BMI Calculator: What is my Body Mass Index score?

What is the Body Mass Index (BMI)?

The Body Mass Index (BMI) is a scale used by health professionals that can help to assess if a person’s weight is within a healthy range and track weight gain. First theorised in the 1800s by statistician and sociologist Adolphe Quetelet, it has been in wide use since the 1970s, when scientists began to notice a correlation between a population’s BMI range score and their risk of developing certain health issues, such as cardiovascular disease, Type 2 diabetes, high blood pressure and some types of cancers. The BMI formula is the same for everyone, regardless of gender, age or body type.

How do I work out my BMI score?

Your BMI score is used to plot where you sit on the Body Mass Index, which categorises scores into ranges: underweight, normal weight, overweight and obese. The score is worked out via an equation.

First, square your height in metres (height multiplied by height). Then take your weight in kilograms and divide it by that answer (your height squared). So, if a person weighs 45kg and is 150cm tall, the equation would be: 1.5m x 1.5m = 2.25. 45 (kg) divided by 2.25 (height squared) = 20. This means a person with a height of 150cm and a weight of 45kg would have a BMI score of 20. According to the BMI, that person would be in the “healthy weight range”. Medical experts advise that this equation only applies to adults and should not be used as a guide for children.

What does my BMI score mean?

It’s important to remember that researchers have found BMI to be a good indicator for the health and lifespan of adults – not necessarily for an individual, but for a group of people who have the same BMI. This means that while it can be a useful indicator when thinking about a large group of people, it may not necessarily line up with how healthy you are personally. The calculation used to find BMI is the same for men and for women, for example.

The different categories for BMI are:

• Less than 18.5 – Underweight
• 18.5 – 24.9 – Healthy weight range
• 25-29.9 – Overweight
• 30 – 34.9 – Obese class one
• 35 – 39.9 – Obese class two
• 40+ – Obese class three

Does the BMI calculator work for everyone?

BMI scores can be a useful indicator of a person’s risk of developing chronic lifestyle diseases and other health problems. However, the index has its limitations and should be used in conjunction with other health data, such as waist measurements, activity levels, diet and general fitness. For example, if someone has a very high muscle mass, they’ll weigh more than someone less muscular of the same height and stature.

Should everyone have the same BMI?

The healthy weight range also changes with age. For those over 65 years old, the healthy weight range is considered to be a BMI score of between 22 and 30.

Differences in BMI between individual adults of the same age and sex are usually due to body fat. However there are many exceptions to this rule, which is why a BMI figure may not always be accurate. Generally speaking, the more body fat you’re carrying, the higher your health risk. However, BMI cannot differentiate between body fat and muscle mass.

BMI calculations will overestimate the amount of body fat for body builders, high-performance athletes and pregnant women, while underestimating it for the elderly and those with disabilities. Height also impacts accuracy, as it can overestimate obesity for shorter people (less than 150cm) and overestimate it for tall people (more than 190cm).

Why is my waist circumference important?

It can be useful to combine BMI with waist measurement to work out if you may need to address weight-related health. This is because it has been shown that carrying extra kilos around the middle of the body can be an indication of internal fat deposits around vital organs, such as the heart and liver. These types of deposits have been shown to increase risks of heart disease and stroke, among other illnesses.

Your health may be at risk if your waist circumference is greater than:

• 94cm, if you’re male; or
• 80cm, if you’re female

To measure your waist, find the top of your hip bone and the bottom of your ribs, breathe out normally, and wrap a tape measure between those two points. When it comes to identifying risk of developing chronic disease, waist measurement could often be a more telling indicator than BMI.

How do I get to a healthy weight?

If you decide that you’d like to aim to get your BMI into the “healthy” range, there’s lots of information and help available. A good place to start researching could be the He Puna Waiora Healthify website, which includes practical information about healthy eating, exercise and provides access to community-based health programs. However, before starting on any health improvement program, it’s advised to get the all clear from a health professional, such as a GP or dietician.

Is weight loss medication funded in NZ?

It’s important to note that while your doctor can prescribe weight loss medications, they are not covered by funding, so you will need to pay for them yourself. The cost can range from $100 to $500 per month, depending on the medication prescribed.

Is weight loss surgery funded in NZ?

Bariatric surgery, or weight-loss surgery, may be an option for people who are severely overweight and haven’t been able to lose weight through other methods. It’s especially important for those whose weight is affecting other health conditions, like diabetes, sleep apnoea or heart problems.

In some cases, people aged 18 to 60 may be eligible for publicly funded weight-loss surgery, which means the government covers the cost. To qualify, you must meet certain conditions, such as having a BMI over 40, being overweight for more than five years and having tried to lose weight unsuccessfully for at least two years. You also need to commit to long-term follow-up care and diet changes.

Certain health issues, like smoking, severe heart or lung disease, and active cancer, can disqualify you from publicly funded surgery. Because there are limited spots, there’s a selection process your doctor will guide you through. Some patients might need to travel for surgery, depending on where they live. If you don’t want to wait or can afford it, private surgery is also an option.

About the author of this page

This report was written by Canstar Content Producer, Caitlin Bingham. Caitlin is an experienced writer whose passion for creativity led her to study communication and journalism. She began her career freelancing as a content writer, before joining the Canstar team.