BMI Calculator
Calculate body mass index in metric or imperial units, with your category, the healthy weight range for your height, and an honest account of the limits.
Your BMI
Remember what BMI cannot see. It does not distinguish muscle from fat, ignores where fat is carried, and is not valid for children, pregnancy or highly trained athletes. Treat it as one screening figure, not a diagnosis, and discuss anything that concerns you with a qualified healthcare professional.
About the BMI Calculator
Body mass index is weight in kilograms divided by height in metres squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way of describing populations, and it was adopted for individual screening much later, largely because it is cheap and needs only two measurements that anyone can take.
The standard adult categories are: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or above obese, which is often subdivided into three classes. Several health authorities use lower thresholds for people of South Asian, Chinese and other Asian descent, where cardiometabolic risk rises at a lower BMI — a threshold of 23 rather than 25 is common in those guidelines.
It is worth being clear about what the number cannot do. BMI does not distinguish muscle from fat, so a well-trained athlete is routinely classified as overweight while carrying very little fat. It says nothing about where fat is distributed, and abdominal fat carries considerably more metabolic risk than the same mass elsewhere. It is not valid for children and teenagers, who need age and sex-specific percentile charts, and it is not appropriate during pregnancy. Waist circumference and waist-to-height ratio are better single indicators of risk for most adults — this calculator shows your BMI clearly and tells you honestly where it stops being useful.
How to use it
- Choose your unitsMetric with kilograms and centimetres, or imperial with pounds, feet and inches.
- Enter height and weightUse your most recent measurements. Small errors in height affect the result more than you would expect, because height is squared.
- Read your BMIThe value is shown with one decimal place alongside its standard category.
- See your healthy rangeThe weight range corresponding to a BMI of 18.5 to 24.9 at your specific height.
- Check the position indicatorA visual scale shows where your value sits across the categories.
- Read the limitationsUnderstand what the number does not capture before drawing conclusions from it.
Features
- Metric and imperialKilograms and centimetres, or pounds with feet and inches — converted correctly, not approximated.
- Standard categoriesThe WHO adult classifications, with obesity subdivided into classes I, II and III.
- Healthy weight rangeThe actual weight range for your height that corresponds to a healthy BMI.
- Visual scaleA clear indicator of where your value sits, rather than a bare number.
- BMI PrimeYour BMI as a ratio of the upper healthy limit, which some clinicians find easier to interpret.
- Completely privateHeight and weight are health data. Nothing is transmitted, logged or stored.
Who uses this tool
- General health awarenessGet a quick screening figure and understand where it sits in the standard ranges.
- Setting a weight goalSee the actual weight range for your height rather than working from a number you half-remember.
- Before a medical appointmentArrive knowing the figure your clinician is likely to calculate and discuss.
- Fitness trackingUse it as one input among several, understanding that it will misclassify a muscular build.
Tips worth knowing
- Measure height without shoes and weight at a consistent time of day — body weight varies by one to two kilograms across a normal day.
- If you carry significant muscle, treat BMI as unreliable and use waist-to-height ratio or a body composition measurement instead.
- Waist circumference below half your height is a simple, well-supported risk indicator that BMI misses entirely.
Frequently asked questions
What is a healthy BMI?
For most adults, 18.5 to 24.9. Below 18.5 is classified as underweight, 25 to 29.9 as overweight and 30 or above as obese. Several health authorities apply lower thresholds for people of South Asian and other Asian descent, where risk rises at a lower BMI.
How is BMI calculated?
Weight in kilograms divided by height in metres squared. In imperial units the formula is weight in pounds divided by height in inches squared, multiplied by 703. This calculator converts imperial input precisely rather than using a rounded factor.
Is BMI accurate for athletes?
No. Muscle is denser than fat, so a muscular person can have a high BMI with very low body fat. Bodybuilders and many professional athletes are routinely classified as overweight or obese by BMI alone, which is a limitation of the measure rather than a finding about them.
Does BMI work for children?
Not in this form. Children and teenagers are assessed against age and sex-specific percentile charts, because healthy body composition changes substantially during growth. This calculator is intended for adults aged 18 and over.
What is BMI Prime?
Your BMI divided by 25, the upper limit of the healthy range. A value under 1.0 sits inside the healthy range and a value of 1.2 means you are 20% above the upper limit — some people find that ratio more intuitive than the raw number.
Should I make health decisions based on BMI?
It is a screening tool, not a diagnosis. It does not measure body fat, distribution, fitness or metabolic health. Discuss any concerns with a qualified healthcare professional who can consider the full picture. Nothing here is medical advice.