BBC News Seat Calculator 2010

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.

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.

Media coverage

Your heart might say Clegg. But vote with your head - Polly Toynbee. Sat 24 Apr 2010
"Every time you see a poll, go to the BBC's brilliant election seat calculator for a nasty shock. Work out any variety of options. Labour may yet do far worse – but if so, Cameron wins, not Clegg."
This grotesque and unfair voting system must change - Will Hutton. Sun 25 Apr 2010
"The charts here, drawn from information on the BBC's seat calculator, illustrate just how unfair the results could be. If the votes for the three main parties stood at 30% each on May 6 (with "others" such as the Scottish and Welsh nationalists at 10%), Labour would return to Westminster with 315 MPs, 11 short of an overall majority, while the Conservatives have 206 and the Liberal Democrats a mere 100."

Notes

I made the first prototype of this in 2006 when Actions Script 3 was first released bringing much better performance to Flash and generally making more things possible (I still really miss AS3 and kind of wish the ECMAScript 4 standard which it implements had made it to Javascript, we could have had types as a proper language feature rather than the wierd situation with typescript which we have now, also a nicer OO model IMO)

In the first prototype I used a ternary plot as the control, this was quite cool but not particularly intuitive. (annoyingly I don't have any better quality screenshots of the prototypes)

A pie chart was much easier to understand (I made this second prototype whist on paternity leave)

I built most of this in my spare time as I couldn't get sign off to work on it from the editiorial team until the Swingometer was done. In the end both things shared a good chunk of code.

I put in a cheat code so you coud see a geographical map version. I can activate this by typing 'map' whislt the app is in focus this seems a bit flaky though, perhaps an issue with with Ruffle, the library which emulates Flash player.