BMI Calculator

This works out your body mass index from height and weight, places it on the World Health Organization scale, and returns the weight range that would put you in the healthy band. The chart above colours every height and weight combination at once, so you can see where you sit and how far the nearest boundary is.

What BMI measures

BMI is a measure of leanness or corpulence based on height and weight, intended to stand in for tissue mass. It is the most widely used indicator of whether someone is a healthy weight for their height, because it needs two numbers anyone can obtain and no equipment beyond scales and a tape measure.

It is older than most people assume. The Belgian statistician Adolphe Quetelet described the ratio in 1832 while studying the characteristics of populations, and it circulated for well over a century as the Quetelet Index. The name body mass index was attached by the physiologist Ancel Keys in 1972, in a paper that recommended it for population studies while cautioning against reading too much into any single person's figure. That caution has been repeated ever since and is worth keeping in mind here too.

The formula

In metric units it is weight in kilograms divided by height in metres squared:

BMI = mass (kg) ÷ height² (m)

For someone 1.778 m and 72.57 kg:

72.57 ÷ 1.778² = 23.0

In US customary units the same calculation needs a conversion factor of 703:

BMI = 703 × mass (lb) ÷ height² (in)

703 × 160 ÷ 70² = 23.0

The 703 is not arbitrary. It is what you get from converting both units at once: 0.45359237 kilograms per pound divided by 0.0254 metres per inch squared comes to 703.07. Both routes describe one person, 5 feet 10 inches and 160 pounds, and both return 23.0.

The WHO categories for adults

The table above the chart gives the World Health Organization classification, used for men and women aged 20 or over. The headline boundaries are 18.5 and 25: below 18.5 is underweight, 18.5 to 25 is normal, 25 to 30 is overweight, and 30 and above is obesity, subdivided into three classes at 35 and 40.

Those numbers translate into real weights only once height is fixed, which is what the second chart does. At 5 feet 10 inches the healthy band runs from about 129 to 174 pounds, the overweight threshold is 174, and obesity begins at 209. At 5 feet 4 inches every one of those figures drops by roughly 25 pounds. This is why comparing your weight against someone else's tells you nothing useful unless you are the same height.

One important variation: the WHO has recognised since a 2004 expert consultation that these cut-offs understate risk in many Asian populations, where cardiovascular and metabolic problems appear at lower body weights. Several countries use action points nearer 23 for overweight and 27.5 for obesity. If that applies to you, read the boundaries lower.

Children and teenagers

The adult table does not apply under the age of 20, and applying it anyway is the most common misuse of a BMI calculator. Children are growing, and the amount of body fat that is normal changes substantially with age and differs between boys and girls.

The CDC therefore assesses ages 2 to 20 against BMI-for-age percentiles rather than fixed numbers. A child's BMI is calculated the same way, then plotted on a growth chart against others of the same age and sex. Below the 5th percentile is underweight, the 5th to 85th is a healthy weight, the 85th to 95th is overweight, and at or above the 95th percentile is obesity. Older material sometimes labels the 85th to 95th band "at risk of overweight" and the top band "overweight"; the current terminology is the one above.

Enter an age under 20 above and the calculator says so rather than quietly handing back an adult category. The arithmetic is still shown, because a paediatrician will want the figure to plot, but the classification needs the growth chart.

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
• Higher LDL cholesterol, lower HDL cholesterol, and high triglycerides
• Type 2 diabetes
• Coronary heart disease and stroke
• Gallbladder disease
• Osteoarthritis, caused by the breakdown of joint cartilage
• Sleep apnoea and other breathing problems
• Certain cancers, including endometrial, breast, colon, kidney, gallbladder and liver
• Depression, anxiety and other mental illness
• Body pain, difficulty with physical tasks, and lower quality of life
• An increased risk of death compared with people at a healthy BMI

The general advice is to keep BMI under 25, but the sensible version of that is a conversation with a doctor who can look at the rest of your circumstances rather than a single ratio.

