What is BMI?
Body Mass Index (BMI) is a simple measure that uses your height and weight to estimate whether your weight falls into a healthy range. It's calculated by dividing your weight by the square of your height, and the result places you into one of four categories: underweight, normal weight, overweight, or obese.
Doctors and health organisations use BMI as a quick screening tool because it's easy to calculate and correlates reasonably well with body fat for most people. It doesn't directly measure body fat, so it works best as a starting point rather than a complete picture of health.
BMI categories explained
- Below 18.5 — Underweight. May indicate insufficient nutrient intake.
- 18.5–24.9 — Normal weight. Generally associated with lower health risk.
- 25–29.9 — Overweight. May increase risk of certain conditions over time.
- 30 and above — Obese. Associated with higher risk of conditions such as type 2 diabetes and heart disease.
Limitations of BMI
BMI doesn't distinguish between muscle and fat, so muscular individuals — such as athletes — may show a higher BMI despite having low body fat. It also doesn't account for where fat is distributed on the body, which can matter for health risk. For a fuller picture, BMI is best used alongside other measures like waist circumference, body fat percentage, and overall lifestyle.
Frequently asked questions
What is a healthy BMI range?
For most adults, a BMI between 18.5 and 24.9 is considered within the healthy weight range. Below 18.5 is classified as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Is BMI accurate for everyone?
BMI is a useful screening tool but doesn't account for muscle mass, bone density, age, or sex. Athletes and very muscular people often have a high BMI despite low body fat, so BMI should be considered alongside other health measures.
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by the square of your height in meters (kg/m²). For imperial units, divide weight in pounds by height in inches squared, then multiply by 703.
Does BMI differ for children?
Yes. For children and teens, BMI is interpreted using age- and sex-specific percentiles rather than the fixed adult categories, since healthy body composition changes during growth. This calculator is designed for adults.