To calculate BMI with kg and cm, you divide your weight in kilograms by your height in metres squared — the result is a single number expressed in kg/m² that the World Health Organization then maps onto its standard weight categories. Although the phrase "with kg and cm" suggests a simple two-input problem, the BMI formula does not accept centimetres directly: it needs height in metres, so the first real action is converting your height from cm to m by dividing by 100. Once the units are correct, the arithmetic collapses to a single division — BMI = weight (kg) ÷ height (m)². You can complete the calculation with the BMI Calculator, which updates your BMI value and the matching WHO category the moment you change a number in any input box.
The metric BMI formula in kg/m²
BMI in metric form is one of the simplest health formulas in use: weight in kilograms divided by height in metres squared. Expressed as an equation, BMI = weight (kg) ÷ height (m)². The squared height term means that doubling your height roughly quadruples the denominator and therefore divides your BMI by four for the same weight. The expression is sometimes called the Quetelet index after the Belgian mathematician and astronomer Adolphe Quetelet, who derived the weight-over-height-squared ratio in the 1830s while studying the physical characteristics of large populations. Quetelet himself never called the result "body mass index" — that label came later in the 20th century — but his ratio is the same one used in clinics today. Because the metric expression is unit-consistent (kg divided by m²), the resulting number is itself expressed in kg/m², and that is the convention clinicians and public-health bodies use when they write BMI ranges in their guidelines (Wikipedia: Body mass index).
The imperial equivalent of the same formula uses a conversion constant of 703, so BMI = 703 × weight (lb) ÷ height (in)². The factor 703 is not arbitrary: it scales pounds per square inch up to kilograms per square metre, allowing the imperial version to produce the same numerical result as the metric version for the same person. Both formulas give the same WHO classification, so the choice between metric and imperial is purely a matter of which measurements you already have to hand. If your height is in centimetres and your weight is in kilograms, you stay on the metric side; if your measurements are in inches and pounds, the imperial version avoids any unit conversion.
Why the formula uses metres, not centimetres
BMI uses metres in the denominator because the formula was designed around the SI units of kilograms and metres, and the unit consistency (kilograms per square metre) makes the number meaningful and internationally comparable. Centimetres are a smaller unit than metres by a factor of 100, so squaring the height makes the difference worse by a factor of 10,000. If you accidentally plug centimetres into the formula without converting, your BMI comes out 10,000 times too small — for a 70 kg adult at 175 cm, the wrong result is roughly 0.0023 instead of about 22.86, which would incorrectly appear severely underweight on every chart. The BMI Calculator prevents this confusion by taking metric height in metres inside its metric input boxes; you simply enter 1.75 rather than 175.
Many readers find it easier to read their own height in centimetres because that is how stadiometers, passports, and ID cards display it. The fix is mechanical: divide centimetres by 100, or equivalently move the decimal point two places to the left. 165 cm becomes 1.65 m, 172 cm becomes 1.72 m, 180 cm becomes 1.80 m, and so on. Once the conversion is done, the formula is identical for every adult and produces a number that the WHO has already classified for the world.
Calculate your BMI from kg and cm step by step
- Convert your height from cm to m. Take your height in centimetres and divide by 100. Record the result in metres. For example, 175 cm becomes 1.75 m.
- Square the height in metres. Multiply your height in metres by itself. For 1.75 m that gives 1.75 × 1.75 = 3.0625 m².
- Divide weight in kg by the squared height. Take your weight in kilograms and divide it by the m² figure from step two. For a 70 kg adult at 175 cm: 70 ÷ 3.0625 ≈ 22.86 kg/m².
- Match the number to a WHO category. Read your BMI value against the WHO adult cut-offs in the table below to see whether you fall in the underweight, normal, overweight, or obese range.
A single worked example makes the four steps concrete. For a person weighing 70 kg with a height of 175 cm, the conversion in step one gives 1.75 m. Squaring in step two produces 1.75 × 1.75 = 3.0625. The division in step three gives 70 ÷ 3.0625 = 22.86 (to two decimal places). Step four reads 22.86 against the WHO ranges and finds it sits between 18.5 and 24.9, so the classification is normal weight. The BMI Calculator runs this same arithmetic in your browser and refreshes the result and the category as soon as you type or change any number.
WHO BMI weight categories for adults
The World Health Organization publishes a single set of adult BMI cut-offs that public-health bodies and clinicians use worldwide as a quick screening step. The values below come directly from the WHO fact sheet on obesity and overweight (WHO: Obesity and overweight) and are applied as continuous boundaries so that every possible BMI falls into exactly one category.
| BMI value (kg/m²) | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight (healthy range) |
| 25.0 to 29.9 | Overweight |
| 30.0 or above | Obese |
These thresholds are population-level screening guides, not individual verdicts. Different public-health bodies sometimes recommend slightly different additional cut-offs for Asian populations, where the risk of weight-related disease can rise at lower BMI values, but the WHO adult categories above remain the standard reference used in most clinical settings. Always treat the category as one signal among several rather than a final diagnosis.
Run the math in your browser with the BMI Calculator
The BMI Calculator removes the unit-conversion worry and the arithmetic at the same time. Choose the Metric / Imperial toggle at the top, enter your weight in kilograms and your height in metres inside the matching input boxes, and read your BMI number plus the matching WHO category as they update the moment you change a value. Nothing leaves your browser — the math runs locally on your device, so no weight or height values are uploaded or stored. If you happen to have imperial measurements instead, switch the toggle to Imperial and enter pounds and inches; the calculator applies the 703 conversion automatically and produces the same WHO classification.
For readers who prefer to verify the metric calculation by hand before relying on the tool, the formula above is the entire method: kg ÷ m² with the cm-to-m conversion built in. Anyone with imperial measurements can follow the same procedure after switching units — see How to Calculate BMI in Pounds and Inches for the imperial version of the worked steps.
What BMI does not tell you
BMI is fast, free, and universally recognised, which is exactly why it has survived as the default population-level weight screen for almost a century. It is also blunt, and the limitations are worth naming before you draw any conclusions from a single number. The formula uses only weight and height, so it cannot tell muscle from fat: a lean, muscular athlete and a sedentary person of identical height and weight receive the same BMI, even though their body compositions and health risks differ substantially. BMI also ignores age, sex, ethnicity, bone density, and where fat is carried on the body — waist-to-hip ratio and waist circumference matter for cardiometabolic risk in ways BMI alone cannot capture.
For these reasons, BMI is best read as a triage signal rather than a diagnosis. A BMI in the overweight or obese range is a prompt to talk to a qualified healthcare professional about a fuller assessment, which can include waist circumference, blood pressure, blood lipids, and body-fat percentage. A BMI in the underweight range is similarly a prompt, not an outcome. Use the BMI Calculator as an educational estimate, treat the result as one data point among several, and consult a clinician for anything that affects your health decisions.