BMI Calculator
Calculate body mass index from height and weight, in metric or imperial units, with the WHO classification.
How to use this calculator
- Choose metric or imperial units.
- Enter your weight and height.
- The result shows your BMI and the WHO adult classification band it falls into.
The formula
BMI = weight (kg) ÷ height (m)² or BMI = 703 × weight (lb) ÷ height (in)²- kg/m²
- Metric form
- 703
- Conversion factor that makes the imperial form agree with the metric one
How it works
BMI scales weight against the square of height, which roughly corrects for the fact that taller people naturally weigh more.
The WHO bands for adults are: underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, and obesity 30 and above.
Worked example
An adult weighing 70 kg with a height of 175 cm.
- 1Height in meters: 1.75
- 21.75² = 3.0625
- 370 ÷ 3.0625 ≈ 22.86
BMI ≈ 22.9 — within the healthy-weight band.
When this calculator is useful
- A quick population-standard weight screening
- Tracking change over time
- A starting point for a conversation with a health professional
Frequently asked questions
Is BMI accurate for athletes?
Often not. Dense muscle raises weight without the associated health risks of fat, so very muscular people can score 'overweight' or 'obese' while being metabolically healthy.
What is a healthy BMI?
The WHO classifies 18.5–24.9 as the healthy range for adults, but it is a screening band, not an individual diagnosis.
How do I enter 5 feet 9 inches?
In imperial mode enter total inches: 5 × 12 + 9 = 69.