Determining the width of grommet curtains is the most important preparation you'll make besides choosing the right fabric and curtain rod. If you don't measure the width properly, you're likely to start sewing only to find out when you go to hang the grommet curtains that they are not full enough for the window. To avoid this, install your curtain rod first so you'll take the most accurate measurements. Also, if the grommet curtains will cover more than 1 window (perhaps a pair), measure each window separately, even if they appear the same, because rarely are two separately installed windows exactly equal.
Make a large drawing of the window (or set of windows) you will make grommet curtains for. Don't draw the window(s) to scale, but make the drawing large enough so you can easily note various measurements.
Measure the inside width of each window by measuring first at the bottom, then at the middle, and then at the top, and note the widest measurement on your drawing.
Draw a heavy line across the top of the windows to represent the curtain rod. If you have more than 1 window, add the inside widths together plus 8 to 10 inches for the curtain rod -- it needs to extend 4 to 5 inches beyond the window frame on either side -- and note this on your drawing.
Multiply the total width measurement by 1.5, 2 or 2.5, depending on how full you want the grommet curtains. Write this measurement above the curtain rod measurement on your drawing, and circle it.
Divide the circled measurement by either 48 or 54 (the standard widths of drapery fabric), and round up to the nearest whole number to determine how many fabric widths you need to make your grommet curtains. If you don't know what size your fabric will be, take your measurements to the fabric store and compute this final measurement on site.