BMI Calculator

What this calculator does

This tool calculates Body Mass Index (BMI) from a height and a weight. BMI is a single number that relates body mass to height, and it is the most widely used screening figure in public health. You enter your measurements in either metric or imperial units, and the calculator instantly shows your BMI to one decimal place, the World Health Organization category that the number falls into, and the range of weights that would correspond to a “normal weight” BMI for your height. Every figure updates live as you type, and you can copy the BMI value with one click.

Why you might need it

BMI appears constantly: on health forms, in fitness apps, in insurance paperwork, and in medical records. Being able to work it out quickly is useful when you only have a height and weight to hand and want the standardised number rather than a rough guess. The healthy weight range is handy in a different way — instead of asking “what is my BMI”, it answers “what weights fall inside the normal band for someone my height”, which is a more concrete piece of information. Because the calculator handles both unit systems, it is also convenient when a document gives measurements in pounds and inches but you need the metric BMI, or vice versa.

How to use it

1. Choose Metric or Imperial with the units toggle at the top.
2. In metric mode, enter your height in centimetres and weight in kilograms. In imperial mode, enter height in feet and inches and weight in pounds.
3. Read the BMI figure and its WHO category in the result card.
4. Check the healthy weight range card to see the kilogram and pound band that maps to a normal-weight BMI for your height.
5. Use the Copy BMI button to copy the number, or Reset to return every field to its default.

How it’s calculated

BMI uses one formula: weight in kilograms divided by the square of height in metres, written BMI = kg ÷ m². A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. When you work in imperial units the calculator first converts: it multiplies feet by 12, adds the inches, multiplies by 2.54 to get centimetres, and divides by 100 for metres; pounds are multiplied by 0.45359237 to get kilograms. The same division then runs. The WHO adult classification splits the result into four bands: below 18.5 is underweight, 18.5 to 24.9 is normal weight, 25.0 to 29.9 is overweight, and 30.0 and above is obese. The healthy weight range is found by rearranging the formula to solve for weight — multiplying 18.5 and 24.9 by height in metres squared gives the lower and upper kilogram limits, which are then converted to pounds.

Common pitfalls

The most important thing to understand about BMI is that it measures mass, not body composition. It cannot tell muscle from fat or where fat is distributed, so a very muscular athlete and a sedentary person can share the same BMI. The WHO bands are defined for adults; they do not apply to children and teenagers, who are assessed with separate percentile charts, nor are they adjusted for pregnancy. Another common slip is a units mismatch — entering a weight in pounds while the toggle still says metric will produce a wildly wrong figure, so check the toggle before reading the result. Finally, BMI is a screening number, not a diagnosis; it places you in a band but does not describe an individual’s health.

Tips and related calculations

If you want to understand the energy side of the picture rather than the mass side, pair this with a BMR calculator, which estimates how many calories your body burns at rest, or a calorie calculator, which scales that figure by an activity level. Those tools answer “how much energy do I use”, while BMI answers “how does my mass compare to my height”. When you record a BMI over time, keep the measurement conditions consistent — same scale, similar time of day — because day-to-day weight fluctuates and small changes can nudge the number across a band boundary. Because the whole calculation is simple arithmetic running locally, you can re-run it as often as you like and nothing ever leaves your browser.