BMI Calculator
Enter your height and weight to instantly calculate your Body Mass Index and see what it means for your health.
What is BMI?
Body Mass Index (BMI) is a simple numerical measure that uses your height and weight to estimate whether you have a healthy body weight. It was developed by Belgian statistician Adolphe Quetelet in the 19th century and is now one of the most widely used screening tools in medicine and public health.
The formula is straightforward: divide your weight in kilograms by the square of your height in metres. A score of 18.5–24.9 is classified as a healthy weight by the World Health Organisation and most national health agencies.
BMI is a screening tool, not a diagnostic measure. It does not directly measure body fat and cannot distinguish between fat mass and lean mass. Athletes with high muscle mass may be classified as overweight despite having low body fat. For a more complete picture, consider also measuring waist circumference or body fat percentage.
How to Interpret Your BMI
The WHO BMI classifications below apply to adults aged 18 and over. Different cut-off points are used for children and some ethnic populations.
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Moderate (malnutrition risk) |
| 18.5 – 24.9 | Healthy weight | Low |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese (Class I) | High |
| 35.0 – 39.9 | Obese (Class II) | Very high |
| 40.0 and above | Obese (Class III) | Extremely high |
Worked Examples
Example 1: Healthy adult male. Height 180 cm (5'11"), weight 78 kg (172 lbs). BMI = 78 ÷ (1.80²) = 78 ÷ 3.24 = 24.1 — Healthy weight.
Example 2: Shorter adult female. Height 160 cm (5'3"), weight 85 kg (187 lbs). BMI = 85 ÷ (1.60²) = 85 ÷ 2.56 = 33.2 — Obese Class I.
Example 3: Tall male athlete. Height 188 cm (6'2"), weight 98 kg (216 lbs). BMI = 98 ÷ (1.88²) = 98 ÷ 3.53 = 27.8 — Overweight, but this person may have high muscle mass, making BMI an unreliable indicator here.
Frequently Asked Questions
-
No. BMI does not distinguish between fat and muscle. Athletes, older adults (who may have lost muscle), and people of some ethnic backgrounds may get misleading results. It's best used as one screening tool alongside waist circumference and clinical assessment.
-
The WHO defines a healthy BMI as 18.5–24.9. However, some research suggests slightly different optimal ranges by age and ethnicity. For adults of Asian descent, for example, overweight is often defined at BMI ≥ 23.
-
Both provide useful information. BMI is free and instant but imprecise. Body fat percentage (measured by DEXA scan, hydrostatic weighing, or estimation methods like the US Navy method) gives a clearer picture of body composition. Ideally use both.
-
Not directly. For children and teens, BMI is plotted on age- and sex-specific growth charts. The resulting "BMI-for-age" percentile is used instead of fixed cut-off points, since healthy BMI ranges change as children grow.
-
Yes. BMI above 25 does not automatically mean poor health. If your waist circumference is within a healthy range, you are metabolically fit, physically active, and have no chronic conditions, a slightly elevated BMI may pose no additional risk.
-
A BMI below 17.5 is associated with significantly increased health risks including malnutrition, bone density loss, heart complications, and immune suppression. Below 15 is considered a medical emergency. If you are concerned about a low BMI, please seek medical advice.