About this calculator

This calculator estimates your Body Mass Index (BMI) — a simple ratio of weight to height that gives a general indication of whether your weight falls within a range associated with healthy outcomes for most adults. Enter your weight in kilograms and your height in centimetres to see your BMI and the WHO category it corresponds to.

How to read your results

The main figure is your BMI, rounded to one decimal place. Below it you will see which WHO adult category your result falls into: Underweight (below 18.5), Healthy weight (18.5–24.9), Overweight (25.0–29.9), Obese class I (30.0–34.9), Obese class II (35.0–39.9), or Obese class III (40 and above). Keep in mind that BMI does not distinguish between muscle and fat — a person with high muscle mass may register as overweight even if their body composition is healthy. Use this estimate as one data point alongside advice from a qualified health professional.

Worked example

A person weighs 70 kg and is 175 cm tall.

BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — Healthy weight category. No units other than kg and cm are needed; the formula produces a dimensionless number.

Frequently asked questions

What does my BMI number actually mean?

BMI is a population-level screening tool, not a diagnostic measure. A result in the healthy range (18.5–24.9) suggests that, statistically, your weight is within a range associated with lower risk of weight-related conditions for most adults. It does not confirm your overall health status — a healthcare provider can interpret the number in context.

Why does BMI not account for muscle versus fat?

BMI uses only weight and height, so it cannot tell how much of your weight is muscle, fat, bone, or water. An athlete with dense muscle mass may have a BMI in the overweight range despite low body fat, while an inactive person may fall in the healthy range with a high proportion of body fat. If you suspect your body composition differs significantly from average, a body-fat measurement gives a more complete picture.

Does BMI apply to children?

No. The WHO adult cut-offs used here (18.5 / 25 / 30) apply only to adults aged 18 and over. For children and teenagers, BMI must be compared to age- and sex-specific growth charts, not fixed thresholds. Consult a paediatrician for an appropriate assessment of a child's weight.

Are there limitations for certain ethnic groups?

Yes. Research suggests that health risks associated with excess body fat can occur at lower BMI values in some Asian populations and at higher values in some Pacific Islander populations. Some health authorities use adjusted thresholds for these groups. The WHO cut-offs shown here are the global standard for general reference.

How often should I recalculate my BMI?

BMI changes whenever your weight or height changes. For most adults, height is stable, so tracking weight periodically — every few months, for example — is enough to spot trends. A single reading is less informative than a series of readings over time.

How it's calculated

BMI is calculated as weight in kilograms divided by the square of height in metres: BMI = weight(kg) ÷ height(m)². Height in centimetres is converted to metres by dividing by 100 before squaring. The result is rounded to one decimal place. The WHO adult classification bands are: below 18.5 = Underweight; 18.5 to below 25 = Healthy weight; 25 to below 30 = Overweight; 30 to below 35 = Obese class I; 35 to below 40 = Obese class II; 40 and above = Obese class III. These thresholds are derived from population studies linking BMI ranges to the risk of weight-related health conditions and are published by the World Health Organization.