FAQs about body mass index (BMI)

Your body mass index (BMI) gives you an idea of whether you're underweight, overweight or a healthy weight for your height.

Your BMI is calculated using your weight and your height and is best done using a calculator (see BMI calculator). BMI is a useful measurement of growth for most people. However, it is only an estimate and it can't tell how much of your weight is body fat and how much is muscle mass.

What does your BMI result mean?

BMI result What does the result mean?
Less than 18.5 Underweight
This result means that you may be underweight. Being underweight can be associated with a range of health issues. If you're concerned about your weight, talk about your result with your GP, practice nurse or dietitian.
18.5–24.9 Normal weight
This result means that you are a healthy body weight, which is generally good for your health. Keep up the great work!
25–29.9 Overweight
This result means that you may be overweight. Carrying extra weight is associated with a range of health issues, including being at increased risk of heart disease. If you're concerned about your weight, talk about your result with your GP, practice nurse or dietitian
30 or more Obese
This result means that you may be obese. Obesity is associated with a range of health issues, including being at an increased risk of heart disease. If you're concerned about your weight, talk about your result with your GP, practice nurse or dietitian.

Who is BMI measurement not suitable for?

The BMI measurement is not suitable for some people:  

  • women who are more than 10 weeks pregnant
  • children under 2 years of age
  • people, particularly children, with certain health conditions such as some endocrine or genetic conditions.

Talk to your healthcare provider about your weight if you have any questions or concerns.

How accurate is BMI?

Although BMI is not a perfect measure, it's generally agreed to be the most useful and valid measure for most people. However, there are some limitations.

Adults with higher than normal levels of muscle mass
BMI is less accurate as an indicator of being overweight in adults with higher than normal levels of lean body tissue (muscle mass). This is because muscle weighs more than fat and the BMI does not take this into consideration.

Ethnic groups with smaller body stature
BMI is less accurate as an indicator of being overweight for people in ethnic groups with smaller body stature (eg, Asian ethnic groups). Along with BMI, measuring your waist can help you work out whether you are carrying excess weight around your middle and therefore at risk of heart disease and type 2 diabetes.

Is it healthy to be underweight?

Being underweight can increase your risk of, or be a sign of, ill health. Talk to your doctor or practice nurse for further advice.

What does ‘obese’ or ‘overweight’ mean?

'Overweight' and 'obese' are medical terms used to describe a person who has body fat at a level above what is considered healthy. BMI provides an indication of excess weight for height, but can't tell the difference between fat and muscle mass.

Being obese is not good for your health. It can increase your risk of developing high blood pressure and serious health conditions, including type 2 diabetes, arthritis, heart disease and cancer.

Why should I measure my waist size?

Measuring your waist is a good way to check that you don't have too much fat around your stomach (tummy). It is a better estimate of visceral fat, the dangerous internal fat that coats your organs. It's therefore a more accurate predictor of your risk of heart disease, type 2 diabetes and stroke

How do I measure my waist size?

To measure your waist:

  • find the top of your hip bone and the bottom of your ribs (this is usually at the level of your belly button)
  • wrap a tape measure around your waist midway between these points
  • breathe out normally before taking the measurement.

Waist circumference and disease risk

No matter what your BMI is, or how tall you are, you should try to lose weight if your waist size is:
Men 94 cm (37 in)
Women 80 cm (31.5 in)
You're at very high risk of serious health conditions and should talk to your GP or practice nurse if your waist size is:
Men 102 cm (40 in)
Women 88 cm (34 in)

References

  1. Healthy weight BMI calculator Ministry of Health, NZ
  2. What is my BMI? Heart Foundation, NZ
  3. Measuring weight Ministry of Health, NZ
Credits: Health Navigator Editorial Team.