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BMI Calculator
Body Mass Index (BMI) is a screening number that relates your weight to your height: it is your mass in kilograms divided by the square of your height in meters. This tool computes your BMI live and places it on the six World Health Organization adult categories, and it also tells you the weight range that sits inside the normal band for your height. Everything happens in your browser – none of your measurements are sent anywhere.
How to use
- Choose the unit system — Metric (centimeters and kilograms) or US (feet, inches, and pounds). Switching tabs converts your current numbers, so you can compare the same physique under both systems without retyping.
- Enter height and weight — the BMI value and category update instantly as you type.
- Read the result: the Body Mass Index, the category label (for example "Normal weight" or "Obesity (Class I)"), and the exact WHO range that category covers.
- The tool then shows the normal-weight band for your height — the weight in kilograms from a BMI of 18.5 to a BMI of 24.9 — so you can see where you stand relative to the healthy window without doing the arithmetic yourself.
- The WHO adult classification table highlights your category in the full six-step scale, from "Underweight" through "Obesity (Class III)".
The widget accepts any height and weight you type; a missing or non-positive value simply leaves the result empty until it becomes usable.
The six WHO categories
| Category | Range (kg/m²) |
|---|---|
| Underweight | below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity (Class I) | 30.0 – 34.9 |
| Obesity (Class II) | 35.0 – 39.9 |
| Obesity (Class III) | 40.0 and above |
The boundary values follow the WHO cut-offs published for adults aged 18 and over: exactly 18.5 counts as normal, exactly 25 counts as overweight, and exactly 30 counts as obesity class I. For a worked example, a person of 175 cm and 70 kg has a BMI of 22.9 and falls in the (fairly wide) normal band, while the same height with 55 kg gives 18.0 and tips over into underweight.
What BMI is good for
BMI was designed as a population screening measure, not a diagnosis. It needs no equipment beyond a scale and a tape measure, which is exactly why it is still the first number used in epidemiological studies, insurance underwriting, and office health checks. Tracking your BMI over months is a convenient proxy for whether your weight is trending up or down, and it costs nothing to recompute as often as you like. Compare it with the site's BMR calculator if you want the next step: knowing how many calories your body burns at rest changes how you react to that trend.
Limitations — read this first
BMI uses only weight and height and therefore says nothing about body composition. A trained athlete with a high proportion of muscle routinely lands in "Overweight" while being extremely lean; equally, two people with the same BMI can have very different amounts of visceral fat. Because fat distribution matters for health, BMI does not substitute for waist measurement, blood pressure, or blood markers. The categories also apply to adults; children and adolescents use age- and sex-specific percentile charts instead. Pregnant women and very muscular individuals are two classic cases where the plain BMI number misleads. Treat the classification here as an orientation point, and pair it with other measurements — or professional advice — before drawing conclusions.