Start your measurement on the bottom side of the ottoman where the fabric begins. Stretch the measuring tape from that point over the top of the ottoman to the opposite bottom side of the fabric. Add 1 inch to the total and write down the number.
Measure the other side of the ottoman, from where the bottom of the fabric begins over the top of the ottoman to where it ends on the opposite bottom side. Add 1 inch to this measurement as well and write it down.
Convert the inches to yards by dividing the largest number by 36. The yards will be the length of the fabric you will need. The smaller number in inches is how wide the fabric will need to be to cover the the ottoman.