat than men at the same BMI.
• It is not built for children, teenagers, or pregnant women. Children and teens are compared with age- and sex-specific growth charts instead of the adult cutoffs.

Waist circumference, waist-to-hip ratio, and direct body-fat measurements, such as skinfold calipers, bioelectrical impedance, or a DEXA scan, give a fuller picture alongside BMI.

History

The Belgian scientist Adolphe Quetelet described the ratio of weight to height squared in the 1830s, as part of his broader work on the statistics of the "average man." It became known as the Quetelet Index. In 1972, the American physiologist Ancel Keys and his colleagues studied several weight-height formulas and found this one tracked body fat more closely than the alternatives. They renamed it the Body Mass Index, and it has been the standard population weight screen since.

Frequently asked questions

What counts as a healthy BMI? For most adults, a BMI from 18.5 to 24.9 is classed as normal weight.

Is BMI accurate for everyone? It works well as a screening tool across large groups of people. For one individual, especially someone very muscular or very lean, it is best combined with a waist measurement or a body-fat test.

Does BMI apply to children? No. Children and teenagers are assessed against BMI-for-age percentile charts specific to their sex, not the fixed adult cutoffs.

Is the BMI formula different for men and women? No. Adults of any sex use the same formula and the same category cutoffs. Body composition differs between men and women, which is one reason BMI is treated as a rough guide rather than a precise health measure.

Why does the imperial formula use 703? It is the constant that converts pounds and inches into the same scale as kilograms and metres, so the imperial and metric formulas give matching results.

References

• World Health Organization — Body mass index (BMI)
• U.S. CDC — About Adult BMI
• NIH / NHLBI — Calculate Your BMI