Assuming you are talking about decomposed granite, if you want 3 inches of decomposed granite and you want the same elevation (good idea), then take out 3 inches of soil. Then use as much granite as it takes to bring the level up to the previous level after compacting.

Thank you!

I hope you have an accurate way to measure how much you've actually removed. I guess it would be fairly simple over a flat area because you can establish a benchmark and measure everything off that. If you're doing it by hand or may be a bobcat, so much the better, but anything larger than that and you'll have trouble with that level of accuracy.

Thank you, yes we will be removing the soil by hand. It’s a pretty small area of our yard and relatively flat. We plan to set up a datum with level to keep track of how deep we are going. Hoping it works well...

Haha archaeologist actually

Perfect. Well, don't overthink the concept of putting 3 inches of granite into a 3-inch deep hole. I am curious. Why do you want decomposed granite and why 3 inches of it? We have a couple places where we used it but we just dumped it on the surface and walked on it to compact.

Here's something else to consider when doing it your way. After you dig out the 3 inches of soil, start watering lightly 3x per day for a week to see if there are any weed seeds exposed while digging. If anything grows, spray it with Round Up. If the watering brings up nutgrass, then hit that with Sedgehammer, Image, or another sedge specific killer. Then continue watering another week to see if any more weeds germinate. Repeat the RU if necessary.

Also do not make the mistake of thinking the granite will be the end of weeds. They can sprout in the granite as easily as topsoil.

So did I just suggest not overthinking it and then give you a bunch of stuff to think about?

Thank you so much for the tips! good to know about the weeds, we will do that and spread pre-emergent too I think. we decided to dig out the 3” because our soil is already pretty high in our yard so adding the extra DG even compacted would likely spill over our concrete patio and walkway

Agreed, you don't want that. We put a 4 inch steel border around ours to contain it.

Just a note, I hope you know you don't have to set up a 10 ft square grid and sift all the soil you dig out for potsherds and bone fragments. LOL

Doesn't have to but it's good practice. ;-)

Hahah!