What is BMI?
BMI stands for Body Mass Index. It is a simple measure that uses your height and weight to estimate whether your weight falls into a healthy range. Doctors, nutritionists and health organisations around the world use BMI as a quick screening tool because it requires only two numbers and no specialist equipment.
How BMI is Calculated
The standard formula divides your weight in kilograms by the square of your height in metres:
BMI = weight (kg) ÷ height² (m²)
For imperial units, the formula is slightly different: multiply your weight in pounds by 703, then divide by the square of your height in inches. This calculator handles the conversion automatically, so you can enter your numbers in the units you already use.
BMI Categories
- Below 18.5: Underweight
- 18.5 – 24.9: Normal weight
- 25.0 – 29.9: Overweight
- 30.0 and above: Obese
Limitations of BMI
BMI is a general guide, not a diagnosis. It does not distinguish between muscle and fat, so athletes and very muscular people may have a high BMI without excess body fat. It also does not account for age, sex, bone density or body composition. Always treat BMI as one signal among many and talk to a healthcare professional for a complete assessment.
Frequently Asked Questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy.
Is BMI accurate for children?
No. Children and teenagers need age- and sex-specific BMI percentiles, which are different from adult BMI categories.
Does BMI differ by sex?
Adult BMI categories are the same for men and women, although body composition differs. Some clinicians prefer waist-to-height ratio for a more nuanced view.
How often should I check my BMI?
Occasional checks are fine. Weekly or monthly tracking is usually enough for general awareness unless a doctor has advised otherwise.