The hole tells you the depth, so the hole comes first
Nobody can answer this over the phone, and the reason is straightforward. The number is not decided by the slab, it is decided by what comes out of the ground under it. We strip the sod and the topsoil, dig out the soft spots and anything organic, and keep going until we are down on ground that will actually hold. Whatever depth that turns out to be is the hole we have to fill back.
On ground that has never been disturbed and comes up firm a few inches down, that is a modest amount of stone. On a lot that was graded years ago and has fill in it, or a corner where a stump got buried, or a wet spot that never drains, it is a good deal more. Same slab, same building, different amount of stone, and the whole difference is in what we found.
- Sod and topsoil, all of it, because organic material rots and shrinks
- Old fill from whenever the lot was last graded
- Buried stumps, brush and construction debris
- Wet or spongy pockets that never dry out
- Old gravel with mud worked up through it
The load adds to it, on top of whatever the ground asked for
Once the soft material is out, the second question is what is going on top. A walkway carrying people asks less of the base than a shop floor a loaded trailer backs onto. Heavier loads press further down into the ground, so more of that ground has to be firm and uniform, and stone is one of the ways you get there.
That is why two slabs on the same property can call for different base depths. It is not inconsistency. It is the load.
Stone that packs, not stone that rolls
The layer of stone between the ground and the concrete is the subbase, and how much of it you have matters a lot less once the material is wrong. Clean, single sized gravel, the round decorative kind, never locks together. You can run a compactor over it all afternoon and it stays a pile of marbles.
What goes under a slab is dense graded crushed stone, crusher run or 21A, which arrives with everything from rock down to stone dust in the same load. The fines fill the gaps between the bigger pieces and the whole thing keys together under a compactor into something you can walk across without leaving a boot print. A thinner layer of the right stone will outwork a thick layer of the wrong stone, so the material named on an estimate is worth reading as closely as the depth beside it.
Compacted depth is the number that counts
When a base is called out at a depth, that means compacted depth, in place, after the plate or the roller has been over it. Stone dumped loose and raked flat measures deeper on the day and settles once it gets loaded, and you lose part of what you paid for.
It is also why a deep base gets built up in layers instead of one dump. A compactor only reaches so far below the surface, so the depth you are buying only exists if it was built in stages with a pass over each one.
Working out roughly how much stone a pad takes
If you want a ballpark before we come out, the arithmetic is the same as it is for concrete. Convert the base depth to feet, multiply by the square footage of the pad, then divide by 27 to get cubic yards.
Say the base is called out at six inches under a 600 square foot shop pad. Six inches is half a foot, so 600 times 0.5 is 300 cubic feet, and 300 divided by 27 is about 11 cubic yards of stone in place. That six inches is an example so the math reads clearly, not a standard. Your depth comes off your own written estimate.
Two things that figure does not account for. Stone is sold by weight rather than by volume, so a supplier converts it for you when you order. And in place is not the same as delivered, because the material packs down under the compactor. Both are good reasons the stone on a job gets ordered by the crew doing the compacting rather than by a homeowner with a calculator.