BMI (Body Mass Index) is a number calculated from your weight and height. It provides a simple, widely used screening tool to categorise a person as underweight, normal weight, overweight, or obese. It was developed by Adolphe Quetelet in the 19th century and adopted by health organisations worldwide as a quick measure of body fatness.
May indicate malnutrition, an eating disorder, or other underlying health condition. Can be associated with weakened immune function and bone density loss.
Associated with the lowest risk of weight-related health problems. The goal for most adults. Maintaining this range through diet and exercise is recommended.
Increased risk of heart disease, type 2 diabetes, and high blood pressure. Some people in this range, particularly those with high muscle mass, may not face elevated risks.
Significantly elevated risk of cardiovascular disease, diabetes, sleep apnoea, joint problems, and certain cancers. Medical guidance is recommended. Further divided into Class I (30–34.9), Class II (35–39.9).
Also called Class III obesity. Severely elevated health risks. Strongly associated with reduced life expectancy. Medical intervention, such as weight management programmes or bariatric surgery, may be considered.
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | Below 18.5 | Low weight-related risk, other health concerns possible |
| Normal weight | 18.5 – 24.9 | Lowest risk of weight-related conditions |
| Overweight | 25.0 – 29.9 | Moderately increased risk |
| Obese — Class I | 30.0 – 34.9 | High risk |
| Obese — Class II | 35.0 – 39.9 | Very high risk |
| Obese — Class III (Morbid) | 40.0 and above | Extremely high risk |
While BMI is a useful and widely used tool, it has important limitations that you should be aware of:
BMI is best used as one part of a broader health assessment. For a more complete picture, consider also looking at waist circumference, body fat percentage, blood pressure, cholesterol levels, and overall fitness.
The BMI formula is straightforward:
Metric: BMI = weight (kg) ÷ height (m)²
Example: 70 kg ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²
Example: (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7
Both formulas give the same result for the same person — the factor 703 converts between the two unit systems.
A BMI of 25 sits exactly at the boundary between "Normal weight" and "Overweight" according to WHO classifications. It is not considered problematic on its own, but it indicates the upper end of the normal range. Factors like muscle mass, age, and body fat distribution should also be considered.
A BMI of 30 marks the threshold for obesity (Class I). At this level, the risk of type 2 diabetes, heart disease, high blood pressure, sleep apnoea, and certain cancers is meaningfully elevated compared to normal weight. Healthcare professionals generally recommend weight management intervention at this level.
Not necessarily. BMI is a population-level screening tool, not a definitive individual health measure. Very muscular athletes often have BMIs in the overweight range despite excellent health. Conversely, someone with a normal BMI can still have high body fat (known as "normal weight obesity"). BMI should be interpreted alongside other health markers.
The standard BMI categories (18.5–24.9 for normal weight) apply to both men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI. Some researchers suggest that the thresholds for health risk may differ slightly by sex and age, but the standard WHO categories are used universally in most clinical settings.
Yes. This is called "metabolically obese normal weight" (MONW) or "normal weight obesity." A person may have a normal BMI but carry excess visceral fat, have insulin resistance, high blood pressure, or abnormal cholesterol. Conversely, a person with a slightly elevated BMI but high muscle mass, low body fat, and excellent fitness markers may be very healthy. BMI is just one data point.