For most standard width panels, the rule of thumb is to calculate your finished curtain length in inches, add 16, then divide by 36 to get the number of yards of fabric you need.
For 60” curtains the equation would look like this:
60 + 16 = 76 inches
78/36 = 2.16 yards
For large patterned fabric, it's recommended that you add 1 extra yard to account for vertical repetition of the curtain fabric.