How do I measure for curtains?

[toiletpaper]

I'm buying curtains and poles for the first time in my life (have blinds currently) and really have no idea. My downstairs window is 140cm ish wide when I add 15cm or so at the side so am I right in thinking the standard size of 167cm width will be ok? The drop is 135cm so I'll be getting the 137cm drop to sit just below the windowsill and above the radiator.

My patio doors are 160cm wide with the extra added at the sides so will 167cm width be ok for these too or should it be a lot more than the actual width? The drop to just above the floor is 215cm or so. The closest drop to buy is 228cm so I'm hoping that should be ok as I can just make sure the pole is high enough so that the curtain isn't touching the floor.

Thanks for any advice, I'm not much of a DIY/house improver

Depends on how much gather you like.
I buy 1.5 or 2x width and down to the floor exact.

@TheQueef when you say gather do you mean when they're 'bunched up' (can't think of a better word sorry) when the curtains are pulled shut? So if I bought 167cm width for a 145cm window, when they're shut they would be quite straight with little gather?

Yep bunched up.
I don't have any rugs or carpets downstairs so always go for a bunched up big pair so I have pleats when they are closed.
So it feels opulent
It depends how flat they will be when closed and the style.

IIRC the John Lewis website has a section on how to measure up correctly.

It would be normal to buy 2 or 2.5 times the width you wish to cover. Less will look a bit sparse. Remember that isn't only the window but also 15cm of wall either side. Also remember to allow height from the top of the window up to the curtain pole or rail. Do you want the curtain to sit above the pole or hang below it?

JLP have a good guide. Also re: linings, curtain hooks, tie backs etc.

I worked for years for a company who sold fabric and did made to measure curtains.

Measure the width of your track or pole, or the size of the aperture plus 30cm. Divide that by 120. That will give you the number of widths if fabric for each curtain, for pencil please curtains, allowing for gather.

Measure from the top of your pole or track to the point you want the curtain to reach.

Normally the sizes on the packet are for each curtain, so if your window is 160cm wide then 167cm curtains will be perfect.

I got John Lewis over to measure up and fit runners (poles didn't work for various reasons) for two giant windows. Absolutely worth it. Circa £350 all in. Then bought cheap curtains from IKEA and Dunelm.

Thank you everyone! Makes a lot more sense now. @Reachersloveinterest I didn't realise that the width in the pack was per curtain, I thought it was between the two. That's really helped me out. Thanks again

Let me give you a piece of advice, buy bigger than you need/ bigger than you think.
I bought too small and had to keep buying bigger.