Measure the cushions from the very back of the seat to the top of the cushion with a measuring tape. Write this measurement down as the height.
Measure from the back of the seat to the front end of the seat. Mark this measurement down as the depth.
Measure for the width of the cushion by measuring from the inside edge of one arm, or side edge if the chair or bench has no arms, to the opposite edge. Write this measurement down as the width.
Add the depth and height measurements together in inches and divide by 36 to find the number of yards your cushions require. Round up the number of yards to the next 1/2 yard to allow for seams and mistakes. Double this final number to also cover the backs of the cushions, as these measurements are only for one side.
Divide the width measurement by the bolt size of the fabric you are using. If the width measurement is 24 inches, a 44-inch bolt is more than enough fabric. If you have a wide cushion that measures more than 44 inches, you need to add another bolt for every extra 44 inches. These extra bolts must be the same yardage as the first bolt. Double the final number of bolts, as you did with the yardage, to cover the backs of the cushions as well.
Leave the width's measurement in inches, as fabric is sold in yards for length, while the width is determined by the bolt size.
Take your measurements with you to the fabric supply store. Once you decide on a fabric, check the bolt size against your width measurement. If the bolt size is 44 inches and your cushion width measurement is 75 inches, you need to double your yardage to cover the width, as the length must be covered along the width as well. If your cushion width is within the bolt size and you have a few extra inches for seams and mistakes, the yardage you require for the length is all you need.