I have a customer who is looking to put down Bank Run Gravel (varies in size from quarter size to tennis ball) in his beds instead of mulch. I have not done alot with the gravel to be able to measure exactly how much to purchase for the job. The area would normally require about 4 yards of mulch. Can anyone give me a formula to figure it up? or some suggestions on how much you would think I need? Thanks in advance for the help!