To determine the volume of mulch you will need for a specific area use the following formula:

Volume = Length x Width x Depth (depth would be in a percentage) / 27

Inches as a decimal are:

1" =.083     2" =.167     3" =.25     4" =.33     5" =.417     6" =.5     7" =.583     8" =.67       9" =.75     10" =.833     11" =.917     12" =1