Is BMI Accurate? What Your BMI Tells You and What It Doesn't

Body mass index (BMI) is one of the most common health measures, and one of the most misunderstood. As a dietitian, here's what your BMI can and can't tell you, and what to look at alongside it.

What is BMI and how is it calculated?

BMI uses your weight and height to estimate whether you're in a healthy weight range. It's your weight in kilograms divided by your height in metres squared.⁵

For adults, the Australian BMI ranges are:⁵

• Under 18.5: underweight
• 18.5 to 24.9: healthy weight
• 25 to 29.9: overweight
• 30 to 34.9: obese class I
• 35 to 39.9: obese class II
• 40 or more: obese class III

Is BMI accurate?

Not on its own. BMI can't tell the difference between fat and muscle, so a muscular person can land in the overweight range with little body fat. It also misses many people who carry too much fat. A meta-analysis of 32 studies found the usual BMI cut-offs failed to identify about half of adults with excess body fat.³

A 2025 Lancet Commission of 58 experts recommended using BMI as a screening tool rather than an individual measure of health. They advise confirming excess body fat with a waist measurement or a direct body fat measurement, except when BMI is over 40.¹

The healthy BMI range is also different for some people. It's generally:⁵

• lower for people of Asian background;
• higher for people of Polynesian background, such as Māori and Pacific Islander peoples;
• higher for older people;
• higher for elite athletes with more muscle;
• higher in pregnancy.

What is a healthy waist measurement?

Where you carry fat matters. Fat around your middle can coat organs such as the heart, liver, kidneys and pancreas, raising the risk of type 2 diabetes, heart disease and some cancers.⁵ An international expert consensus concluded that waist size adds information to BMI for predicting health risk and should be measured routinely.²

In Australia, the waist sizes linked with higher risk are:⁵

• Men: 94 cm or more (increased risk); 102 cm or more (greatly increased risk)
• Women: 80 cm or more (increased risk); 88 cm or more (greatly increased risk)

These don't apply to children or during pregnancy.

How to measure your waist

Take off bulky clothing, loosen your belt and stand with your feet shoulder-width apart. Wrap a tape measure around your belly in line with your belly button, loose enough to fit one finger underneath.⁵

Are there better ways to measure body fat?

Scans such as DEXA, CT and MRI separate fat from muscle and give a much clearer picture of body composition. But they're expensive and not always available, which is why BMI and waist measurement remain the quick, low-cost tools used in most clinics.

Should you use BMI for children and teenagers?

Adult BMI ranges aren't suitable for children.⁵ Children are still growing, body fat changes with age and differs between boys and girls, and teenagers have growth spurts at different times. Health professionals use age- and sex-specific growth charts instead. Even then, BMI misses over a quarter of children and teenagers who carry excess body fat.⁴

Comparing BMI with friends and classmates can also harm a young person's wellbeing. If you're worried about your child's weight, talk to your practitioner.

Frequently asked questions

Is BMI accurate?

Not on its own. It can't tell fat from muscle and misses about half of adults with excess body fat, so it's best used alongside a waist measurement.

What is a healthy BMI?

For most adults, 18.5 to 24.9. The range is generally lower for people of Asian background and higher for people of Polynesian background, older people, elite athletes and pregnant women.

What is a healthy waist measurement?

Risk increases from 94 cm for men and 80 cm for women, and rises further from 102 cm for men and 88 cm for women.

Is waist measurement better than BMI?

They work best together. Waist size shows where you carry fat and adds information about health risk that BMI misses.

Can I use BMI for my child?

Not with adult ranges. Children need age- and sex-specific charts, so talk to your practitioner if you're concerned.

References

Research (PubMed)

1. Rubino F, Cummings DE, Eckel RH, et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol. 2025;13(3):221–262. PubMed 39824205
2. Ross R, Neeland IJ, Yamashita S, et al. Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity. Nat Rev Endocrinol. 2020;16(3):177–189. PubMed 32020062
3. Okorodudu DO, Jumean MF, Montori VM, et al. Diagnostic performance of body mass index to identify obesity as defined by body adiposity: a systematic review and meta-analysis. Int J Obes (Lond). 2010;34(5):791–799. PubMed 20125098
4. Javed A, Jumean M, Murad MH, et al. Diagnostic performance of body mass index to identify obesity as defined by body adiposity in children and adolescents: a systematic review and meta-analysis. Pediatr Obes. 2015;10(3):234–244. PubMed 24961794

Australian guidelines

5. Australian Government Department of Health, Disability and Ageing. Body mass index (BMI) and waist measurement. View source

This article is general information and isn't a substitute for personalised advice. Speak to your practitioner about your situation.