Mulch is usually sold measured in cubic yards or cubic feet, but it's not too hard to figure out how many square yards that translates to. Remember that a yard is three feet long, so a square yard is 3 feet long by 3 feet wide, for a total of 9 square feet.
Measure the area you want to cover in mulch in feet. Use a measuring tape for irregular areas or curved lines, or a yardstick if straight lines will work for you. Take measurements at the longest and widest parts for irregular areas.
Multiply the length times width to get the total area in square feet. If your measurements include inches, convert them to fractions of feet. For instance, 6 inches is half a foot; or 8 inches is two-thirds of a foot.
Divide the number of square feet by 9 to find out how many square yards of mulch you will need.
Figure out how many bags of mulch this means. Most bags contain 2 or 3 cubic feet of mulch. Take the number of square yards you already figured out and multiply it by the depth in feet you want to spread the mulch. For instance, if you want to spread it 2 inches deep, multiply the square yards by one-sixth of a foot, or 0.167. This gives you the amount in cubic feet.
Divide the number of cubic feet you figured out by the amount in each bag to get how many bags of mulch you should buy. Or, if buying in bulk, convert cubic feet to cubic yards. There are 27 cubic feet in a cubic yard, so divide the number of cubic feet by 27 to get cubic yards.