BMI calculator
Calculate your Body Mass Index and see where you fall in the healthy weight range.
PhD Sports Science, Registered Nutritionist (RNutr)
Sports scientist and registered nutritionist specialising in metabolic health, athletic performance and dietary analysis.
What if?
Your BMI on the scale
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About the BMI calculator
Body Mass Index (BMI) is a widely used screening tool that estimates whether a person's weight is in a healthy range relative to their height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it is calculated by dividing weight in kilograms by the square of height in metres.
BMI is used by healthcare professionals and researchers as a quick, population-level indicator of potential weight-related health risks. People with a BMI below 18.5 are considered underweight; 18.5–24.9 is healthy; 25–29.9 is overweight; 30 or above is classified as obese. These thresholds are the same globally, though some health bodies use lower cutoffs for Asian populations.
It's important to understand what BMI does not measure: it cannot distinguish between fat mass and lean muscle mass, and it ignores fat distribution (visceral fat around the abdomen is more harmful than subcutaneous fat). A rugby player with exceptional muscle mass may score an "overweight" BMI despite having very low body fat. For a more complete picture, consider also measuring waist circumference and, if available, body fat percentage.
How it works
BMI = weight (kg) / height (m)² Where: weight is in kilograms height is in metres (divide centimetres by 100) Imperial conversion: BMI = (weight in pounds × 703) / height in inches²
Where
kgBody weight in kilograms (1 lb = 0.4536 kg)m²Height in metres, squared (1 inch = 0.0254 m)Worked example
Weight: 80 kg
Height: 178 cm → 1.78 m
BMI = 80 / (1.78 × 1.78)
= 80 / 3.1684
= 25.25
Category: Overweight (BMI 25–29.9)
Healthy weight range for 1.78 m:
Minimum: 18.5 × 1.78² = 58.6 kg
Maximum: 24.9 × 1.78² = 78.9 kg
Weight to reach healthy range: 80 − 78.9 = 1.1 kg
Tips to improve your result
- 1.
BMI is a starting point, not a verdict. Always consider it alongside other measures like waist circumference (aim for under 94 cm for men, under 80 cm for women), blood pressure, cholesterol and blood glucose.
- 2.
If your BMI shows overweight but you are very muscular, get your body fat percentage measured instead (DEXA scan, hydrostatic weighing or BodPod for accuracy; consumer scales for rough tracking).
- 3.
Losing as little as 5–10% of your body weight if overweight significantly reduces the risk of type 2 diabetes, heart disease and certain cancers — even if you remain in the "overweight" BMI range.
- 4.
BMI thresholds were set for European populations. If you are of South Asian, East Asian or Middle Eastern background, a BMI of 23+ may indicate increased risk. Some clinicians use adjusted cutoffs for these groups.
- 5.
Children and teenagers have different healthy BMI ranges that change with age and sex. Use a dedicated age-adjusted BMI-for-age calculator rather than the adult thresholds.
Reference table
WHO BMI classification for adults
| Category | BMI range | Health implications |
|---|---|---|
| Severely underweight | Below 16.0 | High risk of malnutrition, organ failure |
| Underweight | 16.0 – 18.4 | May indicate nutritional deficiency or illness |
| Normal weight | 18.5 – 24.9 | Lowest health risk in population studies |
| Overweight (Pre-obese) | 25.0 – 29.9 | Increased risk of metabolic conditions |
| Obese Class I | 30.0 – 34.9 | Moderate increase in health risks |
| Obese Class II | 35.0 – 39.9 | Severe increase in health risks |
| Obese Class III | 40.0+ | Very severe health risks; interventions advised |