BBC News: Election 2010 BBC News

Election seat calculator

The seat calculator is a rough way of converting percentage support for political parties into numbers of seats in parliament.

It allows you to get an idea of what the next parliament might look like, and what sort of percentage support a party will need to win a majority.

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How it works

Party seat totals are calculated by applying a uniform national swing. This assumes that for every seat in the country, each party's vote share changes by the same amount.

So this is a crude model - in reality every seat is unique. For past elections, it's also important to remember that only the proportions of each party's seats is being shown.

Because of boundary changes - where seats are created, abolished and merged - the total number of seats in the House of Commons has changed over time.

In the past therefore, parties have required different numbers of seats to win a majority and form a government.

To make comparison easier we have taken the % vote share from these historic elections and shown what would happen if that result occurred with today's 650-seat House of Commons. Northern Ireland appears as grey because its 18 seats are all held by parties grouped within the "other" category.

If you are wondering how it is possible a party can win an election but get fewer votes than other parties read this simple guide to a first-past-the-post election.

More interactive guides

Check out opinion polls going back to the Thatcher years with the interactive poll tracker.

Start swinging with the Jeremy Vine swingometer and see which constituencies could change hands.

Or find out about what a hung parliament could mean.

But now comes the difficult part - making it work
Why has Eton College produced 18 British PMs?
Frantic talks on who will form the next government


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