I'm putting in a stepping stone path. The stones will be on top of 4" of packed, 3/4"- (three quarter minus) stone – which packs to a nice, solid base.
My question is, if I know that the volume of the stone in the path is going to be (for example) one cubic yard (3'x3'x3'), what amount of stone should I order?
In other words, if I ordered one cubic yard, spread it out, and tamped it down, how close would the resulting volume be to one cubic yard? Is it close enough to not worry? Or would it compact down to only be 75% of the cubic yard?
Edited to add:
For the sake of this question, assume that I've already added a buffer in my order for voids I've not accounted for, that I've already adjusted for loss while wheel-barrowing the stone to the area I'm going to put it, that I've accounted for deviations in how much the gravel company delivers, and that I've already accounted for extra slough off the shoulder of the base layer.
The question is NOT about "how to build a path?" or "it's just a path, why do you care?" – the question is simply, how much volume is lost when compacting 3/4"-? It's obviously non-zero because it actually does compact (as opposed to river rock which does not compact).
Best Answer
When something calls for x" of compacted y, you typically calculate the amount of y prior to compacting.
So if something calls for a 4" compacted base, order enough to cover your area with 4". Then compact.
(Actually, order to cover 4", but the only lay down half, compact that, then lay down the other half, then compact that.)
In other words, you don't typically order the stone to allow for compaction. You order the stone based on the volume needed prior to compaction.
UPDATE:
If you're just asking about the rate of compaction 3/4 minus has, the answer is...actually, that's a tough one. There seems to be a lot of opinion, but no specific engineering spec that I can find. General googling seems to imply you'd be in the 20% loss due to compaction range, but I can't find any hard data to back that up.
Keep in mind that there's also issues of your soil base compacting as well.
So based on that limited info, I'd suggest ordering 20% over. Keep in mind that this may be a trial-and-error process that may not be worth the headache. If you can't get 20% compaction, then you're left with extra crushed stone that may or may not be easy to get rid of.