How to Measure for Curtains
People always ask, "How do I measure for curtains?"
Measuring for curtains is easy, just follow these simple steps
1. All sizes are stated WIDTH (left to right) by LENGTH (top to bottom)
Curtain or valance sizes:
The width of rod pocket curtains is stated as the flat out width. You would never use the curtain flat out so keep in mind that a 60" wide panel will not look good if you try to cover a 60" window with it. You need to provide fullness by using 
multiple panels for a window that size. Certain items are used "flat out" or close to it and we will state that in the product description.
Pinch pleated drapes on the other hand have the fullness sewn in. So a pair of 48" wide drapes can cover 48". You need to factor in the overlaps which we usually consider as 12” (center area where the left and right panel should overlap each other when closed) and returns (the "wrap around" part which returns from the face of the rod to the wall) so a 48” pinch pleat pair really covers about 36” of area.
Measuring Your Window
WIDTH
On most curtains & window treatments, it takes two to three times more material/curtain to provide the proper fullness in appearance when installed/hanging on rod than the actual window width.
Measure the width of the rod from left to right. A common rule for displaying curtains properly says the curtains finished width should be at least 2 times the width of your window (if not more - sheers can be 3 times the window width) to achieve a look of proper fullness.
Example: If your window measures 36" wide (window width x 2 = 72"), you need curtains that will give a minimum width of 72" or 2 panels for that window. In this case 2 panels will give about 100" to 120" of width which will look nice and full. Always round up to the next full number.
Common Fullness multipliers (multiply your measured width by):
Standard fullness multiply by 2
Deluxe Fullness multiply 2.5
Ultra fullness multiply by 3
LENGTH
Measure the length of the window or area you want to cover from top to bottom. Overall curtain length, anywhere from the sill to the apron or to the floor is a personal preference and need not be exact. By moving your rod up or down a little you can adjust the position of the curtain at your window.
If sill length is desired, just below the bottom of the sill is correct.
Other hanging styles bring the curtain to the floor in order to “puddle”