Those sites are great what about if you need it in the field.
This is what the book says
Site Lenght x site width x .inches divide by 27
But usually I divide by 30
Heres an example
100 x 100 = 10,000 sqf. x.3=3,000 / 30 = 100 yards of mulch
You can do this up to nine inches after that you use 1 as 10 inches instead of .10 and 1.1 as 11 inches 1.2 as 12" and so on
This a rough estimates assuming a yard would cover 100 sqf. at 3" deep
But works real good for giving quick quote estimates