Some of the concrete, sand and aggregate sellers have handy
little pocket guides that they can give to you when you stop by
to check on prices or to make a purchase. These guides make
it easy to calculate yards of materials and to estimate the weight
of the materials. Obviously, you aren't the first customer of theirs
who needs to do those calculations.
Of course, if you are like me and you keep misplacing the pocket
guide, then you still need to do it the hard way. For weight, I
generally just appoximate with 2500 pounds per yard for dry sand,
gravel, stone, etc. Dry topsoil is about 1500 pounds/yard and wet
sand is approximately double that at 3000 pounds/yard (although
wet sand seems to weight about ten times that amount when you
are shoveling it and hauling it in a wheelbarrel.) I also figure 4000
pounds per yard for wet concrete mix and for cured concrete.
Using these numbers as guidelines will give you a good ballpark
estimates for most common materials that you buy by the yard.
Buying a yard of mulch? I'd say it is lighter than dry topsoil, and
I'd ballpark it at 1000 pounds/yard.