I want to cover a 12 foot by 25 foot area with 3 inches of mulch. How many yards should I get?
The first thing to decide is how deep you want your mulch, which you have determined to be 3 inches (or 1/4 foot). Then use this formula to calculate the cubic yards: (length x width x depth)/27. For your 12 x 25 foot area: 12 x 25 x .25 = 75. 75/27 = 2.77. So, you need 3 cubic yards. Of course, this formula will work for any material, such as topsoil, compost, mulch, etc.
Need to figure out how much mulch you need for your project? Try out the handy Michigan Gardener soil and much calculator.