BMI Calculator
Calculate your Body Mass Index using Australian health guidelines. Supports metric and imperial units.
Your details
Your BMI
28.7
Category
25.0 – 30.0
Healthy range for your height
54.1 – 72.8 kg
BMI Prime
1.15
BMI is a screening tool for adults (18+). It does not measure body fat directly or account for muscle mass, bone density, age, sex, or ethnicity. Discuss results with a healthcare professional.
What is BMI?
Body Mass Index (BMI) is a numerical value calculated from your weight and height. It provides a quick way to screen whether an adult falls into a weight category that may be associated with health risks. BMI is used worldwide by doctors, researchers, and public health bodies as a population-level indicator of weight status.
The concept was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a tool for studying population trends, not individual diagnosis.[8] It was never intended to be a definitive measure of health, and it remains exactly that: a screening tool that can flag the need for further assessment.
BMI does not measure body fat directly. Two people with the same BMI can have very different body compositions. It is one piece of information, best interpreted alongside other measures such as waist circumference, blood pressure, and blood tests.
How is BMI Calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared. The result is a single number that places you into one of several weight categories. The formula is the same regardless of age or sex.
Metric formula
BMI = weight (kg) ÷ [height (m)]²
Worked example
A person who weighs 70 kg and is 170 cm tall (1.70 m):
70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2
Imperial formula
BMI = [weight (lbs) ÷ height (in)²] × 703
Worked example
A person who weighs 154 lbs and is 5'7" (67 inches):
(154 ÷ 67²) × 703 = (154 ÷ 4489) × 703 = 24.1
The constant 703 in the imperial formula converts the result to match the metric scale. Both formulas produce the same BMI value for equivalent measurements. The calculator above handles the conversion automatically when you switch between metric and imperial units.
BMI Prime
BMI Prime is your BMI divided by 25 (the upper limit of the healthy range). A value of 1.00 means you are at the threshold, below 1.00 is within the healthy range, and above 1.00 is overweight. It provides a quick way to see how far your BMI is from the healthy upper limit as a simple ratio.
Adult BMI Categories
The World Health Organisation classifies adult BMI into six categories.[1] These are used internationally, including by Australian health authorities, as a guide for assessing weight-related health risk.[2]
| Category | BMI (kg/m²) |
|---|---|
| Underweight | < 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese class I | 30.0 – 34.9 |
| Obese class II | 35.0 – 39.9 |
| Obese class III | ≥ 40.0 |
Underweight (BMI below 18.5) may indicate insufficient nutrition and is associated with weakened immune function, reduced bone density, and reproductive difficulties. If your BMI falls below this threshold, a GP can help identify contributing factors.
Healthy weight (18.5 to 24.9) is the range associated with the lowest statistical risk of weight-related health problems for most adults. Maintaining this range through balanced nutrition and regular physical activity supports long-term health.
Overweight (25.0 to 29.9) carries an increased risk of developing type 2 diabetes, cardiovascular disease, and high blood pressure. Small, sustained changes to diet and activity levels can meaningfully reduce this risk.
Obese classes I through III (30.0 and above) represent progressively higher risk for serious conditions including heart disease, stroke, certain cancers, sleep apnoea, and osteoarthritis. Speaking with a GP is recommended to explore management options, which may include dietary support, exercise programs, or medical interventions.
Waist circumference
Australian guidelines flag increased health risk when waist circumference exceeds 94 cm for men and 80 cm for women.[2] BMI and waist circumference together give a more complete picture of weight-related health risk than either measure alone.
BMI and Health in Australia
According to the Australian Institute of Health and Welfare (AIHW), approximately two in three Australian adults (67%) are classified as overweight or obese.[3] This makes weight management one of the most significant public health challenges in the country. BMI is routinely measured during GP health assessments and Medicare-funded health checks as a starting point for evaluating weight status.[2]
Standard BMI cut-offs were developed from data on populations of predominantly European descent. Research has since shown that health risks can vary at different BMI levels depending on ethnic background.[4] The World Health Organisation and Australian clinical guidelines recognise these differences.
