We are constantly being urged to keep healthy, and one of the quickest ways of calculating how healthy you are is your BMI - your body mass index.
It is a crude way of telling whether you are overweight.
But over the years the BMI has come in for some criticism for not telling the whole story.
Now, Massey University researchers have brought in some Space Age technology to determine whether your BNMI really matches how much fat you are carrying around.
Chrissy is a 25-year-old receptionist at a prestige motoring company, and part-time model who also runs who her own promo agency.
"I would say that I am pretty healthy," she says. "I am active, go walking after work every day and I eat my greens and my meat and don't have take outs, and I don't smoke, and I have a glass of wine with friends."
Jade is a 20-year-old accountancy student at university who is also a part-time model.
"I am quite the opposite to Chrissy, she says. "I do smoke, and I do drink two or three nights a week, and I have never done any exercise since I can remember, probably from first form when I was forced."
Chrissy and Jade have come to Massey University's Dr Rozanne Kruger to see if their body mass index accurately reflects their body fat.
First their BMI is calculated by taking height and weight measurements. Jade is shorter than Chrissy, but weighs almost the same. Her BMI is 20, within the healthy range, whilst Chrissy's is 18 – suggesting she is underweight.
The next step is calculating their body fat using Massey's new research tool - the 'bod pod'.
Dr Kruger is using it to conduct a study comparing how a woman's BMI stacks up against her body fat reading, and she has been inundated with volunteers.
"I just think most women are conscious of their health and their body, and we are bombarded everyday with messages and images of what is supposed to be healthy and how a healthy woman should look like," says Dr Kruger. "And everybody wants to know, am I healthy?"
But if you rely on the BMI, is it accurate? In Australia last week critics used the BMI to back claims 19-year-old Miss Universe Australia contestant Stephanie Naumoska was too skinny.
Her bmi was 16, in the malnourished category - she disagreed, claiming she was healthy.
"I've been tall and thin my whole life, and anybody who knows me knows that this is just the way my body is," she says. "I'm tall and I'm thin."
And relying on the BMI can make a mockery of being super fit. All Black Piri Weepu's BMI makes him overweight. Richie McCaw with a BMI of 30 is obese, and Tony Woodcock's BMI makes him almost morbidly obese.
Crazy of course, which is why Rozanne Kruger is doing the research. The bod pod measures the volume of air inside and compares that figure to when it is empty.
Chrissy follows computer prompts to breathe through a tube, which calculates the volume of air in her lungs. A formula somehow puts it all together and gives the result.
Her results suggest her percentage of body fat is 22.6 percent, slightly below the normal range for women – aligning with her BMI result.
"I thought that I would be healthy 'cause I feel that I am healthy 'cause I watch what I eat and things like that," she says. "So to find out I was a little bit on the leaner side as they said, I was okay with that. I think I am a small person."
Next up was Jade. Remember she has a BMI at the lower end of the normal range, but does not look after herself.
Her result was 26.9 percent, within the normal healthy range.
Jade seemed to feel that did not match her BMI.
"It was interesting to find out you know, that I did have a little bit of excess fat on me which is you know, good to know."
Dr Krugers' research will take into account diet and exercise but we won't know if there is a dramatic difference between the BMI readings and the bod pod until the findings are released later this year.
For jade though, the results already mean a change in lifestyle.
"My diet never used to concern me at all," says Jade. "What I ate, anything like that, fast food was an everyday kind of thing. Easy - you pop out to uni and grab a bite to eat. Now, definitely looking towards the healthier option."