BMI Calculator

What BMI actually measures

Body mass index is a ratio of your weight to the square of your height. It does not measure fat, muscle or health directly. It is a quick screening number that tells you whether your weight is in a range that, across large populations, is linked with lower or higher health risk.

That makes it useful and limited at the same time. For most adults who do not lift weights seriously, BMI tracks body fat reasonably well. For a lifter carrying a lot of muscle, it can be badly wrong, which is covered further down.

This calculator is built for adults aged 18 and over. Children and teenagers are assessed with age and sex specific BMI percentiles, not the adult categories below.

How to use the calculator

Pick metric or imperial with the unit toggle. In metric, enter height in centimetres and weight in kilograms. In imperial, enter feet and inches plus pounds. The tool converts everything internally, so the result is identical either way.

Weigh yourself in the morning, after the bathroom and before eating, for the most repeatable number. Measure height barefoot against a wall. A 2 cm (about 1 in) error in height moves BMI by roughly half a point, so it is worth getting right.

Choose the cut-offs that fit you: WHO international, or Asian / Indian if you are of South, East or Southeast Asian background. You get your BMI to one decimal place, your category on both standards, the weight range that is healthy for your height (18.5 to 24.9 on WHO, 18.5 to 22.9 on the Asian cut-offs) and your BMI Prime.

The formula

The maths is the same one Adolphe Quetelet described in the 1830s. Only the unit conversion changes between systems.

BMI = weight (kg) / height (m)²

BMI = 703 x weight (lb) / height (in)²

BMI Prime = BMI / 25

BMI Prime expresses your BMI as a fraction of the upper healthy limit. A BMI Prime of 1.00 sits exactly on the line between healthy and overweight. 1.20 means you are 20% above that line, 0.80 means you are 20% below it. It makes progress easier to read than raw BMI points.

The healthy weight range is worked out in reverse: 18.5 x height² and 24.9 x height², giving the lowest and highest weight that keep you in the healthy band.

Reading your result

These are the World Health Organization categories for adults, which the calculator uses:

A result near a boundary means very little on its own. Someone at 25.2 is not meaningfully less healthy than someone at 24.8. Watch the direction your number moves over months rather than reacting to a single reading.

Asian populations use lower cut-offs

A 2004 WHO expert consultation found that many Asian populations carry more body fat and higher risk of type 2 diabetes and heart disease at the same BMI as European populations. It suggested public health action points of 23 for increased risk and 27.5 for high risk.

India's 2009 consensus guidelines for obesity (Misra et al., published in the Journal of the Association of Physicians of India) adopted lower cut-offs for Indian adults, in line with the WHO Asia-Pacific classification. These are the cut-offs the calculator uses when you pick Asian / Indian:

In practice this changes the healthy weight for a given height quite a lot. At 165 cm (5 ft 5 in), the WHO healthy range runs up to 67.8 kg, but the Asian range stops at 62.3 kg. At 175 cm (5 ft 9 in), it is 76.3 kg on WHO and 70.1 kg on the Asian cut-offs.

The reason is body composition. At the same BMI, South Asian adults tend to carry more body fat, and more of it around the abdomen, than white Europeans. Type 2 diabetes, high blood pressure and heart disease start showing up at lower BMIs as a result, which is why a BMI of 23 to 24.9 already counts as overweight for Indians.

Why lifters often get misclassified

BMI cannot tell muscle from fat. A 180 cm (5 ft 11 in) man who trains hard and weighs 90 kg (198 lb) at 13% body fat has a BMI of 27.8, which the table calls overweight. His waist and health markers may say otherwise.

If you have been lifting consistently for a year or more, pair BMI with a measure that separates fat from muscle. The body fat calculator estimates fat percentage from a few tape measurements, and the FFMI calculator shows how much lean mass you carry for your height.

BMI also tends to underestimate risk in older adults, who lose muscle and gain fat while their weight stays the same. Waist size is a useful second check; the waist-to-hip ratio calculator covers that.

Waist size: the check BMI misses

Where you carry fat matters as much as how much you carry. Fat stored deep around the organs, which shows up as a larger waist, is more strongly linked with type 2 diabetes and heart disease than fat on the hips and thighs. Two people with the same BMI can have very different risk because of this.

A simple rule used by the UK's NICE guidance is to keep your waist below half your height. A 170 cm (5 ft 7 in) person would aim for a waist under 85 cm (33.5 in). Measure at the midpoint between the bottom of your ribs and the top of your hip bone, relaxed, after breathing out.

If your BMI is in the healthy range but your waist is above that line, it is worth paying attention to. If your BMI says overweight but your waist is comfortably under it and you train regularly, your extra weight is probably muscle.

What to do with your number

If you are above the healthy range and want to change it, start by finding your maintenance calories with the TDEE calculator, then set a modest deficit with the calorie calculator. Our guide to calculating calories for weight loss explains the process step by step.

If you are underweight, the same tools work in reverse: eat above maintenance and lift to make the gain mostly muscle. The beginner full body workout is a sensible place to start.

Either way, read BMI vs body fat percentage to understand when each number is the better guide.