BMI Calculator

What Is BMI?

Body Mass Index (BMI) is a simple numerical measure derived from a person's height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and later adopted by the World Health Organisation as a population-level screening tool for weight-related health risks.

The formula is straightforward: BMI = weight in kilograms ÷ height in metres squared. A person weighing 75 kg at 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 24.5 — comfortably within the healthy range.

How to Interpret Your BMI Result

The World Health Organisation classifies BMI into four categories:

• Under 18.5: Underweight — may indicate nutritional deficiency or underlying health conditions
• 18.5–24.9: Healthy weight — associated with the lowest risk of weight-related health conditions
• 25.0–29.9: Overweight — associated with moderately elevated risk of metabolic conditions
• 30.0 and above: Obese — associated with significantly elevated cardiovascular and metabolic risk

Important Australian context: The National Health and Medical Research Council (NHMRC) notes that people of Asian descent may experience elevated metabolic risk at a BMI of 23 or above, below the standard threshold. This reflects differences in body composition at equivalent BMI values.

The Limitations of BMI

BMI is a useful screening tool but it has significant limitations that all users should understand. It cannot distinguish between muscle mass and fat mass — a professional athlete or bodybuilder may have a BMI in the "overweight" range despite having very low body fat. Conversely, an older adult may have a BMI in the healthy range despite having low muscle mass and high body fat (sometimes called "sarcopenic obesity" or "skinny fat").

BMI also does not account for where fat is stored. Visceral fat — the fat deposited around abdominal organs — is metabolically active and associated with insulin resistance, type 2 diabetes, and cardiovascular disease. Two people with identical BMIs can have very different health profiles depending on their fat distribution. This is why measuring your waist-to-hip ratio and waist circumference alongside BMI gives a more complete picture of your metabolic health.

What to Do With Your BMI Result

If your BMI falls outside the healthy range, the next step is to combine it with additional measurements for a fuller picture. Use our Body Fat Percentage Calculator to estimate your body composition, and our Waist-to-Hip Ratio Calculator to assess metabolic risk. If you're looking to move your BMI toward a healthier range, our TDEE Calculator and Calorie Deficit Calculator will give you precise, personalised nutrition targets to get there safely and sustainably.

Remember: a single number does not define your health. BMI is a starting point for a conversation — with yourself and ideally with a qualified health professional — not a diagnosis.