BMI Calculator
Body Mass Index (BMI) is a widely used screening tool that estimates whether your weight is healthy relative to your height, using the formula: weight (kg) divided by height (m) squared. The Australian National Health and Medical Research Council (NHMRC) defines healthy BMI as 18.5–24.9; overweight as 25–29.9; and obese as 30 and above. BMI is a population-level screening tool, not a diagnostic measure, it has well-known limitations. It doesn't distinguish between fat and muscle mass (an athlete can have a high BMI with very little body fat), it doesn't account for where fat is stored (visceral abdominal fat is higher-risk than subcutaneous fat), and it may not be appropriate for certain ethnic groups or older adults. The healthy weight range shown below gives the weight span that falls within BMI 18.5–24.9 for your specific height.
Frequently Asked Questions
What is BMI and how is it calculated?
BMI (Body Mass Index) divides your weight in kilograms by the square of your height in metres. The formula is: weight (kg) divided by (height in metres squared). A BMI of 25 or above indicates overweight, while 30 or above indicates obesity. While BMI does not measure body fat directly, it correlates reasonably well with direct measures of body fat in most adults and is the most widely used population-level screening tool for weight-related health risk.
What are the BMI categories for adults?
Under 18.5 = Underweight (may indicate nutritional deficiency or underlying health issue). 18.5-24.9 = Healthy weight (lower risk of weight-related disease). 25-29.9 = Overweight (increased risk of heart disease, type 2 diabetes, and some cancers). 30+ = Obese, divided into Class 1 (30-34.9), Class 2 (35-39.9), and Class 3 (40+, sometimes called morbid obesity). These ranges apply to all adults aged 18 and over regardless of sex or ethnicity.
Is BMI an accurate measure of health?
BMI has significant limitations. It does not distinguish between muscle and fat, so a muscular athlete can have a high BMI despite having very low body fat. It also does not account for where fat is stored on your body: visceral fat around organs is more dangerous than subcutaneous fat under the skin. Some ethnic groups also have different health risk thresholds at the same BMI. For these reasons, BMI should be used as a screening tool only, and considered alongside waist circumference, blood pressure, and blood tests for a complete health picture.
What is a healthy waist measurement?
Waist circumference is a useful complement to BMI for assessing health risk. Men should aim for under 94cm (approximately 37 inches) and women under 80cm (about 31.5 inches) to reduce health risks. Above 102cm for men or 88cm for women significantly increases the risk of metabolic disease. Waist-to-height ratio may be even more useful than BMI or waist circumference alone: keeping your waist circumference to less than half your height is associated with lower health risk across different populations.
What is waist-to-height ratio (WHtR)?
Waist-to-height ratio (WHtR) divides your waist circumference by your height, both in the same units. It is calculated as waist (cm) divided by height (cm). Unlike BMI, WHtR accounts for where body fat is stored - visceral fat around the abdomen is more dangerous than fat elsewhere. WHtR categories are: under 0.40 = Underweight, 0.40–0.50 = Healthy, 0.50–0.60 = Increased risk, over 0.60 = High risk. Keeping your waist to less than half your height is a simple rule associated with lower risk of diabetes, heart disease, and early mortality across populations.
Does BMI change with age?
BMI ranges are the same for all adults regardless of age, as the thresholds were developed based on health outcomes across all adult ages. However, body composition naturally changes with age: muscle mass tends to decrease (called sarcopenia) while fat mass increases even if your weight stays constant. This means an older person with a normal BMI may actually have higher body fat than a younger person with the same BMI. Some researchers argue that slightly higher BMI cutoffs for older adults would be more appropriate, though this is not yet standard clinical practice.
What should I do if my BMI is outside the healthy range?
BMI is a screening tool, not a diagnosis. If your BMI is under 18.5 or 25 or above, consider consulting your doctor or a dietitian who can assess your full health picture including diet quality, physical activity levels, family history, blood pressure, and blood markers like cholesterol and blood sugar. A BMI of 30 or above, or a BMI of 27 or more combined with weight-related health conditions, may make you eligible for structured weight management programs including dietary counselling and medical support.