BMI Calculator

Health disclaimer

BMI is not a complete health assessment. It does not diagnose disease and should not replace professional medical advice. If you have health concerns, consult a qualified clinician.

Weight divided by height squared, then a WHO adult band

BMI is your weight in kilograms divided by the square of your height in metres. In imperial mode the page first converts, using 1 pound = 0.45359237 kg and 1 inch = 2.54 cm, and then applies the same formula. The defaults show it: 70 kg at 175 cm is a BMI of 22.9, and 154 lb at 5 ft 9 in is 22.7, because those convert to 69.9 kg and 175.3 cm rather than to exactly the metric defaults.

The band printed beside the number is the World Health Organization's adult definition. WHO: for adults, "overweight is a BMI greater than or equal to 25; and obesity is a BMI greater than or equal to 30". With underweight below 18.5, that gives four bands:

Under 18.5: underweight.
18.5 to under 25: normal weight. The CDC uses the same range and calls it "Healthy Weight".
25 to under 30: overweight.
30 and above: obesity.

The band is decided before the number is rounded

The page decides the band on the exact BMI and only then rounds it for display, so a BMI of 24.96 is reported as normal weight even though it would round to 25.0. To stop the number contradicting the band beside it, the page shows two decimals near a boundary, and cuts rather than rounds them. Measured at 180 cm:

59.93 kg is a BMI of 18.497 and shows 18.49, Underweight.
59.95 kg reaches 18.5 and shows 18.5, Normal weight.
80.9 kg shows 24.96 and 97.18 kg shows 29.99, each in the lower band.
81 kg and 97.2 kg show 25.0 and 30.0, and move up a band.

Until 18 October 2026 the two decimals were rounded, and in the last 0.005 below each boundary the page printed, for example, 18.50 beside "Underweight". That has been corrected and is now covered by a test.

Bands and groups the page does not show

Obesity classes. The CDC divides obesity further: class 1 is 30 to under 35, class 2 is 35 to under 40, and class 3, severe obesity, is 40 or greater. This page says "Obesity" for a BMI of 31 and of 55 alike.
Asian populations. A WHO expert consultation published in 2004 concluded that "the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs lower than the existing WHO cut-off point for overweight". It kept the WHO cut-offs as the international classification and identified further "public health action points" at 23.0, 27.5, 32.5 and 37.5. Some countries use lower thresholds as a result. This page applies only the international ones.
Children and teenagers. Adult bands do not apply. The CDC explains that because children are growing, "their BMI values are expressed relative to others of the same sex and age", as percentiles for ages 2 through 19, and WHO uses BMI-for-age for ages 5 to 19. This page asks for no age or sex and applies the adult bands to whatever you type: 3 kg at 50 cm, the size of a newborn, comes out as 12.0 and "Underweight", which means nothing for a baby.

Switching units does not carry your numbers across

Metric and imperial each have their own boxes, and changing the menu switches between them without converting anything. Measured: we entered 95 kg and 180 cm, a BMI of 29.3 and overweight, then switched to imperial. The page immediately showed 22.7, Normal weight, calculated from the imperial boxes, which still held 154 lb and 5 ft 9 in. Nothing on screen says the person has changed.

The imperial boxes have their own traps:

Leaving inches blank is an error. 180 lb at 6 ft with the inches box empty gives "Enter positive weight and height." Type 0 inches and it works: a BMI of 24.4.
Leaving feet blank is also an error, but 0 ft and 72 in works and gives the same 24.4.
Inches above 11 are accepted: 5 ft 15 in is calculated as 190.5 cm, the same as 6 ft 3 in.
Decimal feet are accepted: 5.5 ft is 5 ft 6 in, 167.6 cm, not 5 ft 5 in.

In metric mode, height must be in centimetres and there is no check that the result is plausible. Typing a height of 1.75 meaning metres gives a BMI of 228,571.4, labelled "Obesity". The result updates as you type, so watch the height shown under the result, which the page reports in centimetres.

What BMI can and cannot tell you

BMI uses only weight and height, so it cannot see what that weight is made of or where it is carried. The CDC is direct about its role: "BMI is a screening measure. BMI should be considered with other factors when assessing an individual's health." WHO notes that "Other measures, such as waist circumference, can also be used to identify overweight or obesity."

So the number is a starting point for a conversation with a clinician, not a verdict, which is also what the disclaimer above says.

What this tool deliberately does not do

It does not ask your age or sex, so it cannot give a child's percentile.
It does not show obesity classes or any national cut-offs other than WHO's.
It does not estimate body fat, and takes no waist or hip measurement.
It does not reject implausible values, or convert between metric and imperial when you switch.
It does not keep a history. Nothing is saved between visits.
Nothing is uploaded. The calculation runs in your browser and your weight and height stay on your device.

Sources

Adult definitions of overweight and obesity, BMI-for-age for ages 5 to 19, and waist circumference as another measure: World Health Organization, "Obesity and overweight" fact sheet. who.int
Adult categories including the three obesity classes, and "BMI is a screening measure": CDC, "Adult BMI Categories". cdc.gov
Children and teens 2 through 19 and BMI-for-age percentiles: CDC, "Child and Teen BMI Categories". cdc.gov
Lower risk thresholds in Asian populations: WHO Expert Consultation, "Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies", The Lancet, 2004. doi.org
The inch as exactly 2.54 cm, and the pound as 0.4535924 kg to seven figures: NIST Special Publication 811, Appendix B.8. nist.gov

Every behaviour and every value on this page was measured against this tool as it is published, in a browser, on 18 October 2026.

Frequently asked questions

What BMI counts as overweight?

For adults, WHO defines overweight as a BMI of 25 or more and obesity as 30 or more, with underweight below 18.5. The CDC uses the same boundaries. These are adult definitions and do not apply to anyone under 20.

Why does it sometimes show two decimals?

Near a boundary, one decimal could contradict the band. A BMI of 24.96 would round to 25.0 beside "Normal weight", so the page shows two decimals, cut rather than rounded, whenever the one-decimal figure would land on the other side of a boundary.

Can I use it for my child?

No. Children's BMI is judged as a percentile for their age and sex, which this page does not ask for. It will apply adult bands to any numbers you enter, so use a child and teen calculator such as the CDC's instead.

Are the cut-offs different for Asian people?

A 2004 WHO expert consultation found high risk of type 2 diabetes and cardiovascular disease in Asian populations at BMIs below 25, and named action points at 23 and 27.5. WHO kept 25 and 30 as the international bands, which are what this page uses.

Why did my result change when I switched units?

Because metric and imperial have separate boxes and switching does not convert between them. The new result comes from whatever the other boxes hold, 154 lb and 5 ft 9 in unless you changed them. Re-enter your figures after switching.

I am exactly six feet. Why does it say to enter a positive height?

The inches box was probably left empty, which the page treats as invalid. Type 0 in the inches box. Inches above 11 and decimal feet are accepted too, so 5 ft 15 in and 6 ft 3 in give the same answer.

Does it tell me which class of obesity I am in?

No. It reports any BMI of 30 or more as "Obesity". The CDC's classes are 30 to under 35 for class 1, 35 to under 40 for class 2, and 40 or more for class 3, so you can read your class from the number.

Is my weight saved or sent anywhere?

No. The calculation happens in your browser, nothing is uploaded, and nothing is kept once you leave the page. Each visit starts again from the default 70 kg and 175 cm.