BMI Calculator

Turn a weight and a height into a body mass index, its category, and the healthy weight range that goes with it.

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What this tool does

BMI Calculator divides weight in kilograms by height in metres squared, then places the result on the WHO adult scale: under 18.5 underweight, 18.5 to 24.9 normal weight, 25 to 29.9 overweight, and 30 upwards obesity, split into classes I, II and III. Underweight is broken down the same way, into mild, moderate and severe thinness. Alongside the number comes a table row per person — weight, height, the index, its category and the healthy weight range for that height — written in kilograms and centimetres whatever units went in.

There are two ways in. The Settings panel under the two boxes takes one person — metric asks for kilograms and centimetres, imperial for pounds plus feet and inches — and that is the whole interaction if you want a single figure. The input box takes a list instead: one person per line with units written out, optionally with a name before a colon, so "Ada: 70 kg 175 cm", "Ben: 154 lb 5 ft 9 in" and "Cleo: 11 st 4 lb, 1.68 m" can share a list and come back as one table. Anything typed there replaces the fields entirely.

What the measure cannot see is worth stating plainly. It is a ratio of weight to height and knows nothing about body composition, so a muscular build reads high without excess fat, and someone who has lost muscle with age can read as normal while carrying more of it than the number implies. The adult cut-offs are applied to every entry without adjustment, which makes the result unsuitable for children and teenagers, who are assessed against age-and-sex percentile charts, and for pregnancy. Some health services also use lower overweight and obesity thresholds for people of South Asian and Chinese descent; those are not applied here.

The arithmetic happens in the page. No measurement is transmitted or stored, and the tool keeps working with the network off, which is the right arrangement for a figure about your own body. One strictness to know: every line in the input box needs units on both values. "70 175" is refused with a message naming the line, because there is no safe way to guess whether that means kilograms and centimetres or pounds and inches.

How to use it

  1. Choose metric or imperial. The Units setting swaps the fields between kilograms with centimetres and pounds with feet and inches. Only the pair matching your choice is read.
  2. Enter the weight and the height. Type them into the Settings panel under the input and output boxes. Both must be greater than zero — a height of zero is refused rather than returning a meaningless figure.
  3. Or paste several people instead. One per line in the input box, each carrying units and an optional name before a colon. Lines starting with # or // are skipped, so notes can stay in the list.
  4. Read the row that comes back. The output is a table: weight, height, the index to one decimal place, its category and the healthy weight range in kilograms. Copy output puts that same table on the clipboard as CSV.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Filling in a figure a form has asked for

Insurance applications and clinic forms often want the index itself rather than a weight and height, and this returns it with the category attached.

Working from British measurements

Stones, pounds, feet and inches get mixed in everyday use. Written into the input box with their units, each is converted before the division rather than after — though the row that comes back is metric.

Comparing a group on one scale

A labelled line per person returns a table of weight, height, body mass index, category and healthy range, all measured the same way.

Turning a category into a target weight

"Overweight" on its own is abstract. The healthy range in kilograms for your own height is a number you can actually weigh against.

Questions

Are my weight and height sent anywhere?
No. The calculation is a division carried out in this browser tab, so the weight and height you type are never transmitted, logged or stored, and the page goes on working after the network is gone. Measurements of your own body should not need a round trip to somebody else's machine.
How is the figure calculated?
Weight in kilograms divided by height in metres squared. Imperial entries are converted first, at 0.45359237 kg per pound and 2.54 cm per inch, so metric and imperial descriptions of the same person produce the same result.
Is it accurate for athletes or older people?
Not on its own. It weighs the whole body without separating muscle from fat, so a heavily muscled person can land in the overweight band carrying little excess fat, while someone who has lost muscle with age can sit inside the normal band carrying more. It is a screening measure, not a diagnosis.
Can I use it for a child?
No. The categories here are the adult cut-offs, applied unchanged to every entry. Children and teenagers are assessed against age-and-sex percentile charts, and the number this tool returns will not correspond to a reading from one.
Why was one of my lines rejected?
Each line needs units on both values: "70 kg 175 cm" is read, "70 175" is not, and the error names the line so you can see which to fix. Stones are understood in the input box, though the imperial fields themselves ask for pounds only.
What counts as the healthy weight range?
The weights that put you between 18.5 and 24.9 at your height, quoted in kilograms. The upper figure is the rounded band edge rather than the 25 boundary itself, so the very top of the range sits marginally below where overweight begins.
Does it adjust for sex, ethnicity or waist size?
No. There is a single scale and no adjustment for any of them. Several health services apply lower thresholds for people of South Asian and Chinese descent, and waist circumference is a separate measurement this tool does not take.
Why is the answer in kilograms when I entered pounds?
The table is built in metric whatever you type: an imperial entry is converted to kilograms and centimetres first, and those converted figures are what the row shows beside the index. 154 lb at 5 ft 9 in comes back as 69.9 kg at 175.3 cm, and the healthy range beside it is in kilograms too, so a target weight in pounds is a conversion you make yourself.

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