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BMI Calculator

Get your BMI and WHO weight category instantly in metric or imperial units.

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Choose your unit system with the Metric / Imperial toggle.
  2. 2.Enter your weight and height in the matching boxes (kg and m for metric, lb and in for imperial).
  3. 3.Read your BMI value and WHO weight category, which update instantly as you type.

About BMI Calculator

This free BMI calculator turns your weight and height into a single number — your Body Mass Index — and maps it onto the World Health Organization's standard weight categories. BMI is also known as the Quetelet index, named after the Belgian mathematician and astronomer Adolphe Quetelet, who devised the weight-divided-by-height-squared ratio in the 1830s while studying the physical characteristics of populations. It was popularised as the 'body mass index' in the 20th century and remains the most widely used population-level screening measure for weight status.

The math is deliberately simple. In metric units, BMI equals your weight in kilograms divided by the square of your height in metres (kg/m²). In imperial units, the same ratio is scaled by a conversion constant of 703, so BMI equals 703 times your weight in pounds divided by the square of your height in inches. This calculator computes both entirely in your browser — nothing is uploaded — and updates the result the moment you change a value or switch units.

Once your BMI is known, it is compared against the WHO categories for adults: below 18.5 is classified as underweight, 18.5 to 24.9 as normal (healthy) weight, 25.0 to 29.9 as overweight, and 30.0 or above as obese. These cut-offs are used by clinicians and public-health bodies worldwide as a quick, low-cost triage step, not as a final verdict.

BMI has a well-known and important limitation: it is calculated from only weight and height, so it cannot distinguish muscle from fat. A muscular athlete and a sedentary person of the same height and weight receive the same BMI, even though their body composition and health risks differ. BMI also does not account for fat distribution, bone density, age, sex, or ethnicity, all of which affect what a given number means for a given individual. For these reasons a raw BMI figure should be read as a screening signal rather than a diagnosis. Treat this tool as an educational estimate and speak to a qualified healthcare professional for anything that affects your health decisions.

Methodology & sources

BMI (the Quetelet index, devised by Adolphe Quetelet in the 1830s) is computed as weight ÷ height². Metric: BMI = weightKg / heightM². Imperial: BMI = 703 × weightLb / heightIn², where 703 converts lb/in² to kg/m². The result is classified with the World Health Organization adult cut-offs: <18.5 underweight, 18.5–24.9 normal, 25.0–29.9 overweight, ≥30.0 obese (boundaries applied as <18.5 / <25 / <30 / ≥30 so the ranges are continuous). Assumptions: adult (non-pregnant) subjects; BMI reflects weight-for-height only and cannot distinguish muscle from fat or account for age, sex, or ethnicity. This tool is a screening estimate for education only — it is not a diagnosis or medical advice; consult a qualified healthcare professional.

Frequently asked questions

How is BMI calculated?
BMI is your weight divided by the square of your height. In metric units it is weight in kilograms divided by height in metres squared (kg/m²); in imperial units it is 703 multiplied by weight in pounds, divided by height in inches squared. This is the classic Quetelet-index formula.
What is a healthy BMI range?
Using the WHO categories for adults, a BMI below 18.5 is underweight, 18.5 to 24.9 is the normal or healthy range, 25.0 to 29.9 is overweight, and 30.0 or above is obese. These ranges are screening guides for populations, not individual diagnoses.
Is BMI accurate for everyone?
No. Because BMI uses only weight and height, it cannot tell muscle from fat and ignores age, sex, ethnicity, and fat distribution. Very muscular people can register as overweight while carrying little fat. BMI is a general screening estimate, not medical advice — consult a qualified healthcare professional for a real assessment.

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