Adjusted thresholds for different populations
For people of Asian, South Asian, and Middle Eastern backgrounds, health risks may increase at lower BMI thresholds. The WHO suggests considering overweight from a BMI of 23 (rather than 25) and obesity from 27.5 (rather than 30) for these populations.[4]
For Aboriginal and Torres Strait Islander peoples, BMI should be interpreted alongside other clinical measures. Standard cut-offs may not fully capture health risk in these communities, and culturally appropriate health assessments are recommended.[5]
For older Australians (aged 74 and above), some researchers and Australian health resources suggest that a slightly higher BMI range of 22 to 26 may be acceptable.[6] In older age, general health, functional capacity, and nutritional status can be more relevant indicators than strict BMI targets. A GP can help weigh these factors in context.
Limitations of BMI
While BMI is a useful screening tool, it has well-documented limitations that are important to understand when interpreting your result.
- Muscle mass. BMI cannot distinguish between fat and muscle. Athletes and people who train regularly may have a BMI in the overweight or obese range despite having low body fat and excellent cardiovascular health.[7]
- Age and sex. Older adults tend to have more body fat at the same BMI compared to younger adults. Women typically carry a higher percentage of body fat than men at the same BMI.[7] These differences mean the same number can represent different levels of health risk.
- Body composition. BMI measures total weight relative to height. It cannot account for how that weight is distributed between fat, muscle, bone, and water. Two people with identical BMIs may have very different health profiles.[1, 7]
- Children and teenagers. Adult BMI categories do not apply to anyone under 18. Clinicians use age-and-sex-specific growth percentile charts for children and adolescents, which account for normal developmental variation.[9]
- Pregnancy. BMI is not interpreted in the standard way during pregnancy. Weight gain is expected, and antenatal care teams use different frameworks to assess healthy weight during pregnancy.[2]
Alternative measures
For a more complete picture of body composition and health risk, healthcare professionals may use waist circumference, waist-to-hip ratio, body fat percentage (via skinfold callipers or bioelectrical impedance), or DEXA scans. These measures help contextualise a BMI result, particularly for individuals where BMI alone may be misleading.
Healthy Weight Range by Height
The table below shows the approximate healthy weight range (BMI 18.5 to 24.9) for common adult heights. Use this as a general reference and consider your individual circumstances.
| Height (cm) | Healthy weight range (kg) |
|---|---|
| 150 | 41.6 – 56.0 |
| 155 | 44.4 – 59.8 |
| 160 | 47.4 – 63.7 |
| 165 | 50.4 – 67.8 |
| 170 | 53.5 – 72.0 |
| 175 | 56.7 – 76.3 |
| 180 | 59.9 – 80.7 |
| 185 | 63.3 – 85.2 |
| 190 | 66.8 – 89.9 |
| 195 | 70.3 – 94.7 |
References
- 1.Body mass index (BMI). World Health Organisation. www.who.int/data/gho/data/themes/topics/topic-details/GHO/body-mass-index
- 2.Body mass index (BMI) and waist measurement. Australian Government Department of Health. www.health.gov.au/topics/overweight-and-obesity/bmi-and-waist
- 3.Overweight and obesity. Australian Institute of Health and Welfare. www.aihw.gov.au/reports/overweight-obesity/overweight-and-obesity/contents/about
- 4.Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 2004; 363(9403): 157–163. www.sciencedirect.com/science/article/abs/pii/S0140673603152683
- 5.What’s your body mass index (BMI)?. National Heart Foundation of Australia. www.heartfoundation.org.au/bmi-calculator
- 6.BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr, 2014; 99(4): 875–890. pubmed.ncbi.nlm.nih.gov/24452240/
- 7.About Adult BMI. Centers for Disease Control and Prevention. www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
- 8.Adolphe Quetelet (1796–1874) — the average man and indices of obesity. Nephrol Dial Transplant, 2008; 23(1): 47–51. pubmed.ncbi.nlm.nih.gov/17890752/
- 9.Child and Teen BMI Categories. Centers for Disease Control and Prevention. www.cdc.gov/bmi/child-teen-calculator/bmi-categories.html
Frequently Asked Questions
Common questions about BMI, how it's calculated, and what it means for your health in Australia.
