When calculating fabric quantity for curtains a general rule of thumb is to allow 1 drop per 60cm width of the window.

Therefore a window of 2400mm would need 4 drops off fabric.

This works on widths of fabric around 140cm wide which is the most current width.

Also a wider width fabric would need less drops.

Once drops are calculated, work out cut lengths .

On a plain fabric would be the finished height required plus 35cm for heading and hems.

That is a finished height of 2400mm would need 2750mm as the cut length

So a window of 2400mm wide by 2400cm high would need 4 drops of 2750mm which works out 11 meters in total.