Working out how much cord you need to replace the cord on your curtain track is easy.

Double the length of the track and double the length of the cord drops.

So

If you have a 3m track corded from one side only with a 1.5m drop then

(2x3m) + (2x1.5m) = 9m and give yourself a metre or so spare just in case so buy 10m

If you have a 3m track corded from both sides with a 1.5m drop then

(2x3m) + (2x1.5m) + (2x1.5m) = 12m and give yourself a metre or so spare just in case so buy 13m

and so on.