Basically it is just a little bit of math.
Mulch is often sold by the yard (cubic yard) so this is a volume measurement (3ft x 3ft x 3ft), (which is 27 cubic feet).
If you need to know how much mulch to get, you need two things
1) the square footage you plan to cover with the mulch
2) know how deep you are going to install it (usually 3 or 4 inches deep)
then figure out the volume you need of mulch.
1) 100 square feet of bed to cover
2) 3" deep
100 square feet x 3" (or .25 foot (3/12)) = 25 cubic feet
so with 1 cubic yard = 27 cubic feet, one yard will be enough for you.
Hope this helps.
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