Metric and imperial formulas
Metric: BMI = weight in kilograms / (height in meters) squared. A person weighing 70 kg at 1.75 m has a BMI of 70 / 3.0625 = 22.9.
Imperial: BMI = 703 x weight in pounds / (height in inches) squared. The 703 factor converts the units.
The standard chart
Under 18.5 is classified as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese. These bands come from the WHO and are used consistently across most health systems.
The bands are population-level screening tools. They are useful for spotting trends across large groups and much less useful for judging an individual.
Where BMI falls short
BMI cannot tell muscle from fat. A muscular athlete and a sedentary person of the same height and weight get the same number, even though their body composition is completely different.
It also ignores waist circumference, age and sex. Use BMI as one input next to waist measurement, blood pressure and how you actually feel - not as a verdict.