The debate on proportional representation is not a young one. It emerges every so often, particularly when pundits are feeling a bit pre-millennial in the months before an election is called. A lot of chests get puffed out, a lot of ink is spilled and, duly, nothing gets done: electoral reform is a pressing issue before an election, but it tends to drop a little way down the agenda once a new government is formed. I'm not here to argue the merits of all the different possible kinds of electoral reform, however; John Rentoul did a much better job of that in yesterday's paper. Nor do I intend to speculate on whether or not the system actually will get reformed by any incoming government. I just want to talk about some of the implications of our current system.

We all know that our first-past-the-post system leaves a lot to be desired, but I didn't know quite how much until I saw the Voter Power Index. Designed by the new economics foundation - a left-leaning 'think-and-do tank' - it's a widget that calculates the power of everyone's vote by constituency. And, well, it shocked me. The only people voting on 6 May who will have actual power are those who live in the very marginal constituencies.

This may sound obvious, but the actual implications for our democracy are surprisingly stark: the safer the seat where you live, the less your vote is worth. We all know that under a 'perfect' democratic system, one person has one vote. This Index, however, estimates that the 'average' Briton's vote is actually 'worth' around 0.25 of a vote. They get to this figure by lowering the value of votes from people living in safe seats: the safer the MP in your borough, the less your vote - for them or against them - is actually worth in the big picture. Seeing as fewer than 20% of constituencies can be considered marginal, there are comparatively few voters who have the capacity to affect the election.

This link was given to me by a despairing 18-year-old living in Canterbury. He'll obviously be voting for the first time, but it will be in a borough which has voted blue since 1918, and which has had only one non-Tory MP since 1874. Current incumbent Julian Brazier has a bracing majority of 7,471. According to Voter Power Index, his vote is worth 0.168 of a full vote. In effect, he told me, his vote counts for practically nothing, and he feels there's almost no point in casting it. Canterbury, meanwhile, still comes out in the upper half of constituencies with voter power - votes in most places are worth even less.

The nef makes these calculations using two factors - how marginal a constituency is, and how many registered voters live there. Of course, its maths assumes that the bulk of voters are going to vote the same way as they did last time, and it does not account all that well for factors like popular momentum or big swings, but the point raised is nevertheless a useful one. A Tory living in central Glasgow has as little say in his country's leadership as a Green does living in Henley.

I checked my own constituency, and I was quite dismayed to find that my own vote is worth a less than miraculous 0.168. I've never been willing to take part in a system which doesn't satisfactorally represent me, and I have to say, this calculator is hardly going to speed my heels to the booths.