What is BMI?
Body Mass Index (BMI) is a simple numerical measure that relates a person's body weight to their height. It is widely used as a screening tool to categorize individuals into weight status groups — underweight, normal weight, overweight, and obese. BMI was developed by Belgian statistician Adolphe Quetelet in the 19th century and remains one of the most common health indicators used by doctors and public health professionals worldwide.
While BMI does not directly measure body fat, studies have shown that it correlates well with more direct measures of body fat for most people. It is quick to calculate, requires no special equipment, and provides a useful first-pass assessment of whether someone's weight may be putting them at risk for health conditions such as type 2 diabetes, heart disease, high blood pressure, and certain cancers.
Formula
BMI is calculated using the following formula:
BMI = weight (kg) ÷ height (m)²
In metric units: divide your weight in kilograms by the square of your height in metres.
For example, a person who weighs 70 kg and is 1.75 m tall:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9
BMI categories (WHO standards)
| BMI range | Category | |-----------|----------| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | 30.0 and above | Obese |
Example
Suppose you weigh 85 kg and are 180 cm tall.
- Convert height to metres: 180 cm ÷ 100 = 1.80 m
- Square the height: 1.80 × 1.80 = 3.24
- Divide weight by squared height: 85 ÷ 3.24 ≈ 26.2
A BMI of 26.2 falls in the Overweight category (25.0 – 29.9).
FAQ
Is BMI accurate for everyone? BMI is a screening tool, not a diagnostic measure. It can overestimate body fat in athletes with high muscle mass and underestimate it in older adults who have lost muscle. It also does not account for fat distribution, which is an important health factor.
What is a healthy BMI? According to the World Health Organization, a BMI between 18.5 and 24.9 is considered normal weight for adults. However, optimal ranges can vary slightly by age and ethnicity — for example, lower thresholds are recommended for people of Asian descent.
Does BMI apply to children? Standard adult BMI categories do not apply to children. For individuals under 18, BMI is interpreted using age- and sex-specific percentiles rather than fixed cut-off values.
Can I have a healthy BMI but still have health problems? Yes. BMI is just one indicator among many. People with a "normal" BMI can still have high body fat percentage, high cholesterol, or other risk factors. Always consult a healthcare professional for a full assessment.
Should I use kg/cm or lb/in? This calculator uses metric units (kilograms and centimetres). If you use imperial units, convert first: 1 lb ≈ 0.454 kg and 1 inch = 2.54 cm.