BMI Calculator

Body Mass Index (BMI) is a simple screening measure that relates weight to height. It is widely used to place adults into broad categories: underweight, normal weight, overweight or obese.

This calculator computes your BMI from height and weight in either metric or imperial units, shows the World Health Organization category it falls into, and gives the weight range that would place you in the normal-weight band at your height.

BMI was devised in the 19th century as a population statistic, not an individual diagnostic. It uses height and weight and nothing else, so anything that changes body composition without changing either is invisible to it. That makes BMI a reasonable first filter and a poor final answer.

• Muscle mass: muscle is denser than fat, so lean, heavily trained people frequently read as overweight or obese while carrying very little fat.
• Fat distribution: BMI cannot separate abdominal fat from fat carried on the hips and thighs, and abdominal fat is the more metabolically risky of the two. A waist measurement supplies exactly what BMI misses.
• Age: adults tend to lose muscle and gain fat over time, so an unchanged BMI at 70 usually represents more body fat than the same number did at 30.
• Pregnancy: BMI is not valid during pregnancy. Care is guided by pre-pregnancy BMI and by recommended gestational weight-gain ranges instead.
• Height extremes: squaring height distorts the result at the tails, reading low for very short people and high for very tall people.
• Ethnicity: cardiometabolic risk begins to rise at lower BMI values in several populations, which is why some health bodies use additional lower thresholds.

None of this makes BMI useless. It tracks population-level risk well and costs nothing to measure. It simply cannot carry the weight of an individual diagnosis on its own.

The chart below gives BMI for every combination of height from 145 cm to 195 cm and weight from 45 kg to 130 kg, with both imperial equivalents on the axes. Find your height down the left, your weight across the top, and read the value where they meet.

Two related figures appear alongside the main result. Both are attempts to make the same height-and-weight data easier to read.

• BMI Prime is simply your BMI divided by 25, the upper limit of the normal range. It turns the result into a ratio: exactly 1.00 sits on the boundary, 0.80 means you are 20% below it, and 1.20 means 20% above. The appeal is that it reads the same way regardless of which national threshold you use: a population using 23 as its cut-off can divide by 23 instead.
• The Ponderal Index divides weight by height cubed rather than squared. Because human mass scales closer to the cube of height than the square of it, this distorts less at the extremes, which is why it is preferred for very tall and very short people and used in neonatal assessment. The trade-off is that it has no widely agreed category thresholds, so the number is hard to interpret on its own.

Neither adds information that BMI does not already contain: they are the same two measurements rearranged. They are useful for comparison and for the height extremes where BMI is weakest, not as a second opinion.

Adult categories do not apply to anyone under 20. A child's body composition changes continuously with growth, and it changes differently for boys and girls, so a fixed cut-off such as 25 carries no meaning at age nine.

A child's BMI is calculated with the same formula, then compared against reference data for children of the same age and sex. The resulting percentile (not the raw BMI) is what gets interpreted:

• Below the 5th percentile: underweight
• 5th to below the 85th percentile: healthy weight
• 85th to below the 95th percentile: overweight
• 95th percentile and above: obese

Percentile charts differ between countries because they are built from different reference populations, so the same child can land on slightly different numbers in the UK and the US. Growth is also uneven, which makes a single reading far less informative than the trend across several measurements. This calculator applies adult categories and is not suitable for under-20s.

Type 2 diabetes and cardiovascular disease begin to appear at lower BMI values in people of South Asian, Chinese, Japanese, Korean and South-East Asian descent than in white European populations. At an identical BMI, these groups tend to carry more body fat, and more of it around the abdomen.

A WHO expert consultation examined this in 2004 and deliberately stopped short of issuing a replacement classification, concluding that the available data did not indicate a single clear cut-off point for all Asian populations. It recommended retaining the standard categories for international comparison, while identifying additional potential public health action points along the BMI continuum:

• 23.0: roughly where 25 sits on the standard scale
• 27.5: roughly where 30 sits on the standard scale
• 32.5 and 37.5: the two higher action points identified by the same consultation

In practice the first two are the ones that reached national guidance. In the UK, NICE recommends 23 to indicate increased risk and 27.5 to indicate high risk for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family background.

This calculator reports the standard WHO categories. If you belong to one of these groups, read your result against the lower thresholds as well, and discuss it with a doctor rather than drawing your own conclusion from the number.