BMI Calculator

What BMI measures

Body mass index compares your weight with your height. It does not measure body fat directly, but across large groups of adults a higher BMI goes with a higher risk of conditions such as type 2 diabetes, high blood pressure and heart disease, and a very low BMI goes with its own risks. Because it needs only a tape measure and a scale, doctors use it as a quick first screen, then look at other measures before drawing conclusions about one person.

How to use the calculator

1. Choose Metric for centimetres and kilograms, or Imperial for feet, inches and pounds.
2. Enter your height without shoes and your weight, ideally measured in the morning in light clothing.
3. Read your BMI, its WHO category and the weight range that would give a BMI of 18.5 to 24.9 at your height.

The formula and the categories

BMI = weight in kilograms / (height in metres x height in metres), rounded to one decimal place. For imperial entries the calculator first converts: 1 inch = 2.54 cm and 1 pound = 0.45359237 kg. The World Health Organization's categories for adults are:

• Below 18.5: underweight
• 18.5 to 24.9: healthy (normal) weight
• 25 to 29.9: overweight
• 30 or more: obesity, further split into class I (30 to 34.9), class II (35 to 39.9) and class III (40 or more)

The category is read from the rounded figure you see, so a BMI shown as 25.0 is in the overweight band.

Worked examples

At 168 cm and 72 kg, height in metres squared is 1.68 x 1.68 = 2.8224, and 72 / 2.8224 = 25.5, which falls in the overweight band. The healthy range for that height is 18.5 x 2.8224 = 52.2 kg up to 24.9 x 2.8224 = 70.3 kg, so this person is about 1.7 kg above it. In imperial units, 5 ft 6 in and 160 lb give a BMI of 25.8, with a healthy range of roughly 115 to 154 lb.

Reading your result sensibly

One reading is a snapshot. Weight can swing by a kilogram or more across a day with meals and fluids, so compare readings taken the same way, at the same time of day, over several weeks. If your BMI sits just inside or just outside a band, the trend matters more than the label. The healthy range shown is a span of weights, not a single target, and a doctor may set a different goal for you based on your age, waist size, blood pressure and blood tests.

Asian populations

A WHO expert consultation published in The Lancet in 2004 reviewed evidence that many Asian populations have a higher risk of type 2 diabetes and cardiovascular disease at BMIs below 25. It kept the WHO categories as the international classification, and suggested additional points for public health action, including 23 and 27.5. If your BMI is between 23 and 25, the calculator shows a short note about this. Your doctor can tell you which cut-offs apply to you.

Who should not rely on BMI

• Children and teenagers: they are still growing, so they are assessed with BMI-for-age percentiles on growth charts instead of these adult categories. This calculator is not for them.
• Pregnant women, whose weight changes for reasons BMI does not capture.
• Very muscular people and older adults, because BMI cannot tell muscle from fat and can over- or under-state risk.

BMI is an estimate for general information and is not medical advice or a diagnosis. Please consult a doctor before starting a diet or exercise plan, or if your result worries you.