BMI Calculator
Body mass index (BMI) compares your weight to your height. It's usually the first number an Australian doctor looks at when you raise your weight as a health concern, and it's the measure the NHMRC, healthdirect and Medicare-funded programs all use as a starting point. Enter your height and weight to see your result and where it sits. The calculator is free and there's no sign-up.
Not sure your result means what you think it means? A weight assessment with one of our doctors looks at BMI alongside your waist measurement, body composition, bloods and medical history.
What your BMI result means
These are the adult BMI categories used across Australian health services. They apply to people aged 18 and over who aren't pregnant or breastfeeding.
| BMI (kg/m²) | Category | What it generally indicates |
|---|---|---|
| Under 18.5 | Underweight | Possible risk of nutritional deficiency, reduced bone density and low energy availability. Worth a GP review. |
| 18.5 – 24.9 | Healthy weight range | Lowest average risk of weight-related conditions across the population, though waist measurement still matters. |
| 25.0 – 29.9 | Overweight | Increased risk of type 2 diabetes, high blood pressure, sleep apnoea and cardiovascular disease. |
| 30.0 – 34.9 | Obesity, class I | Substantially increased risk. Medical treatment options, including medication, are generally available from this point. |
| 35.0 – 39.9 | Obesity, class II | High risk. Bariatric surgery may be considered where a weight-related condition is also present. |
| 40.0 and over | Obesity, class III | Very high risk, with a strong case for structured medical management. |
Australian public health resources often group everything from 30 upwards as a single "obesity" category. Clinically it's split into three classes, because treatment options and eligibility thresholds change as the number rises.
Why BMI alone doesn't tell the whole story
BMI is a screening tool, not a diagnosis. It was designed to describe populations, and it can misjudge individuals in fairly predictable ways.
- It can't tell muscle from fat. A rower, a bricklayer or anyone carrying a lot of lean mass can land in the overweight or obese range with a perfectly healthy level of body fat.
- It ignores where fat sits. Fat around the abdomen and organs carries more cardiovascular and diabetes risk than fat elsewhere, and BMI can't see the difference.
- Risk starts earlier for some backgrounds. The NHMRC and WHO recommend lower thresholds for people of South and East Asian descent, with increased risk from a BMI of 23 and higher risk from 27.5.
- It doesn't apply to everyone. Children and adolescents are assessed with age- and sex-specific percentiles. BMI isn't used during pregnancy or breastfeeding, and body composition changes with age shift what the number means for older adults.
This isn't a fringe view. In January 2025 a Lancet Diabetes & Endocrinology Commission, endorsed by more than 75 medical organisations, recommended that obesity be confirmed with a second measure of body fat rather than BMI alone, and drew a line between preclinical obesity (excess body fat, organs still working normally, raised future risk) and clinical obesity (excess body fat that is already causing organ dysfunction or limiting daily activity). Two people with the same BMI can sit on opposite sides of that line, which is why a proper assessment looks well beyond the calculator.
Measure your waist as well
Waist circumference is the single most useful thing to add to your BMI, and you only need a tape measure. Measure directly against your skin, halfway between the bottom of your ribs and the top of your hip bones, breathing out normally without pulling the tape tight.
| Risk level | Men | Women |
|---|---|---|
| Increased risk | 94 cm or more | 80 cm or more |
| Substantially increased risk | 102 cm or more | 88 cm or more |
A healthy BMI paired with a waist above these thresholds is worth acting on. In the ABS National Health Survey, more than two in three Australian adults had a waist measurement in an increased-risk range.
When your BMI is worth talking to a doctor about
Book a weight assessment if any of these apply to you.
- Your BMI is 30 or above.
- Your BMI is 27 or above and you have a weight-related condition such as type 2 diabetes, high blood pressure, sleep apnoea, fatty liver or PCOS.
- Your BMI is in the healthy range but your waist measurement isn't.
- Your BMI is under 18.5, or has dropped without you trying.
- You've lost weight before and regained it, which is a physiological response rather than a failure of willpower.
- You've had bariatric surgery and weight is creeping back.
BMI also feeds directly into what treatment you're eligible for. Weight loss medications are generally considered from a BMI of 30, or from 27 alongside a weight-related condition. Bariatric surgery is generally considered from a BMI of 40, or from 35 with a weight-related condition. Your doctor will weigh up benefits, side effects and your history before recommending anything.
What we do with your numbers at Alevia
Alevia is a medical weight loss clinic staffed by GPs who have chosen to specialise in obesity management, supported by dietitians and psychologists. Your BMI is where the first consultation starts, not where it ends. An initial weight assessment covers what's driving your weight gain, screening for blood pressure, cholesterol, diabetes, thyroid and sleep apnoea, and a set of goals built around your health rather than a target on a chart.
We also run in-house body composition analysis, which separates fat mass from muscle mass and estimates your basal metabolic rate. It answers the questions BMI can't: how much of your weight is fat, where you're carrying it, and whether you're holding onto muscle as you lose weight. From there your plan might involve nutrition support, behaviour and lifestyle work, psychological support, medication, or preparation for surgery.
We see patients in East Melbourne, Moorabbin, Wantirna, Geelong, Mornington, Adelaide and Hobart, and Australia-wide by telehealth.
Ready to look past the number?
A first consultation with an Alevia doctor gives you a full medical picture and a plan built for your situation.
BMI calculator FAQs
What is a healthy BMI for an adult in Australia?
A BMI between 18.5 and 24.9 is classified as the healthy weight range for Australian adults. It's a population-level guide rather than a personal ideal, and it should be read alongside your waist measurement, blood pressure, blood glucose and cholesterol.
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. Someone who is 1.70 m and 78 kg works out at 78 ÷ (1.70 × 1.70) = 27.0. The calculator above does this for you and shows which category the result falls into.
Is BMI accurate?
It's accurate as a screening measure across large groups and much less reliable for any single person. It can't distinguish muscle from fat or tell where fat is stored, so it overestimates risk in very muscular people and can underestimate it in people who carry weight around the abdomen. Treat it as one input among several.
Is BMI different for men and women?
The formula and the adult categories are the same for both. Body composition differs, so a man and a woman with an identical BMI usually have different amounts of body fat. Waist circumference thresholds do differ by sex.
Does BMI change with age?
The categories don't change for adults, but their meaning shifts. Muscle mass tends to fall and fat distribution changes with age, so the same BMI can represent quite different body composition at 30 and at 70. Some evidence suggests slightly higher BMI is not harmful in older adults, which is a conversation to have with your doctor.
Can I use this calculator for my child?
No. Children and adolescents are assessed using age- and sex-specific BMI percentile charts, not adult categories. Speak to your GP or paediatrician.
What BMI do you need for weight loss medication in Australia?
Weight loss medications are generally considered from a BMI of 30, or from a BMI of 27 where a weight-related condition such as high blood pressure, sleep apnoea or type 2 diabetes is also present. Eligibility, benefits and side effects need to be assessed individually by a doctor or pharmacist.
What BMI is needed for bariatric surgery?
Bariatric surgery is generally indicated at a BMI above 40, or above 35 with a weight-related condition such as type 2 diabetes, hypertension or obstructive sleep apnoea. Alevia provides independent information on surgical options plus pre- and post-operative care.
Do I have to reach a "healthy" BMI to see health benefits?
No. Meaningful improvements in blood pressure, blood glucose, cholesterol, sleep apnoea and joint pain typically appear with a 5 to 10 per cent reduction in body weight, well before BMI shifts category. Health markers are a better measure of progress than the number on the chart.