BMI Calculator, Metric and Imperial
Enter your height and weight in whichever units you think in. You get the number, the World Health Organization category it falls into, and the weight range that would put you in the middle band at your height. What BMI can and cannot tell you is set out underneath, and it is worth reading.
Metres work too. 1.75 is read the same as 175.
What every band weighs at your height
BMI is a screening figure calculated from two measurements. It is not a diagnosis and it says nothing about what any individual body is made of.
The formula, in both unit systems
BMI is weight divided by height squared, in kilograms and metres. Someone 1.75 m and 70 kg has a BMI of 70 ÷ 1.75², which is 22.9.
In imperial units the same ratio needs a conversion constant: BMI = 703 × pounds ÷ inches². The 703 is not arbitrary and it is not a fudge — it is what you get converting pounds per square inch into kilograms per square metre. This tool converts to metric internally and converts back for display, so both modes give the same answer for the same body.
Where the categories come from
The ratio was devised in the 1830s by Adolphe Quetelet, a Belgian astronomer and statistician who was looking for a way to describe the distribution of body sizes across a population. He said explicitly that it was not meant for judging individuals. The name "body mass index" came much later, from the physiologist Ancel Keys in a 1972 paper that compared several simple height-weight indices and concluded this one worked best for population studies.
The cutoffs at 18.5, 25 and 30 are WHO conventions drawn from the observed relationship between BMI and mortality in large cohorts. They are round numbers on a smooth curve. Nothing changes about your body when you cross from 24.9 to 25.0, and the risk difference between a BMI of 24 and a BMI of 26 is small enough to be lost in the noise of everything else about you.
What BMI gets wrong about individuals
This is the part worth understanding, because the number on its own is routinely over-read.
- It cannot see what you are made of. Muscle is denser than fat, so a trained athlete and a sedentary person of the same height and weight get the same BMI and are in completely different physical condition. Rugby players and weightlifters routinely register as obese.
- It cannot see where the weight is. Fat around the abdomen carries more metabolic risk than the same amount elsewhere, and BMI is blind to the distinction. Waist-to-height ratio catches this and takes ten seconds to measure: NICE guidance in the UK uses keeping your waist under half your height as the simple threshold.
- The squaring is a compromise. Mass scales roughly with volume, which is a cube of length, while BMI divides by a square. The consequence is systematic: tall people get slightly inflated BMIs and short people slightly deflated ones for the same build. Quetelet chose the square because it fitted his data, not because the geometry demanded it.
- It says nothing about change over time. An older adult losing muscle can keep an identical BMI while their body composition gets meaningfully worse.
Who the standard thresholds do not fit
The 18.5 / 25 / 30 bands are for adults. They are the wrong tool in several common situations:
- Children and teenagers are assessed with BMI-for-age percentile charts, because the normal ratio changes throughout growth. The adult number is meaningless for them.
- People of Asian descent tend to carry more body fat and more cardiometabolic risk at a given BMI. A 2004 WHO expert consultation recommended additional public-health action points at 23 and 27.5, which the summary above shows alongside the standard band.
- Pregnancy makes BMI uninterpretable. Pre-pregnancy BMI is what clinicians use.
- Amputation, significant oedema and some muscle-wasting conditions all break the assumption that weight is a proxy for tissue mass.
BMI survives because it needs two measurements anyone can take and it predicts population-level risk reasonably well. As a personal verdict it is a rough screening figure and nothing more. Nothing you enter here leaves your browser.
Frequently asked questions
What is a healthy BMI?
The WHO puts the adult range at 18.5 to 24.9. Below that is underweight, 25 to 29.9 is overweight and 30 or more is obesity, split into three classes at 35 and 40. The boundaries are conventions on a continuous curve, not switches.
Is BMI accurate for athletes?
No. Muscle weighs more than fat for the same volume, so a lean, heavily trained person often lands in the overweight or obese band while carrying very little fat. If you lift seriously or play a contact sport, BMI is the wrong measurement to be using.
Why is the imperial formula multiplied by 703?
It is the unit conversion. One kilogram per square metre equals about 703 pounds per square inch, so multiplying pounds-over-inches-squared by 703 gives the same number the metric formula does. It is arithmetic, not an adjustment.
Can I use this for my child?
Not as it stands. Children and teenagers are assessed against BMI-for-age percentile charts, because the expected ratio changes as they grow. Running an adult BMI on a twelve-year-old produces a number that means nothing.
What is a better measurement than BMI?
For a quick self-check, waist-to-height ratio — keep your waist measurement under half your height. It picks up abdominal fat, which BMI cannot see, and needs only a tape measure. For anything beyond screening, body composition measurement and a clinician beat both.
Last updated September 19, 2026