BMI Calculator

About this calculator

What body mass index is

Body mass index relates your weight to your height with a simple formula: your weight in kilograms divided by the square of your height in metres. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

It was proposed by the Belgian mathematician Adolphe Quetelet in the 19th century, and its original purpose was to describe populations, not to diagnose individuals. That distinction is still the key to reading it well today.

How to read your result

The World Health Organization uses four ranges for adults:

These thresholds were set from studies in European populations. Some Asian medical societies use lower cut-offs, because in those populations metabolic risk appears at a lower BMI.

What BMI is good for, and what it isn’t

BMI is useful because it is cheap, quick and repeatable. With scales and a tape measure you get a number you can compare between people and over time. In studies with thousands of participants it correlates reasonably well with the risk of type 2 diabetes, high blood pressure and cardiovascular disease.

Now the part that is almost never mentioned: BMI can’t tell what your weight is made of. It doesn’t know how much is muscle, how much is fat, how much water you retain or where that fat is stored. That last part matters a lot, because visceral fat (the fat around the abdominal organs) has a much bigger metabolic impact than subcutaneous fat.

In practice:

• Someone with a lot of muscle mass can be classed as “overweight” with a perfectly healthy body fat percentage.
• Someone with a normal BMI but little muscle and abdominal fat can have a worse metabolic profile than someone with a higher BMI.
• From age 65, a BMI slightly above 25 is associated in several studies with a better outlook, not a worse one.

What to measure besides BMI

For a more accurate picture with home tools, waist circumference is the next most informative measure, and all you need is a tape measure. Measure horizontally at the level of your navel, without pulling tight, at the end of a normal breath out. As a rough guide, values above 88 cm for women and 102 cm for men are considered a warning sign.

The waist-to-height ratio is even simpler: divide your waist by your height, both in the same unit. A result below 0.5 is usually seen as favourable, and it works equally well across different heights and builds.

How to move the number

If your goal is to change your BMI, what works in the long run is not speed but sustainability. A pace of 0.25 to 0.5 kg a week lets you keep muscle mass, which very aggressive diets don’t. Keeping up strength training during that process is what stops a good part of what you lose from being muscle.

And it’s worth stating the obvious: this number is a reference, not a diagnosis. If you have any doubt about your health, the person who can interpret your case with all the context is a healthcare professional.