BMI Calculator

How to use the BMI Calculator

1. 1

   Choose kg / cm or lb / ft + in.

2. 2

   Enter your weight and height.

3. 3

   Keep Asian / Indian selected if you are of South Asian origin, or switch to WHO to compare.

4. 4

   Read your BMI, its category and where it sits on the coloured bar, then check the healthy weight range for your height.

How BMI is calculated, with an example

BMI is your weight in kilograms divided by the square of your height in metres. Take someone who weighs 70 kg and stands 170 cm tall. Height in metres is 1.70, and 1.70 × 1.70 = 2.89. Then 70 ÷ 2.89 = 24.2.

If you use pounds and inches, the formula is 703 × weight in pounds ÷ (height in inches)². At 154 lb and 5 ft 7 in (67 inches), that is 703 × 154 ÷ 4,489 = 24.1. The tiny gap from the metric answer is only because 5 ft 7 in is slightly more than 170 cm. The calculator converts everything internally, so switching units never changes the category for the same person.

Now the interesting part. A BMI of 24.2 is in the normal range under the WHO table, but it counts as overweight under the Indian cut-offs. Same person, same number, different message. That is why the calculator lets you see both.

Why Indian BMI cut-offs are lower than WHO's

The WHO categories (normal up to 24.9, overweight from 25, obese from 30) were built mostly on data from European populations. Research since the 1990s showed that South Asians tend to carry more body fat, and more of it around the abdomen and internal organs, at the same BMI. Indians also tend to develop type 2 diabetes, high blood pressure and heart disease at lower BMI levels and at younger ages. Doctors sometimes describe this as the "thin-fat" pattern: a person who looks slim but has a high proportion of fat.

A WHO expert consultation in 2004 accepted that lower action points made sense for Asian populations. In 2009 a consensus statement by Indian experts, published in the Journal of the Association of Physicians of India, adopted the cut-offs this calculator uses by default:

If you are Indian, Pakistani, Bangladeshi, Sri Lankan or Nepali, the Asian/Indian row is the more useful guide. Many Indian doctors and dietitians use these numbers in their clinics.

Healthy weight range for your height

The calculator turns the normal BMI band back into kilograms for your height, which is often easier to act on than the BMI itself. Under the Asian/Indian cut-offs, the healthy range runs from BMI 18.5 to 22.9:

Treat the range as a broad target, not a precise goal weight. If you are above it, even a modest loss of a few kilos often improves blood sugar and blood pressure readings, so you do not need to reach the middle of the range to see a benefit.

Waist size tells you what BMI can't

BMI only knows your total weight. It cannot tell whether that weight is muscle, bone or fat, or where the fat sits. Fat around the belly is the type most closely linked with diabetes and heart disease, and it is common in Indians even at a normal BMI.

So measure your waist as well. Wrap a tape around your bare abdomen at the level of your navel, breathe out normally and read the number. The Indian consensus guidelines treat a waist of 90 cm or more in men and 80 cm or more in women as abdominal obesity. If your BMI looks fine but your waist is above these numbers, it is worth a conversation with your doctor.

When BMI is the wrong tool

BMI is a screening number for adults aged 18 and over. It is not a diagnosis, and this page is not medical advice. A few groups should be especially careful with it:

• Children and teenagers: their BMI must be compared with age- and sex-specific growth charts, which this calculator does not do.
• Pregnant women: weight gain in pregnancy is expected and should be tracked with your doctor.
• Athletes and people who lift weights: muscle is heavy, so a fit person can score as overweight.
• Older adults: muscle loss with age can hide excess fat behind a normal-looking BMI.

If your result worries you, or you are underweight without trying, see a doctor. They can look at your waist, blood pressure, blood sugar and family history together, which says far more than one number.