What is a healthy BMI for adults in Australia?
For most Australian adults, a BMI between 18.5 and 24.9 is considered the healthy weight range. However, context matters. For older Australians over 74 years, some researchers and the Better Health Channel suggest a slightly broader range of 22 to 26 may be acceptable, as general health and functional capacity can be more important than strict BMI targets. For people of Asian, South Asian, or Middle Eastern backgrounds, health risks may increase at lower thresholds, with overweight starting from a BMI of 23 and obesity from 27.5.
How do I calculate my BMI manually?
BMI is calculated by dividing your weight in kilograms by your height in metres squared. The formula is: BMI = weight (kg) / [height (m)]². For example, if you weigh 70 kg and are 170 cm tall (1.70 m), your BMI is 70 / (1.70 x 1.70) = 24.2. If you use imperial units, the formula is: BMI = [weight (lbs) / height (in)²] x 703. For example, 154 lbs at 5 foot 7 inches (67 in): 154 / (67 x 67) x 703 = 24.1.
Is BMI accurate for muscular people?
BMI can overestimate body fat in people with high muscle mass. Because muscle is denser than fat, athletes and regularly active people may register as overweight or even obese despite having low body fat. In these cases, waist circumference and body fat percentage measurements are more informative. If you exercise regularly and have significant muscle mass, discuss your results with a GP or exercise physiologist who can assess your body composition more accurately.
What BMI is considered obese in Australia?
In Australia, a BMI of 30 or above is classified as obese. Obesity is further divided into three classes: Class I (BMI 30.0 to 34.9), Class II (BMI 35.0 to 39.9), and Class III (BMI 40.0 or above). For Australians of Asian, South Asian, or Middle Eastern descent, the World Health Organisation notes that health risks associated with obesity may begin at a lower BMI of 27.5. As BMI increases through these classes, the risk of conditions like type 2 diabetes, cardiovascular disease, and certain cancers rises significantly.
Does BMI apply to children and teenagers?
No. The standard adult BMI categories do not apply to anyone under 18. Children and teenagers grow at different rates, so clinicians use age-and-sex-specific growth percentile charts rather than fixed BMI cut-offs. A child's BMI is compared against national reference data for their age group to determine whether their weight falls in a healthy range. If you have concerns about a young person's weight, speak with your GP who can interpret their growth trajectory in clinical context.
What are the health risks of a high or low BMI?
A BMI above the healthy range is associated with increased risk of type 2 diabetes, cardiovascular disease, certain cancers (endometrial, breast, colon), sleep apnoea, osteoarthritis, and high blood pressure. A BMI below 18.5 carries its own risks, including malnutrition, vitamin deficiencies, weakened immune function, reduced bone density (osteoporosis), and reproductive issues. Both ends of the spectrum warrant a conversation with a healthcare professional to understand contributing factors and appropriate next steps.
Is BMI different for men and women?
The BMI formula and category cut-offs are the same for men and women. However, body fat distribution differs between sexes. Women typically carry a higher percentage of body fat than men at the same BMI, while men tend to accumulate more visceral fat around the abdomen. This is why waist circumference measurements (above 94 cm for men and 80 cm for women indicating increased risk) are a valuable complement to BMI when assessing health risk.
Should I use BMI during pregnancy?
BMI is not interpreted the same way during pregnancy. Weight gain is expected and healthy during pregnancy, and the standard BMI categories do not apply. Your antenatal care team will advise on appropriate weight gain targets based on your pre-pregnancy BMI and individual circumstances. If you have questions about weight during pregnancy, speak with your midwife or obstetrician rather than relying on a BMI calculator.