BMI Calculator
A BMI calculator that puts your reading on a 15-to-40 scale beside the WHO cut-offs, then works backwards to the weights that count as healthy at your height.
Your BMI
The scale stops at 40.
The arithmetic, run backwards
Where that sits
Units
Switching converts what you have already typed — the reading never changes because of the units it is shown in.
Who these bands are for
Getting your number
- Leave Units on Metric, or pick Imperial for pounds with separate feet and inches boxes; whatever you have already typed is converted, and the choice is remembered on this device.
- Type your weight and height over the defaults. The reading and its band fill in as you go; empty a field, or type a height no person has, and the field says what it needs while the figure waits.
- Check where your mark sits on the scale, read the band rows for the weights that fit your height, and use Copy this reading to keep the result.
About this tool
Two figures are all the BMI calculator wants: what you weigh and how tall you are. At the metric defaults of 70 kg and 175 cm it reads 22.9, with the word Normal beside the figure. Change either box and everything updates while you type. There is no calculate button.
Under the figure sits a scale running from 15 to 40 in one-point cells, cut at 18.5, 25 and 30. Those three cuts are the WHO's, and your reading is printed in its own cell and marked above the scale. That placement is worth more than the number alone. A 22.9 and a 24.6 both come back Normal, and only one of them is a hair from the next band.
The list above the scale is the part I use most: the same arithmetic run backwards, every band as a span of whole kilograms at your height, with the distance to the next band under it. At 175 cm the Normal row reads 57–76 kg, and 77 kg is the first weight that files as Overweight. Each endpoint is checked by running it through the formula, so at 5 ft 9 in the Normal row stops at 168 lb, because 169 lb already reads 25.0.
The formula itself is weight in kilograms over height in metres squared, and nothing else goes into it. Pounds and inches get converted before that division happens, which is why 154 lb at 5 ft 9 in and 69.9 kg at 175.3 cm both come back 22.7.
Where it stops: BMI has no idea what your weight is made of. Muscle and fat weigh the same to it. Clinicians use it as a first screen and then look at waist measurement, blood work and family history before they conclude anything. Read the number here the same way, as one line of evidence.
Good to know
- Muscle reads as fat. The formula sees total weight and nothing else, so a lifter at 95 kg and 180 cm gets 29.3 and the label Overweight on exactly the same evidence as anyone else that heavy.
- Children and teenagers are scored against age and sex percentiles rather than these adult bands, and pregnancy puts the whole measure out of scope.
- The scale only covers 15 to 40. Past 40 the mark parks at the end and a note says so, while the figure above it keeps counting; the figure is the reading to trust.
Frequently asked questions
How do I work out BMI by hand?
Weight in kilograms over height in metres squared. 70 kg at 1.75 m is 70 divided by 3.0625, which gives 22.9. In pounds and inches you need the 703 factor instead: pounds divided by inches squared, times 703. That is 154 divided by 4,761, times 703, or 22.7.
What counts as a healthy BMI?
For adults from 20, 18.5 up to 24.9, which is the green stretch of the scale. Below 18.5 the tool says Underweight, 25 to 29.9 shows as Overweight, and 30 upwards as Obese. Those are the WHO's international cut-offs, but not the only ones in use: NICE in the UK sets lower thresholds (overweight from 23, obesity from 27.5) for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean background, and the panel under the tool lists who the bands are for.
Does switching to imperial convert what I already typed?
Yes. Switching converts the weight and height you have typed, shown to a tenth, so 70 kg becomes 154.3 lb and 175 cm becomes 5 ft 8.9 in. The tool keeps the exact value underneath, which is why the reading never moves when you switch back and forth; only a number you type replaces it.
Why doesn't it ask for my age or sex?
Because neither changes the answer. One formula and one set of adult bands cover both sexes and every adult age. Where sex genuinely matters is body-fat distribution, and BMI cannot see that either, which is a fair summary of what the measure is good for.
I'm 24.9 one week and 25.1 the next. Does that mean anything?
Very little. The bands were drawn from population data and a fifth of a point is well inside the noise of when you weighed yourself and what you had eaten. A run of readings over a few months carries a signal. Two adjacent ones do not.
If I bookmark this, does it keep my height?
It does. The unit system and the visible height and weight go into the URL while you type, so a saved link reopens with the fields filled and the same reading on the scale. They are plain to read in that link, which is worth remembering before pasting it anywhere public.
Missing something in BMI Calculator? Suggest a feature →
Tell someone who needs this.