BMI Calculator
Calculate your Body Mass Index from height and weight, in metric or imperial units, and see your WHO weight category instantly.
BMI is a rough screening measure, not a diagnosis. It doesn't account for muscle mass, body composition, age, or sex. Talk to a healthcare professional to interpret your result.
What is BMI?
Body Mass Index (BMI) is a simple number that relates a person's weight to their height. It was developed in the 1830s by the Belgian mathematician Adolphe Quetelet and is used worldwide as a quick, low-cost way to screen adults for weight categories that may be associated with health risks. Because it needs only two easily measured values — height and weight — it's a convenient first indicator, but it is a screening tool, not a diagnosis of health or body fatness.
The BMI formula
In metric units, BMI is your weight in kilograms divided by the square of your height in metres:
BMI = weight (kg) ÷ height (m)2
In imperial units, the same relationship uses a conversion factor of 703:
BMI = 703 × weight (lb) ÷ height (in)2
This calculator converts your imperial entries (feet, inches, and pounds) to metric internally, so both unit systems give the same result for the same body.
WHO weight categories for adults
The World Health Organization classifies adult BMI into four broad categories:
- Underweight — BMI below 18.5
- Normal weight — BMI from 18.5 to 24.9
- Overweight — BMI from 25 to 29.9
- Obese — BMI of 30 or higher
These thresholds apply to most adults aged 20 and over. They are not appropriate for children and teenagers, who are assessed against age- and sex-specific percentile charts, nor are they adjusted for pregnancy. Some health bodies also use lower cut-off points for people of South Asian and certain other ancestries, where health risks can appear at a lower BMI.
How to use this calculator
Pick your unit system, then enter your height and weight. If you use imperial units, split your height into feet and
inches. Your BMI and its WHO category update instantly as you type. If a field is empty or zero, the calculator
simply waits rather than showing an error — so you'll never see an NaN or "Infinity" result.
Worked example
Suppose you are 170 cm tall and weigh 70 kg. Converting height to metres gives 1.70 m, and 1.70 squared is 2.89. Dividing 70 by 2.89 gives a BMI of about 24.2, which falls in the Normal weight range. The same person entered in imperial units — roughly 5 ft 7 in and 154 lb — produces the same BMI, because the tool converts the values before calculating.
Important limitations
BMI treats all mass the same, so it can't tell the difference between muscle and fat. A very muscular athlete may register as "overweight" or "obese" despite low body fat, while an older adult with reduced muscle mass may have a "normal" BMI yet carry excess fat. BMI also says nothing about where fat is stored — waist circumference and other measures add useful context. Treat your BMI as one data point and discuss it with a qualified healthcare professional before drawing conclusions.