BMI is a screening ratio based on weight and height. It can provide a consistent starting point for population-level comparison, but it does not directly measure body fat, fitness, health, or individual risk.
The BMI formula
With kilograms and metres, BMI = weight ÷ height². A person weighing 70 kg and measuring 1.75 m has BMI = 70 ÷ (1.75 × 1.75) = 22.86. Imperial inputs use a conversion factor after squaring height in inches.
Use the units correctly
Do not enter centimetres where metres are expected, and do not mix pounds with kilograms. A unit error can produce a result that looks numeric but is not meaningful.
Why context matters
Muscle mass, age, pregnancy, ethnicity, medical history, and fat distribution can affect how a BMI value should be interpreted. Cutoffs can also differ by guideline and population.
A screening result is not a diagnosis
Use the result as an educational prompt, not a diagnosis or treatment decision. Discuss concerns, symptoms, goals, and other measurements with a qualified healthcare professional.
Sources
The figures and rules in this guide are checked against:
- About Body Mass Index (BMI) — U.S. Centers for Disease Control and Prevention
- Adult BMI Calculator — U.S. Centers for Disease Control and Prevention
- Obesity and overweight (fact sheet) — World Health Organization
- Calculate your body mass index (BMI) — NHS