BMI Calculator

What BMI measures and where the index came from

Body mass index (BMI) is a single number that relates a person's weight to their height. It is expressed in kilograms per square meter (kg/m²), although the unit is almost always left off when the number is quoted. A higher value means more weight for a given height.

The idea goes back to the Belgian mathematician and statistician Adolphe Quetelet, who noticed in the 1830s and 1840s that adult weight tends to rise with the square of height rather than with height itself. The name "body mass index" was proposed in 1972 by Ancel Keys and colleagues, who compared several weight-for-height indices and concluded that this one was the best simple stand-in for relative fatness.

BMI is popular because it needs only a scale and a tape measure. Health agencies use it to screen populations for underweight and obesity, and clinicians use it as a first prompt for a closer look, not as a diagnosis. The results below are for adults; children and teenagers are judged against growth charts instead.

Entering height and weight in US or metric units

1. Pick US units (feet, inches, pounds) or Metric units (centimeters or meters, kilograms). The weight is read in pounds with US units and in kilograms with metric units, so match the number you type to the system you chose.
2. Enter your height. In US mode you can leave feet or inches blank, so 65 inches alone works. In metric mode choose centimeters or meters with the selector next to the field.
3. Enter your weight. It must be at least 1.
4. Tick Asian or South-Asian if that applies to you. WHO regional guidelines use lower cut-offs for these populations because health risks start at a lower BMI.
5. Press Calculate BMI.

The result table lists your BMI to two decimals, the category, your BMI Prime, how far you are over or under the healthy limits, and two hazard ratios for all-cause mortality (one for men, one for women). Heights that are clearly unrealistic, such as a few centimeters or several meters, are rejected with a message.

Calculating BMI by hand in kilograms or pounds

BMI = weight (kg) ÷ height (m)²Metric version

BMI = 703 × weight (lb) ÷ height (in)²US version. The factor 703 is the pound-to-kilogram and inch-to-meter conversions folded into one constant.

Example in US units. Someone who is 5 ft 10 in (70 in) and weighs 170 lb has BMI = 703 × 170 ÷ 70² = 119,510 ÷ 4,900 ≈ 24.39. Converting first gives the same answer: 70 in is 1.778 m and 170 lb is 77.11 kg, so 77.11 ÷ 1.778² = 77.11 ÷ 3.161 ≈ 24.39.

Example in metric units. A person 1.78 m tall who weighs 95 kg has BMI = 95 ÷ 1.78² = 95 ÷ 3.1684 ≈ 29.98. That is just under 30, the line between overweight and obesity, and the difference between 29.98 and 30.00 is about 50 grams of body weight.

To find the weight that corresponds to a target BMI, rearrange the formula: weight = BMI × height². At 1.75 m a BMI of 22 corresponds to 22 × 1.75² = 22 × 3.0625 ≈ 67.4 kg.

WHO weight categories and the Asian cut-offs

The World Health Organization groups adult BMI values into the bands below. Each band includes its lower limit and stops just short of the next one, exactly as the WHO table is written: the normal range is 18.50 to 24.99. The calculator judges the BMI as it is displayed, rounded to two decimals, so a BMI that shows as 25 is overweight and 24.91 is still normal.

Several Asian populations carry more body fat at the same BMI and develop type 2 diabetes and cardiovascular disease at lower values. For that reason the WHO Western Pacific Region guidelines (2000) use lower cut-offs of 23 for overweight and 25 for obesity, which the calculator applies when you tick the Asian box. The 2004 WHO expert consultation reviewed the same evidence and proposed 23, 27.5, 32.5 and 37.5 as public-health action points; the calculator does not use those.

Healthy weight for your height

Turning the normal band (BMI 18.5 up to the start of the overweight band at 25) back into weights shows how wide the range is. The figures are rounded to the nearest kilogram or pound, and the top of each range is the weight at which BMI reaches 25.

A person who is 5 ft 8 in (68 in) is in the normal band from roughly 122 lb up to about 164 lb: 18.5 × 68² ÷ 703 ≈ 121.7 and 25 × 68² ÷ 703 ≈ 164.4. For a target based on frame size and sex rather than a single band, try the ideal weight calculator or the healthy weight calculator.

BMI Prime and the over/under limit

BMI Prime is your BMI divided by 25, the top of the normal range (23 when the Asian option is on). Because it is a ratio it has no units, and a value of exactly 1 means you are on the upper limit. A prime of 1.30 means 30 percent above that limit; 0.80 means you are 80 percent of the way to it.

BMI Prime = BMI ÷ 25

The Over/under limit row turns the prime into plain words. At or above the upper limit it reports the percentage over it; below the lower limit (prime 0.74, which corresponds to BMI 18.5, or 0.80 with the Asian option on) it reports the percentage under; in between it simply says "optimal weight". For the 29.98 example above the prime is 29.98 ÷ 25 ≈ 1.20, so the row reads 19.9 percent over the upper limit.

Hazard ratios for all-cause mortality

A hazard ratio compares how quickly an event happens in one group against a reference group. Here the event is death from any cause and the reference group is everyone with a BMI from 18.5 up to but not including 25, whose ratio is set to 1. A ratio of 1.12 means a 12 percent higher death rate over follow-up, and 2.00 would mean double.

The ratios come from the Global BMI Mortality Collaboration, which pooled more than ten million adults from 239 prospective studies. The calculator reports the value for the band your BMI falls in, separately for men and women.

The study had too little data below 15 or above 60, so outside that span the calculator says no figure is available. The result also shows a small table of absolute risk of dying between ages 35 and 69 for each sex. Both sets of numbers describe groups, not individuals: they say how risk is distributed across a population, not what will happen to you.

What BMI cannot tell you

BMI ignores what the weight is made of. Muscle is denser than fat, so a heavily trained athlete can land in the overweight or obese band with little body fat, while an older person who has lost muscle can have a normal BMI and still carry excess fat. It also says nothing about where fat is stored, and fat around the abdomen is more strongly linked to metabolic disease than fat on the hips and thighs.

Other situations need a different yardstick. Children and teenagers are assessed with BMI-for-age percentiles. Pregnancy changes weight in ways BMI does not capture. In very tall or very short adults the squared-height scaling is less accurate.

If BMI looks off for you, add a second measure. A body fat calculator estimates fat percentage from tape measurements, and the army body fat calculator applies the military standards.

This calculator gives general information, not a diagnosis or medical advice. Talk to a doctor or dietitian before making health decisions based on your BMI. See the disclaimer.

Worked examples with the BMI Calculator

Example 1. Unit system: US units; Height: 5 ft 10 in; Weight: 170 lb.

Result: BMI 24.39; BMI category Normal (healthy weight); BMI Prime 0.98; Over/under limit optimal weight; Hazard ratio for all-cause mortality (men) 1.

Example 2. Unit system: Metric units; Height: 1.78 m; Weight: 95 kg.

Result: BMI 29.98; BMI category Overweight; BMI Prime 1.20; Over/under limit 19.9% over the upper limit; Hazard ratio for all-cause mortality (men) 1.12.

Example 3. Unit system: Metric units; Height: 1 m; Weight: 17 kg.

Result: BMI 17; BMI category Underweight; BMI Prime 0.68; Over/under limit 8.1% under the lower limit; Hazard ratio for all-cause mortality (men) 1.83.

Common mistakes with the BMI Calculator

• Entering height in centimeters in a box set to meters, or weight in pounds in a kilograms box. Check the unit beside each box.
• Treating BMI as a diagnosis. It ignores muscle mass, age, sex and body shape, so athletes can read as overweight.
• Applying the adult categories to children. Children and teenagers are judged against growth charts for their age.