BMI Calculator

Calculate body mass index in metric or imperial units.

This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.

Works out body mass index from height and weight in either metric or imperial units, and places the result on the standard scale from underweight through to obese.

How to use it

1Choose Metric or Imperial.
2Enter height and weight — centimetres and kilograms, or feet, inches and pounds.
3Read the figure, the coloured category badge, and the marker's position on the 10-to-40 scale beneath.

Example

Input: 175 cm, 70 kg
Output: 22.9 — Normal

The imperial side converts before calculating, at 0.0254 m to the inch and 0.45359237 kg to the pound, so 5 ft 9 in at 155 lbs gives the same 22.9. The inches field may be left empty and is treated as zero; anything zero or negative elsewhere shows "Please enter positive numbers."

What happens to your data

Two numbers typed into two boxes are this tool's entire input — no age, sex, name or date of birth is asked for, so there is nothing here that could identify anyone. The figure is derived on each keystroke rather than on a submit, meaning no form is ever posted; nothing is written to storage either, so reloading returns you to empty fields rather than to your last measurement.

Last updated August 2026

Someone has asked you for a number you do not carry around — a GP's pre-appointment form, an insurance questionnaire, an online consultation that will not move to the next page without it. Body mass index is a ratio of two measurements and nothing else, so it takes seconds once you know which numbers to hand over and in what units.

Settle the units before you type: the fields change with the choice, and the usual mistake here is a units mistake rather than an arithmetic one. Metric wants height in centimetres — 175, not 1.75. Give it metres and the sum treats you as under two centimetres tall, returning a figure in the tens of thousands. Imperial splits height into feet and inches and takes weight in pounds, so a weight you know in stone needs multiplying by 14 first; there is no stone field.

The second thing to settle is whether this is the question you want answered. The calculation is given your height and your weight, and never your age, your sex, or how the weight is distributed. That is what makes it quick and what makes it blunt: two people at the same figure can be in quite different health, and the bands were drawn for adults. If you are checking a child, a heavily built athlete or someone in later life, treat the result as one reading among several rather than a verdict.

How it works

Toolvore recalculates on every keystroke rather than behind a submit button, so the figure appears the moment both boxes hold a usable number. The arithmetic is the standard one, weight in kilograms divided by height in metres squared, with centimetres divided by 100 first. Imperial is converted before anything is worked out, at 0.0254 metres to the inch and 0.45359237 kilograms to the pound, which is why 5 ft 9 in at 155 lbs gives the same 22.9 as 175 cm at 70 kg. The inches box can be left empty and is read as zero, so a flat 6 ft works alone. The number is rounded to one decimal place for display while the category is decided from the unrounded value against 18.5, 25 and 30, so a reading shown as 25.0 can sit on either side of the line. The coloured bar runs from 10 to 40 and no further: a figure outside that range is still printed in full, but the marker stops at the end of it.

Common use cases

Filling in the height and weight boxes on a pre-appointment form
Producing the figure an insurance or travel medical asks for
Turning feet-and-pounds measurements into the same figure as metric ones
Checking where a proposed weight target would place you
Checking a BMI quoted in a health article against your own numbers
Recording a starting figure at the start of a training programme

Frequently asked questions

What is a healthy BMI for an adult?＋

The scale marks three cut-offs — 18.5, 25 and 30 — and the band between 18.5 and 25 is labelled Normal. Those are the adult thresholds used by the WHO and most national health services, and the band is wider than people expect: at 175 cm it runs from about 56.7 kg to about 76.5 kg, a spread of some 20 kg. Crossing a boundary by a kilogram changes a label rather than your health. They are population guidance, drawn where risk rises across large groups, not a target for an individual body.

Is BMI accurate if you are muscular?＋

Not especially, and this is its best-known weakness. Muscle is denser than fat and the sum sees only total weight, so a rower, a sprinter or a front-row forward can land in Overweight while carrying very little fat. The error runs the other way too: someone light, sedentary and with little muscle can sit inside Normal with a high proportion of body fat. Nothing in a height and a weight separates those two cases. If your training makes the number implausible, a body fat or waist measurement will tell you more than recalculating will.

Does BMI work for children and teenagers?＋

Not with these bands. A growing child's healthy ratio shifts month by month and differs between boys and girls, so paediatric BMI is read against age-and-sex growth charts as a percentile — the 85th and 95th are the usual reference points — rather than against a fixed 25 and 30. No age or sex is asked for here, so the adult thresholds get applied to whatever goes in, and a healthy eleven-year-old can come out labelled Underweight. Under 18, take the raw figure and have a GP or school nurse plot it on a growth chart.

Are the thresholds different for South Asian or Chinese people?＋

In several countries' guidance, yes. Type 2 diabetes and cardiovascular risk begin to climb at a lower BMI in South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean populations, and NICE guidance in the UK accordingly uses 23 as the point to start taking notice and 27.5 as the higher-risk mark. The cut-offs drawn here are the fixed 18.5, 25 and 30, with no ethnicity option, so where the lower thresholds apply, read the figure and disregard the label — 24 sits inside the green band on screen while falling above the level that guidance would use.

How do you calculate BMI by hand?＋

In metric it is weight in kilograms divided by height in metres multiplied by itself: 70 kg at 1.75 m is 70 ÷ 3.0625, or 22.9. Imperial folds in a conversion factor — weight in pounds divided by height in inches squared, then multiplied by 703 — so 155 lbs at 69 inches is 155 ÷ 4,761 × 703, again 22.9. That 703 is the two unit conversions collapsed into one figure. Order matters on a phone calculator: square the height before dividing, because entering 70 ÷ 1.75 × 1.75 left to right gives you 70 back.

Is my height and weight sent anywhere or stored?＋

No. The two numbers are read, converted if you are on the imperial side, divided and printed, all inside the page you are looking at. There is no network request behind it, no submit step that could post a form, and nothing written to your browser's storage — reloading returns you to empty boxes rather than your last measurement. Nothing else is asked for either: no name, no date of birth, no age or sex, so there is nothing in the input that could identify a person. Close the tab and the figures go with it.

What is worth measuring alongside BMI?＋

Waist size, because it captures the thing a height-to-weight ratio cannot see — where the weight sits. Fat around the abdomen carries more metabolic risk than the same weight on hips and thighs, and two people with identical BMI can differ sharply on that. The simplest version needs no chart: keep your waist under half your height. Measure around the middle, roughly level with the navel, standing normally rather than holding your breath in. Blood pressure and a blood glucose or cholesterol reading each tell you something the ratio never will.

Does BMI apply during pregnancy or in later life?＋

Neither is well served by it. During pregnancy the weight gain is expected and the ratio stops meaning what it usually means, which is why antenatal care records a booking BMI from pre-pregnancy or early-pregnancy weight and works from that throughout. In later life the drift runs the other way: muscle is lost with age while fat is kept, and height falls a centimetre or two through spinal compression, lifting the number without anything having changed for the worse. Some guidance uses a slightly higher healthy range past 65 for that reason, and no age is asked for here.