Risks associated with being underweight

The other end of the scale carries its own risks, which get far less attention:

• Malnutrition, vitamin deficiencies and anaemia
• Osteoporosis, which weakens bone and raises fracture risk
• Reduced immune function
• Growth and development problems in children and teenagers
• Fertility problems in women, where hormonal disruption can interrupt the menstrual cycle, and a higher chance of first-trimester miscarriage
• Complications following surgery
• An increased risk of death compared with people at a healthy BMI

Being underweight can also point to an underlying condition, including anorexia nervosa, which carries the highest mortality of any psychiatric illness. If weight has fallen without an obvious cause, that is worth medical attention rather than reassurance from a calculator.

Where BMI falls down

BMI is an estimate that cannot see body composition. It measures excess weight, not excess fat, and it says nothing at all about where fat sits on the body.

In adults. Muscle is denser than fat, so muscular people and trained athletes routinely land in the overweight or obese range while carrying very little fat. The reverse also happens: older adults tend to carry more fat than younger adults at the same BMI, because muscle mass falls with age, and someone can have a perfectly normal BMI alongside too little muscle and too much fat. Women generally carry more body fat than men at an equivalent BMI. Age, sex, ethnicity and activity level all shift the relationship.

In children and adolescents. Everything above applies, plus height and stage of sexual maturation. BMI tracks body fat better in children who are already overweight than in those who are not, which is part of why percentiles rather than thresholds are used.

Fat distribution. Two people with identical BMIs can have very different risk profiles depending on whether fat sits under the skin or around the organs. Visceral fat around the abdomen is the metabolically dangerous kind, and BMI cannot detect it.

These limits are now formally acknowledged. The American Medical Association adopted a policy in 2023 describing BMI as an imperfect measure and recommending it be used alongside other indicators rather than on its own. A Lancet Commission on clinical obesity published in January 2025 went further, proposing that obesity be diagnosed from measures of body fat and actual organ or functional impairment, and distinguishing clinical obesity, where illness is present, from pre-clinical obesity, where the risk is raised but function is intact.

None of that makes BMI useless. It corresponds reasonably well to body fat for the large majority of people and remains a sound first screen. It is a starting point, not a diagnosis.

Two measurements worth taking alongside it

Waist circumference gets at the visceral fat BMI misses. Measured at the top of the hip bone, above about 40 inches (102 cm) for men or 35 inches (88 cm) for women indicates raised risk, whatever the BMI says.

Waist-to-height ratio is simpler still and needs no table: keep your waist under half your height. At 5 feet 10 inches that means under 35 inches. UK NICE guidance recommends this ratio precisely because one rule covers adults of every height, and it flags central fat that BMI passes over.

BMI prime

BMI prime is your BMI divided by 25, the upper limit of the normal range:

BMI prime = BMI ÷ 25

Because it is a ratio of two BMIs it has no units, and it reads as a straightforward proportion. A BMI prime of 0.92 means you are at 92% of the upper healthy limit; 1.2 means 20% above it. Anything below 0.74 is underweight, 0.74 to 1 is normal, above 1 is overweight, and above 1.2 is obese. The table above lists the full set alongside the BMI ranges.

Its real use is comparison. Because populations with different upper limits, such as the lower Asian cut-offs, can each divide by their own threshold, BMI prime lets groups be compared on a common scale that plain BMI does not provide.

Ponderal Index

The Ponderal Index measures the same thing as BMI but cubes height instead of squaring it:

PI = mass (kg) ÷ height³ (m)

72.57 ÷ 1.778³ = 12.9

In US units, divide height in inches by the cube root of weight in pounds: 70 ÷ ∛160 also gives 12.9.

Cubing matters because BMI systematically flatters short people and penalises tall ones. Human bodies scale in three dimensions, not two, so a taller person of identical proportions gets a higher BMI purely from the arithmetic. The Ponderal Index is more reliable at the extremes of height for that reason, and is used in neonatal medicine to assess newborns. For people of average height the two broadly agree, and BMI's ubiquity keeps it the default.

Reading your result

The panel gives your BMI, your category, and the weight range that corresponds to a BMI of 18.5 to 25 at your height, which is the number to act on. The map shows how much movement in either direction would change your category, and the bar chart converts every WHO threshold into a weight for your height specifically.

If you sit near a boundary, do not over-read the single decimal. A pound of water weight or a differently calibrated set of scales moves it. The trend across months tells you more than any one reading. Our Body Fat Calculator, Ideal Weight Calculator and Calorie Calculator cover the measurements BMI cannot.