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One of the reasons why pensions have been described - by Nick Clegg, among others - as "unaffordable" is because people are living longer and placing an extra burden on the pension pot. So what is our average life expectancy now?

Life expectancy figures can be misleading.

One ����ý report, for example, claimed that life expectancy for a man in the UK is 77, meaning that after working to 66, I would have only 11 years left. That seems a bit harsh.

But wait. Another report says that life expectancy for men in this country is 89. Now that's a bit more like it. But they can't both be right. Or can they?

To get a clearer picture, I consulted Professor Paul Sweeting, professor of actuarial science at the University of Kent. He says the 77 figure comes from a set of tables produced by the Office of National Statistics that give "period" life expectancy.

They show an average life expectancy by finding the mean of the probabilities of living from one year to the next. So the probability of living from 0-1, then from 1-2 then from 2-3 and so on. These are added together and then averaged out to find life expectancy.

But Professor Sweeting describes this figure as conservative. The problem is the tables assume that there will be no improvements in the mortality rate.

'Mortality hump'

He pointed me to the "cohort" tables. These work in the same way as the period tables but are adjusted for improvements in the mortality rate over time. That table gives me a more satisfactory 89 years.

Professor Sweeting points out that these figures give life expectancy from birth for someone born today. Taking a look back at the predictions for 1976, the year I was born, I was estimated to live to 83.

That was then. Now at 34, according to Professor Sweeting, I have survived some of the riskier phases of my life and consequently my life expectancy has improved.

"Mortality in the first year of birth is higher than it is for age two, three and 4. For men in particular, at about age 15 mortality rates increase as testosterone kicks in and people start taking more risk and you get what's called the mortality hump," he says.

Although living well into my 80s isn't too bad, what about all those centenarians out there? Can't I expect more? Interestingly, the ����ý pension managers think so. The Pensions Regulator now requires pension schemes to make realistic forecasts of life expectancy. My pension fund managers think members will live to an average of 90.

But of course it is obvious that applying averages to individuals (in this case me) is problematic. Enter Professor James Vaupel, director of the Max Planck Institute for Demographic Research in Germany. He's designed his own system that looks at many variables.

He starts with a base figure. "Life expectancy has been increasing by about 2.5 years per decade, that's three months per year, six hours per day." Using this information he estimates I could live to 89. But then he adds extra information like how much I exercise, drink and how healthily I eat. I score quite well here which, he says, will add another four to five years - so now I'm up to 94.

But he doesn't stop there. He points out this is only the mean average. I actually have a 50/50 chance of getting to the median age for life expectancy - this is higher at about 97 - and I could realistically hope to reach the mode, the age when most people of my age will die, which adds another three years. Now I'm up to 100. I've gained 23 years. Not bad for a few days work.