BMI Calculator: What Your Body Mass Index Really Tells You

What BMI is and how it is calculated

Body Mass Index, or BMI, is a simple numerical measure derived from a person's weight and height. The formula divides weight in kilograms by the square of height in metres. Despite being over 150 years old, BMI remains one of the most widely used screening tools in public health because it is accessible, quick to compute, and broadly comparable across large populations.

The metric itself does not directly measure body fat, but it correlates strongly enough with direct fat measurements that doctors and researchers continue to use it as a cost-effective first filter for identifying weight-related health risks.

• BMI = weight (kg) ÷ height² (m²)
• Example: a person weighing 70 kg at 1.75 m has a BMI of 22.9
• In the US customary system: BMI = 703 × weight (lb) ÷ height² (inches²)

Understanding the BMI ranges

The World Health Organisation classifies BMI into four main categories for adults. A BMI below 18.5 suggests underweight status, while the range from 18.5 to 24.9 is considered normal or healthy. Values from 25 to 29.9 fall into the overweight category, and anything 30 or above is classified as obese.

Some health organisations further subdivide the obese range into Class 1 (30–34.9), Class 2 (35–39.9), and Class 3 (40 and above), with higher classes associated with significantly elevated health risks including cardiovascular disease, type 2 diabetes, and certain cancers.

• Underweight: BMI below 18.5
• Normal / Healthy: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese Class 1: 30 to 34.9
• Obese Class 2: 35 to 39.9
• Obese Class 3 (Severe): 40 and above

Worked BMI calculation example

Consider someone who weighs 82 kilograms and is 1.72 metres tall. First, square the height: 1.72 × 1.72 = 2.9584. Then divide weight by this value: 82 ÷ 2.9584 ≈ 27.7. A BMI of 27.7 falls in the overweight category, suggesting that weight management discussions with a healthcare provider would be appropriate.

A second example: a person at 58 kg and 1.65 m has a BMI of 58 ÷ 2.7225 ≈ 21.3, which sits comfortably in the normal range.

Why BMI has important limitations

BMI is a population-level screening tool, not a personal health diagnosis. It does not distinguish between muscle and fat, so a highly muscular athlete can register as overweight or obese by BMI alone despite having very low body fat. Conversely, someone with a normal BMI can still carry unhealthy levels of visceral fat, particularly in the abdomen.

The metric also does not account for age-related changes in body composition, ethnic differences in risk thresholds, or sex-based differences in fat distribution. Research has shown that South Asian populations, for example, face higher metabolic risk at lower BMI values than Western reference ranges suggest.

For a fuller picture, healthcare professionals typically combine BMI with waist circumference measurements, body fat percentage estimates, blood tests, and other clinical assessments.

Using BMI alongside other health tools

A BMI calculator is usually the starting point rather than the final word on weight-related health. Pairing it with a BMR (Basal Metabolic Rate) calculator helps estimate how many calories the body needs at rest, which informs dietary planning. A calorie calculator then provides context for daily energy needs based on activity level.

Body fat calculators using skinfold or bioelectrical impedance methods add another layer of specificity. Used together, these tools give a more balanced view of body composition than BMI alone.