You could be much younger than you think!

We all know how many years we’ve clocked up on this planet. But what about where it really counts: on the inside?

A new online fitness calendar has been developed that helps you figure out your personal fitness age. No running on tread mills is required; all you need is a tape measure and the guts to use it!

ABC Health and Wellbeing reports that online calculator was developed by Professor Ulrik Wisloff and a team from the KG Jebsen Centre for Exercise at the Norwegian University of Science and Technology.

Professor  Wisloff uses the term ‘fitness age’ rather than biological age because he says your fitness level says a lot about the health of your body. Your fitness age has actually been shown in studies to be a meaningful guide to how many years of life you’ve got left.

Using the answers to some simple questions like your age, sex, height, waist size and exercise regime, the online tool compares your individual fitness level with the average fitness level of others the same age to give you a measure of your personal fitness age.

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If your fitness is below average for your age, then your fitness age is older than your actual age. If you are fitter than the norm, you have turned back the clock.

Three million people have already used the calculator.

However, Professor Wisloff stresses it is not just an algorithm that is for fun.

“It’s based on 60,000 people’s lives over 27 years,” he said. “It’s for real and it’s so easy to use.

“Every GP or health professional all over the world can use it as a first-line risk assessment without taking a blood sample or measuring blood pressure. You can do it at home.”

Okay, so grab that tape measure and get ready to find out the truth about your own fitness age: you’ll find the fitness calculator here.

How did you go? Good or not so good? Will this inspire you to make any changes?