Body Mass Index (BMI) is a quick screening number that relates your weight to your height. It's easy to calculate by hand, and knowing the formula helps you understand what the result does — and doesn't — tell you. Here's how to work it out in both metric and imperial units.
The BMI formula (metric)
In metric units, BMI is your weight in kilograms divided by the square of your height in metres: BMI = weight(kg) ÷ height(m)². Square the height once, then divide. The result has units of kg/m², but it's almost always written as a plain number.
The BMI formula (imperial)
If you measure in pounds and inches, multiply your weight by 703 and divide by your height in inches squared: BMI = (weight(lbs) × 703) ÷ height(in)². The 703 factor simply converts pounds-and-inches into the same kg/m² result, so both formulas give an identical number.
Worked example
Take someone who weighs 70 kg and is 175 cm (1.75 m) tall. First square the height: 1.75 × 1.75 = 3.0625. Then divide: 70 ÷ 3.0625 = 22.86, which rounds to 22.9. A BMI of 22.9 falls in the normal (healthy) weight range. The same person in imperial units (154 lbs, 68.9 in) gives (154 × 703) ÷ 68.9² = 108,262 ÷ 4,747 = 22.8 — the same answer apart from rounding.
BMI classification chart
The World Health Organization uses these adult ranges, the same for men and women aged 20 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
The limitations of BMI
BMI uses only height and weight, so it can't tell what your weight is made of or where it sits. Muscle is denser than fat, so very muscular people can register as "overweight" despite low body fat. It also ignores fat distribution (abdominal fat is riskier than fat elsewhere), and health risk at a given BMI can differ by age and ethnicity. Treat it as one rough signal, not a verdict.
BMI vs body fat percentage
Body fat percentage measures the proportion of your weight that is fat, which BMI can't see. It's more informative about body composition but harder to measure accurately (it needs calipers, a smart scale, or a scan). BMI is popular precisely because it needs only a tape measure and a scale. For a deeper look at what BMI captures and misses, read Understanding BMI.
Where the BMI formula comes from
BMI isn't new — it was devised in the 1830s by the Belgian mathematician Adolphe Quetelet and was originally called the Quetelet Index. He was studying populations, not diagnosing individuals, which is the key to understanding the formula's strengths and limits. Dividing weight by height squared roughly cancels out the effect of height, so people of different heights can be compared on one scale. It became "Body Mass Index" in the 1970s and is now the standard population-level screening measure precisely because it needs nothing more than a scale and a tape measure.
BMI for children, teens, and older adults
The adult categories above apply to people aged 20 and over. For children and teens, the same weight and height go into the formula, but the result is read against age- and sex-specific percentile charts rather than fixed cutoffs, because healthy body composition changes rapidly during growth. In older adults, a slightly higher BMI is sometimes associated with better outcomes, and muscle loss can hide changes in body fat. For these groups especially, treat BMI as a conversation starter with a professional, not a standalone verdict.
A useful companion: waist measurement
Because BMI can't see where fat sits, pairing it with a waist measurement adds important context. Abdominal (visceral) fat carries more metabolic risk than fat on the hips or limbs, so a large waist can flag risk even at a "normal" BMI. A simple rule of thumb is to keep your waist under half your height — a waist-to-height ratio below 0.5. Together, BMI and waist give a fuller picture than either alone.
What to do with your number
If your BMI sits in the normal range, it's one reassuring signal among many — keep an eye on trends rather than a single reading. If it falls outside that range, resist both panic and dismissal: it's a prompt to look at the fuller picture (waist, fitness, blood markers, family history, how you feel) and, if you're unsure, to ask a doctor or dietitian. Sustainable changes to diet, movement, and sleep move the number far more reliably than crash approaches, and a professional can tell you whether the number means anything for you specifically.
Do doctors still use BMI?
Yes — but as a first-pass screen, not a diagnosis. Clinicians use BMI to quickly flag who might benefit from a closer look, then layer in measurements it can't capture: waist circumference, blood pressure, blood sugar and cholesterol, family history, and lifestyle. Major health bodies have explicitly cautioned against treating BMI as a standalone measure of an individual's health, recommending it be used alongside other indicators. So if a BMI reading surprises you, treat it as a reason to gather more information rather than a conclusion in itself — the number is a starting point for a conversation, not the end of one.