It seems like such a simple task to make a set of curtains, and it can be. However, it is important to take several things into consideration as you begin this task.
The first step is to take measurements of the window. In taking your measurement of the window, generally you will want to measure the full length of the curtain rod you plan to use to determine your width. Your measurement for length will depend on the length of the style you have chosen. Kitchen curtains generally reach only to the bottom window sash. In other room settings, however, your length is totally up to you to determine: sash length, a few inches below the sash or full floor length. In the length measurement, be sure to measure from the mounting height of the curtain rod to where you want the curtain hem to hang.
Now that you have your measurements, the next determining factor will be the overall look you desire to achieve. Regardless of the length you choose for your curtains, you will have to plan on adding several inches to this measurement to allow for the hem at the bottom and the curtain rod casing at the top.
For the bottom of the curtains plan on a wide hem. This will help the curtains to hang straight. Adding four to five inches to your length measurement should provide plenty of hemming material. The amount of additional fabric needed for the rod casing will depend on the type of rod you are using. A traditional rod would require at least six additional inches added to the length. If you are using a round rod, you may want to consider attaching rings or fabric loops to thread over the rod rather than the traditional pocket casing.
Now that you have determined the length of the materials needed, you will need to determine the width. Since curtains gather along the rod to create their folds and fullness, simply using the width of the window measurements will not be sufficient. Depending on how full you want your gathers and how heavy of material you plan to use will also have an impact. A general rule of thumb, however, is to double the width of your window measurement and then add another six to eight inches to each panel (most windows have two curtain panels that meet in the center of the window) to allow for the hems on each side.
Now that you have determine your full length, including hem and casing, plus the width of each panel, including hems, you are ready to calculate the the yards of fabric needed for your project. Since fabric can be purchased in several different widths, you will need to determine the width of the fabric you plan to purchase. Take the width measurement for ONE curtain panel and divide this measurement by the width of your fabric and then round up to the next whole number. (Example: Panel width is 48″ and your fabric width is 36″. Since the width needed for the panel is greater than one width of fabric but less than two widths, you will need 2 widths of 36″ fabric for each panel.)
Now that you have determined the number of widths for each panel, multiply your number of widths (per the example, four total, for the set of curtains) times the length in inches you had previously determined. Divide this total number of inches by 36 to determine the full amount of fabric yardage you will need to complete your curtain project.