BMI Calculator

About this calculator

What body mass index measures

Body mass index relates your weight to your height. In pounds and inches, the formula is 703 × weight in pounds ÷ height in inches squared. Someone who weighs 154 lb and stands 5 ft 9 in (69 inches) has a BMI of 703 × 154 ÷ 69² = 22.7.

The 703 is a conversion factor, not a fudge: the original formula is metric, weight in kilograms divided by height in meters squared, and 703 is what turns pounds per square inch into kilograms per square meter. The calculator above works in either system — switch to metric if you know your numbers in kg and cm, and anything you have typed is converted for you.

The Belgian mathematician Adolphe Quetelet proposed it in the 19th century, and his purpose was to describe populations, not diagnose individuals. That distinction is still the key to reading it sensibly.

Reading your result

The World Health Organization uses four adult ranges:

These cut-offs were derived largely from European populations. Several Asian medical bodies use lower thresholds, because metabolic risk in those populations appears at a lower BMI.

What BMI is good for, and what it misses

BMI earns its place because it is cheap, fast and reproducible. A set of scales and a tape measure give you a number that is comparable between people and over time. Across large studies it correlates reasonably well with the risk of type 2 diabetes, high blood pressure and cardiovascular disease.

Here is the part that usually goes unsaid: BMI has no idea what your weight is made of. It cannot tell muscle from fat, it does not account for water retention, and it does not know where that fat sits. That last point matters a great deal, because visceral fat — the kind packed around the abdominal organs — carries far more metabolic consequence than fat under the skin.

In practice that means:

• A muscular person can be classed as overweight while carrying a perfectly healthy body fat percentage.
• Someone with a normal BMI but little muscle and abdominal fat can have a worse metabolic profile than a heavier person.
• Past the age of 65, a BMI slightly above 25 is associated in several studies with better outcomes, not worse.

What to measure alongside it

If you want a truer picture using only what you have at home, waist circumference is the next most informative measurement. Take it horizontally at navel height, without pulling the tape tight, at the end of a normal breath out. As a rough guide, above 35 inches (88 cm) for women and 40 inches (102 cm) for men is treated as a warning sign.

The waist-to-height ratio is simpler still: divide your waist by your height in the same units. A figure below 0.5 is generally read as favourable, and it has the advantage of working equally well across different heights and builds.

Moving the number

If your goal is to shift your BMI, what works long term is not speed but sustainability. Losing 0.25 to 0.5 kg per week preserves muscle mass in a way that aggressive dieting does not. Keeping up resistance training through that process is what stops a large share of the loss coming from muscle rather than fat.

And the obvious caveat still stands: this number is a reference point, not a diagnosis. If you have any concern about your health, a clinician can read your situation with all the context this calculator does not have.