How to calculate BMI
Body mass index (BMI) is your weight in kilograms divided by your height in metres, squared. For 70 kg and 175 cm it is 22.9. Imperial units use a different constant, shown below.
Updated 21 October 2026. Sources named on the page.
- The formula in metric and imperial
- Two worked examples you can check
- What the usual adult ranges are
- A screening number, not a diagnosis
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Enter your height and weight in either unit and the calculator does the steps for you. Nothing you type is saved or sent.
Check your working
Use these known answers to test your own sums.
Known answers
| Weight and height | Working | BMI |
|---|---|---|
| 70 kg, 175 cm | 70 / 1.75 squared | 22.9 |
| 90 kg, 180 cm | 90 / 1.8 squared | 27.8 |
| 160 lb, 70 in | 703 x 160 / 70 squared | 23.0 |
Sources: World Health Organization, adult BMI classification; Adolphe Quetelet's index, as adopted for population screening
Figures and sources checked 21 October 2026. General information, not medical advice.
The formula
BMI = weight (kg) / height (m) x height (m). Write the height in metres, so 175 cm is 1.75 m, then multiply it by itself and divide your weight by the result.
- Step 1: change your height to metres (divide centimetres by 100).
- Step 2: multiply that height by itself.
- Step 3: divide your weight in kilograms by the step 2 answer.
- Step 4: round to one decimal place.
Worked example in metric
A person who weighs 70 kg and is 175 cm tall: 1.75 x 1.75 = 3.0625. Then 70 / 3.0625 = 22.86, which rounds to 22.9.
Worked example in imperial
With pounds and inches, BMI = 703 x weight (lb) / height (in) squared. For 160 lb and 70 in: 70 x 70 = 4,900, and 703 x 160 = 112,480. Then 112,480 / 4,900 = 22.96, which rounds to 23.0.
The 703 is there because pounds and inches are different units from kilograms and metres. The answer is the same number either way.
What the number means
For adults, a BMI from 18.5 up to but not including 25 is usually called the usual adult range. Below 18.5 is below it, 25 up to 30 is above it, and 30 or more is well above it. These bands were set for populations, so they are a way to screen, not a verdict on one person.
Who it is for
The ranges on this page are for adults aged 19 and over. Children and teenagers are assessed against growth charts, not these bands. If you are worried about your weight or health, a doctor can look at the whole picture.
Questions
Is BMI the same for men and women?
The formula and the adult bands are the same for women and men. How well it reflects body fat can differ, which is one reason it is only a screening number.
Do I need to measure in metres?
Use whichever unit you have. Metric needs kilograms and metres. Imperial uses pounds and inches with the constant 703.
How accurate is my result?
It is exact arithmetic on the numbers you enter, so a rounding or measurement slip is the usual cause of a surprise. Measure height without shoes and weigh yourself the same way each time.
Is a BMI in the usual range always healthy?
Not always. BMI does not see where fat sits, how much muscle you carry, or your blood pressure and fitness. See the limits guide for more.
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