Article — BMI Calculator
BMI: what your number means and where it falls short
What BMI actually measures
BMI divides your weight by the square of your height. That is the entire formula. It does not measure fat, muscle, bone density, or anything else people assume it does. It is a ratio, and a crude one.
The number is useful because it correlates with body fat at a population level. For most adults who do not lift weights seriously, a BMI of 22 means something different from a BMI of 35, and doctors can screen thousands of patients quickly using it. That is why every annual checkup starts with one.
But it was never designed to diagnose individuals. It was designed to study populations. Keep that distinction in mind.
The formula uses height squared, not height cubed. If it used cubing (which would track volume more accurately), tall people would almost never register as overweight. Quetelet picked squaring because it fit his 19th-century data better. Modern researchers debate whether an exponent of 2.5 would be more accurate, but the WHO has stuck with 2.
The WHO categories
The World Health Organization splits BMI into six brackets. The cutoffs are the same for men and women, which is one of the criticisms (more on that later).
Underweight (below 18.5) carries its own risks: weakened immunity, bone loss, fertility problems. Most nutrition research focuses on overweight and obesity because that is where the numbers are going globally, but being underweight is not safe either.
If your BMI lands right on a boundary (say, 24.8 or 25.1), do not panic. A single digit either side of 25 is meaningless on its own. Look at the trend over months, not a single weigh-in.
How a 19th-century astronomer invented it
Adolphe Quetelet (1796-1874) was a Belgian mathematician and astronomer. He was not a doctor. He was interested in the concept of the "average man" and collected body measurements from thousands of people to find statistical patterns.
In 1832, he published his observation that weight scales with the square of height across populations. He called it the Quetelet Index. The formula sat mostly unused for 140 years until American physiologist Ancel Keys gave it the name "Body Mass Index" in a 1972 paper and recommended it as a cheap screening tool.
The fact that a formula from 1832 is still your doctor's first stop in 2024 says more about how convenient it is than how accurate it is.
Quetelet also founded the Royal Observatory of Belgium and pioneered the use of statistics in criminology. BMI was a side project. He was trying to mathematically define the "average man," not create a health metric.
Where BMI breaks down
BMI does not distinguish between muscle and fat. This is the number-one complaint about it, and it is valid.
The WHO suggests lower thresholds for Asian populations: overweight at BMI 23 (not 25) and obese at BMI 27.5 (not 30). These adjusted cutoffs better predict diabetes and cardiovascular risk in East and South Asian adults.
BMI around the world
Average BMI varies wildly by country. The US has been trending upward since the 1970s. Japan has barely moved.
In 1970, about 15% of US adults were obese (BMI 30+). Today it is 42%. Genes do not change that fast. What changed: 60% of American calories now come from ultra-processed food, average daily steps dropped from 6,000 to 3,000, and portion sizes roughly doubled.
Japan has the lowest average BMI in the developed world and the longest life expectancy. School lunch programs there are standardized by nutritionists, and children walk to school. The obesity rate is under 5%, compared to 42% in the US.
When to use it (and when not to)
BMI works well as a first pass. It is free, takes five seconds, and correlates with health outcomes for most people. But treat it as one data point, not a verdict.
BMI Free, fast, population-level screeningWaist circumference Better predictor of visceral fatWaist-to-hip ratio Accounts for body shapeBody fat % What BMI tries to estimateDEXA scan Gold standard, but expensiveIf your BMI is 23 and you feel good, skip the DEXA scan. If your BMI is 32 and you want to know whether it is muscle or fat, waist circumference and a body fat measurement will tell you more than BMI ever will.
Common mistakes
5.8 feet is 5 feet 9.6 inches (176.8 cm), not 5 feet 8 inches (172.7 cm). The difference is 4 cm, which shifts BMI by about 0.5 points. Always enter feet and inches separately.
Your weight swings 1-2 kg (2-4 lb) from morning to evening due to food, water, and digestion. Always weigh yourself at the same time. Morning, after the bathroom, before eating.
A BMI of 26 does not mean you are unhealthy. A BMI of 22 does not mean you are healthy. BMI correlates with risk at the population level. For individual health, you need blood work, blood pressure, activity level, and a doctor who looks at the whole picture.