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HealthBody Mass Index (BMI) is one of the most quoted — and most misunderstood — numbers in health. It is not a scan, a lab test, or a diagnosis. It is a simple ratio of your weight to your height that has survived nearly 200 years because it takes ten seconds to compute and needs nothing more than a scale and a tape measure.
This guide walks through the exact formula in both metric and imperial units, works two examples step by step, explains the categories, and shows exactly where the math starts lying to you.
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The formula
BMI = weight ÷ height²
— weight in kilograms, height in meters —
— weight in kilograms, height in meters —
That's the whole thing. Square your height, divide your weight by it. The result has units of kg/m², which nobody ever says out loud — everyone just reports the number.
Height is squared because body mass scales with volume, and volume grows roughly with the square of height. Without the square, taller people would systematically score higher simply for being tall, and the index couldn't compare people of different statures.
Imperial (US) version
If you measure in pounds and inches, the formula needs a conversion factor of 703:
BMI = 703 × weight (lb) ÷ height (in)²
The 703 is just unit conversion folded into one constant: it converts pounds to kilograms and inches to meters inside the formula so the result matches the metric version exactly.
Worked example 1: metric
A person weighs 70 kg and is 1.75 m tall:
- Square the height: 1.75 × 1.75 = 3.0625
- Divide: 70 ÷ 3.0625 = 22.9
- BMI = 22.9 — inside the healthy range
Worked example 2: imperial
A person weighs 180 lb and is 5 ft 9 in tall (69 inches):
- Square the height: 69 × 69 = 4,761
- Multiply weight by 703: 180 × 703 = 126,540
- Divide: 126,540 ÷ 4,761 = 26.6
- BMI = 26.6 — the "overweight" category
What the categories mean
| BMI | WHO category | What it means |
|---|---|---|
| Below 18.5 | Underweight | May indicate inadequate nutrition or muscle mass |
| 18.5 – 24.9 | Healthy weight | Lowest population-level health risk band |
| 25 – 29.9 | Overweight | Elevated risk flag — worth a closer look |
| 30 and above | Obesity | Higher risk band; subdivided into classes I, II, III |
These cutoffs come from the World Health Organization and apply to adults 20 and older. Children and teens use age- and sex-specific percentiles, not these fixed bands — see our BMI for children and teens guide.
A brief history of the formula
Belgian statistician Adolphe Quetelet devised the ratio in the 1830s as a way to describe the "average man" in population studies — it was originally called the Quetelet Index. It stayed an academic curiosity until 1972, when researcher Ancel Keys published a landmark paper studying thousands of men and found the index correlated well with body fat at the population level. He gave it its modern name: body mass index.
That origin matters. BMI was designed to describe populations, and it does that job well. Applying it to individuals is where the trouble starts.
Where BMI misleads
- Muscular builds: muscle is denser than fat, so athletes and weightlifters often score "overweight" with very low body fat. This is the formula's biggest blind spot.
- Fat distribution: two people with the same BMI can carry fat very differently — visceral fat around the organs is riskier than subcutaneous fat, and BMI sees neither.
- Age: older adults naturally lose muscle and gain fat; BMI can read "healthy" while body composition has shifted.
- Pregnancy and athletes, and very short or tall people: the formula is calibrated for average adult builds and gets less reliable at the extremes.
When BMI's reading conflicts with how you look and feel, compare BMI against body fat percentage or take a waist measurement — both add the context the formula lacks.
How to use the number sensibly
Treat BMI as a screening flag, not a verdict. If it lands outside the healthy band, treat it as a prompt to look closer — check body fat percentage, waist circumference, blood pressure, and activity level — rather than as proof something is wrong. And if it lands inside the band while other signals (breathlessness on stairs, blood sugar, sleep) suggest otherwise, the flag can miss just as easily as it can misfire.
Frequently asked questions
Because weight scales with body volume, which grows roughly with the square of height. If height were not squared, taller people would systematically score higher just for being tall. Squaring height keeps the index comparable across different statures.
Not very. BMI cannot distinguish muscle from fat, so athletes and people who lift weights often land in the "overweight" range despite low body fat. For muscular builds, body fat percentage or waist measurements give a clearer picture.
The World Health Organization defines 18.5 to 24.9 as the healthy range for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obesity. These are screening categories, not personal diagnoses.
Belgian statistician Adolphe Quetelet devised it in the 1830s as a population-level index (the Quetelet Index). Researcher Ancel Keys popularized it under the name "body mass index" in a landmark 1972 paper studying thousands of men.
Disclaimer: Calculator health content is for education only — not medical advice. Body metrics are screening tools, not diagnoses. If you have concerns about your health, speak with a qualified health professional.