I’m making curtains for our living room bay window. I wanted something on a large scale - not just curtains on the windows themselves, but around the full bay. I decided on a “theater” style, for lack of a better description:

Very basic outline above. The general plan is to have 2 long panels on either side, and 2 short panels in the center. I may need more short panels, if I need more fullness.
So - where do you start making basic curtains? Well, I measured and determined that the curtains, at finished length, need to be 91″ long from the curtain rod to the floor. This does not include the extra fabric above - the “header”, or full gathering at the top.

I did some research and determined that, if I use a 3/4″ curtain rod (a pretty standard size), I will need to make a casing of 1″ - 2″, depending on the amount of ease I want. I went with 2″, because I don’t have the curtain rod yet, and I want to be safe.
I also went with a 2″ header. If it is much higher, it can look messy. I wanted a little header, but nothing crazy, and I don’t want it to flop over.
Hems are often double folded and then stitched down, so I decided on a standard 3″ hem, and doubled that to add 6″ to the bottom.
After adding all that together … the length of the fabric for a long panel should be 103 1/2″ (the extra 1/2″ is for stitching the top down), but I rounded up to 105″, for 8.75 feet even.
I followed the same plan for the shorter panels, adding extra measurements for the header, casing, and hem (all the same as listed above), but obviously the length is shorter because it will just frame the center top of the window opening.
Last night, I cut all pieces, and I even managed to sew one of the shorter panels, minus the hem. I’m holding off on the hems until I get all the panels sewn. Then I’m going to hang them on the rod and double check my measurements. I’d hate to do all this work and end up with curtains that are just this-much too short.
Correction to my measurements …. I bought a 1 1/4″ diameter curtain rod, and … oops, this casing is too small. I opened it up by 1/2″ and it works now. So the minimum casing you’ll want for a 1 1/4″ diameter curtain rod is 2 1/2″, and maybe as much as 3″ …