Compacted depth and loose depth are different numbers
Everything about a gravel drive is specified in compacted depth — four inches of base means four inches after it has been rolled — but everything is sold in loose volume, straight off a stockpile with air between every stone. The gap between the two is the compaction allowance, and for angular crushed stone laid in reasonable lifts it usually lands somewhere around fifteen to twenty-five percent.
That is a rule of thumb and not a constant. It varies with the gradation of the material, how wet it is, how deep each lift is and what you compact it with. A plate compactor on three-inch lifts achieves something quite different from a tipped load raked flat and driven over a few times. Twenty percent is the default here because it is the middle of the working range, and it is the sort of number worth checking against reality: after the first layer is down and compacted, measure the depth you actually got and adjust the order for the rest.
Two courses, doing two different jobs
A gravel drive that lasts is normally not one material. The lower layer is coarse, angular and well graded — crusher run, road base, or whatever the local quarry calls the equivalent — and its job is to spread wheel loads out over the subgrade and lock together so it does not move. The upper layer is finer, cheaper to replace, and its job is to be the surface you drive and walk on and rake back into place every year or two.
| Layer | Typical material | What it is doing |
|---|---|---|
| Subgrade | The ground you have | Everything above depends on it; soft ground needs more base or a geotextile |
| Base course | Angular crushed stone with fines, compacted in lifts | Load spreading, interlock, drainage |
| Top course | Cleaner, finer angular stone | Surface, appearance, replaceable wear layer |
Rounded pea gravel and river stone are the common mistake in the top layer. They look good in the yard and they never lock together, so they migrate under tyres, pile up at the edges and rut. Angular stone with sharp faces wedges against itself and stays where it is put. If you want the look of rounded stone, it belongs on a path nobody drives on.
Tons, yards and which one you are being quoted
Quarries sell by weight because the loader crosses a weighbridge. Homeowners think in yards because that is what a pile looks like. The conversion for crushed stone is commonly quoted somewhere in the region of 2,400 to 2,900 pounds a cubic yard, so a yard is broadly a ton and a quarter to a ton and a half, but the honest answer is that the figure belongs to the material and the weather, not to a table. Dense limestone and light volcanic aggregate are not the same product by weight, and neither is the same pile before and after rain.
Get the conversion from the supplier you are actually buying from. Ask for it in the form how many tons per cubic yard for this product, put that figure into the density field above, and the tonnage on this page becomes something you can order against instead of an estimate.
Before the first load arrives
The work that determines whether a gravel drive survives happens before any stone is delivered: stripping the sod and organic topsoil off, because gravel placed on soft organic material will sink into it; getting the fall right so water runs off rather than sitting; and deciding whether the subgrade needs a separation fabric under the base. Ordering the stone is the easy part and the last part. If the drive you are replacing rutted or held puddles, the reason is almost certainly underneath it, and another two inches of new stone on top will buy you one season.
Questions people ask
How many tons of gravel do I need for a 40 by 10 driveway?
For 400 square feet at 6 inches compacted total, with a 20 percent compaction allowance, it is about 8.9 cubic yards loose, which at 2,800 pounds a yard is roughly 12.4 tons. Change any of those three assumptions and the answer moves: at 4 inches it drops to about 8.3 tons, and at a lighter 2,400 pounds a yard the same volume is 10.7 tons. That is why the density is an input on this page rather than a fixed constant — the quarry knows what their material weighs and you should use their number.
How thick should a gravel driveway be?
This calculator will not tell you, because the answer is set by the ground underneath and the traffic on top, neither of which it can see. What people commonly build is a coarse compacted base with a finer top course over it, and more total depth over soft or clay subgrade than over firm well-drained ground. A drive that carries occasional delivery trucks needs more than one that carries a car. If the existing surface ruts or holds water, extra depth alone will not fix it — the problem is drainage or subgrade and it needs solving first.
Why is my gravel order short when the maths said it was right?
Three usual reasons, in order of frequency. The compaction allowance was too low or omitted, so the loose depth you ordered compacted down below the depth you wanted. The subgrade was soft and the first load pumped into it rather than sitting on it, which can swallow a surprising amount before any depth registers. Or the area was bigger than the rectangle — turnarounds, the flare at the road, the widening at a parked car — and a driveway measured as one clean rectangle almost always is. Measure the widened parts separately and add them.
Should I put fabric under the gravel?
A geotextile separation fabric between the subgrade and the base course does one specific thing well: it stops the stone and the soil mixing, which is what makes a driveway on soft or clay ground disappear over a few years. On firm, well-drained, granular subgrade it matters much less. This is a judgement about your ground rather than a universal rule, and it is worth asking someone local who has seen what soils do in your area. What it is not is a substitute for depth or drainage.