Formula
How do I calculate BMI?
To compute BMI, divide your weight in kilograms by your height in meters, then divide the answer by your height again. Expressed as a single formula: BMI = weight (kg) ÷ height (m)². If your height is in centimeters, divide it by 100 before squaring. A person weighing 70 kg with a height of 170 cm has a BMI of 70 ÷ 1.70² = 24.2.
What is a normal BMI?
For most adults, the World Health Organization defines a normal (healthy) BMI as 18.5 to 24.9. This range is the same for men and women and does not depend on age within the adult population. Children and teenagers use percentile-based curves instead, because their bodies are still growing.
What are the BMI ranges?
There are five basic ranges within the BMI scale: Underweight is below 18.5. Normal weight is 18.5 to 24.9. Overweight is 25 to 29.9. Obesity is 30 to 34.9. Severe obesity is 35 and above. BMI Prime is a related figure - the BMI divided by 25. A BMI Prime of 1.0 sits at the upper limit of the normal range, so anything above 1 indicates you're carrying some excess weight.
Using the BMI calculator
Enter your height and weight, and you'll instantly see your BMI, your BMI Prime, and a short message telling you which range you fall into. The optional sex selector is there for context - adult BMI itself does not depend on gender, but children's and teens' BMI charts do, and we may add those calculators next.
Other considerations
BMI is a screening tool, not a diagnosis. It can overestimate body fat in well-muscled people (such as bodybuilders or athletes) and underestimate it in older adults who have lost muscle mass. If you want a fuller picture, combine BMI with measurements like waist circumference, body fat percentage, or your basal metabolic rate. Always consult a healthcare professional before making weight-related decisions.
Frequently asked questions
BMI is a number derived from your weight and height. The World Health Organization (WHO) uses it as a first-line screening tool for underweight, healthy weight, overweight, and obesity. It doesn't measure body fat directly.
Divide your weight in kilograms by the square of your height in meters. Example: 70 kg ÷ (1.70 m)² = 24.2, which falls in the normal range.
Per the WHO: under 18.5 is underweight, 18.5–24.9 is a normal/healthy weight, 25–29.9 is overweight, and 30 or above is classified as obese.
BMI is a screening tool, not a diagnosis. It can overestimate body fat in muscular athletes and may not apply well to pregnant people, the elderly, or very short individuals. Always combine it with other clinical measures.
Sources
- A healthy lifestyle - WHO recommendations on Body Mass Index— World Health Organization
- About Adult BMI— Centers for Disease Control and Prevention
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