BMI Calculator
Work out your body mass index in metric or imperial units. You will get the number, the category the WHO puts it in, and a clear note on what BMI can and cannot tell you.
Explain like I'm 5 (what even is this calculator?)
BMI is a quick sum that compares your weight to your height. Type in two numbers, get one number back. Doctors use it as a rough signpost: is someone likely carrying too little weight for their height, about right, or too much? It is a signpost, not a verdict.
Calculate
Enter your weight and height, then press Calculate.
Prove it
Metric formula: BMI = weight (kg) ÷ height (m)². Imperial formula: BMI = (weight (lb) × 703) ÷ height (in)². Categories: under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, 30 and over obese. Source: World Health Organisation.
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What BMI actually measures
BMI, body mass index, is a number that compares weight to height. Nothing more, nothing less. It was devised in the 1830s by a Belgian statistician who was looking for a tidy way to describe the "average man" in a population. It was never meant to be a health diagnosis for an individual. Somewhere along the way, that got forgotten.
As a rough filter across millions of people, it is useful. As a description of the person standing in front of you, it has serious limitations. Knowing both at once is how you use it sensibly.
When BMI is a bad fit
BMI uses only weight and height, so any person whose body composition sits outside the statistical middle is poorly described by it.
- Athletes and very muscular people routinely score as overweight or obese while carrying very little body fat. Muscle is denser than fat; the scale can't tell the difference.
- Older adults tend to lose muscle mass over time, so a "normal" BMI can mask a higher body-fat percentage than the number suggests.
- Children and teenagers need BMI-for-age percentiles that account for growth. The adult cut-offs used here do not apply.
- Pregnant and post-natal women are not well described by BMI for obvious reasons.
- People of different ethnic backgrounds carry body fat differently, and the WHO itself has flagged that standard thresholds may overestimate healthy ranges for some Asian populations and underestimate risk for others. A GP who knows you will interpret the number more carefully than a generic formula can.
How to use the number honestly
Treat BMI as one reading on one gauge. If it lands well outside the "normal" band, it is worth a conversation with a doctor who can look at the rest of the picture: waist measurement, blood pressure, blood tests, family history, how you actually feel. If it lands near the middle, that is reassuring but not a green light to ignore everything else. Plenty of people with a "normal" BMI have poor underlying health; plenty with a "high" BMI are fit and well.
Nothing on this page is medical advice. If you are worried about your weight, see a professional who can look at you, not a form on a website.
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BMI is a blunt measure. These are the more useful daily numbers.
Frequently asked questions
What is a healthy BMI?
The WHO classifies 18.5 to 24.9 as "normal" for most adults. Below 18.5 is underweight, 25 to 29.9 is overweight, 30 and above is obese. These are screening categories, not a diagnosis.
How is BMI calculated?
Metric: weight in kilograms divided by the square of your height in metres. Imperial: weight in pounds times 703, divided by the square of your height in inches.
Is BMI accurate for athletes or very muscular people?
Often not. BMI only knows weight and height, so it cannot tell muscle from fat. A lean rugby player or bodybuilder will often score in the overweight or obese range.
Does BMI apply to children?
No. Children and teenagers need age- and sex-specific BMI-for-age percentiles rather than the adult cut-offs used here. Speak to a GP or paediatrician.
Does this calculator send my numbers anywhere?
No. Everything runs in your browser. The numbers you type never leave your device.