This procedure only works for rectangular-shaped windows. Do this for each window, noting the total yards for each. After you are done with all windows, just add the totals. Voila! ... that's how much yardage of fabric you need.
If your curtain is a Roman shade, click here to calculate your fabric requirements.
Please note: If your window's frame has an arch, do not use this tool. If your fabric has a repeat pattern that you specifically want to be centered, do not use this tool. Email me instead.
Fill in the empty boxes below to see the total yardage you will require.
It's Spring! 
It's Spring!