How do you measure for curtains?

[twinky]

I have measured from the bottom of the curtain rail to the floor - that's 2.77m.Then I measured from one side of the window to the other - not the glass, but the timber frame - and that's 2.27m. But someone said I should add on an extra 30cm or so for them to be a generous fit. So how many metres do you think I need. I haven't got a clue how you make the calculation and don't want to buy material the wrong size! Any help would be very welcome.

If you go to the Channel 4 website to the 'Homes' bit there are instructions on there on how to measure for curtains and they do work, cos I used them (and the making instructions) to make some curtains a couple of weeks ago! Sorry I am too thick to work out how to post a link but it's channel4.com/homes and if you google 'channel 4 homes measure for curtains' it's the first link that comes up.

Thanks for than emsyj. Will have a look.