Australia
BMI calculator
Body Mass Index (BMI) gives a quick read on whether your weight sits in a healthy range for your height. Type in your height and weight and you get a single number plus the category it falls into: underweight, healthy, overweight or obese. The formula is identical worldwide, so the result does not depend on which country you choose. People use it as a first check before a GP visit, to track a trend over months, or to set a rough weight target. It is a screen, not a diagnosis.
How it works
- Take your weight in kilograms and divide it by your height in metres squared. So 70 kg at 1.75 m is 70 divided by (1.75 x 1.75).
- The result lands in one of four adult bands: below 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obese.
- Working in pounds and feet or inches is fine; the units are converted to metric before the division so the number matches.
- The same band thresholds apply to adult men and women, which is why the calculator does not ask for your sex.
BMI = weight (kg) / height (m)²
Square your height in metres, then divide your weight in kilograms by that figure. Height is squared because a taller frame carries more mass at the same build, and squaring keeps the index roughly steady across heights. The answer is read against four fixed adult bands. Pounds and inches are converted to kilograms and metres first, so the band the number lands in is the same either way.
- weight
- body mass in kilograms (lb is divided by 2.205 first)
- height
- standing height in metres (inches are multiplied by 0.0254)
- m²
- height multiplied by itself, the denominator of the index
Adult BMI bands used here
| Underweight | below 18.5 | may point to under-nutrition or another cause |
| Healthy weight | 18.5 to 24.9 | the range most adults are screened against |
| Overweight | 25 to 29.9 | raised risk for some conditions |
| Obese | 30 and above | often split further into class 1, 2 and 3 |
Worked example
Someone 1.75 m tall weighing 70 kg has a BMI of 22.9 (70 divided by 3.0625). That falls inside the healthy band of 18.5 to 24.9. Add 15 kg at the same height and the figure climbs to 27.8, which crosses into the overweight range.
Key facts
- The maths is metric at heart: weight in kilograms over height in metres squared, so 70 kg at 1.75 m gives 22.9.
- The four adult bands are fixed numbers, not percentiles, which is why two people of very different builds can share a reading.
- A muscular build raises weight without raising fat, so athletes often read higher than their body fat would suggest.
- Some health bodies lower the overweight cut-off to 23 for people of South Asian, Chinese and other backgrounds, where risk rises earlier.
Tips
- Weigh yourself at the same time of day, ideally in the morning before eating, so month-to-month numbers compare fairly.
- Pair the reading with a waist measurement; a waist above half your height flags risk that BMI alone can miss.
- Track the direction of travel over several months rather than reacting to one figure that may reflect a heavy meal or water.
- If your result sits near a band edge, recheck your height, as a 2 cm error shifts the number more than most people expect.
BMI at 1.75 m for a range of weights
| Weight (kg) | BMI | Band |
|---|---|---|
| 50 | 16.3 | Underweight |
| 60 | 19.6 | Healthy weight |
| 70 | 22.9 | Healthy weight |
| 80 | 26.1 | Overweight |
| 95 | 31 | Obese |
Frequently asked questions
What counts as a healthy BMI?+
For most adults, 18.5 to 24.9 is treated as healthy. The cut-offs can shift for older adults, very muscular people and some ethnic groups, where a lower or adjusted threshold is sometimes used.
Why might BMI be wrong for me?+
It cannot tell muscle from fat. A lean athlete with heavy muscle can read as overweight, while someone with little muscle can read as healthy despite high body fat. Treat an unexpected result with caution.
Can I use this for my child?+
No. Under-18s are assessed with age and sex specific percentile charts, not the fixed adult bands here. Ask a GP or paediatric service for a child reading.
Should I worry about a single high reading?+
One number is a snapshot. A trend over several months matters more, and waist measurement and blood markers add context. A doctor can interpret the full picture.
Things to watch
- BMI is general guidance, not medical advice, and a single reading cannot diagnose any condition. A GP can interpret it alongside your wider health.
- It says nothing about where fat sits, and visceral fat around the organs carries risk that the same BMI on the hips does not.
- The adult bands do not apply during pregnancy or to under-18s, who are assessed on age and sex specific charts instead.
Sources
- A healthy lifestyle: BMI recommendations · World Health Organization
Last updated: 2026-01-01
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with the official sources listed before making decisions.
- BMI is general guidance, not medical advice. Speak to a healthcare professional about your own health.
- It does not measure body fat, fat distribution or fitness, and is not validated for pregnancy or under-18s.
- Results are rounded to one decimal place.
Reviewed by Vikas Dulgunde.