How Much Gravel Under a Concrete Slab?
Updated November 1, 2026
Gravel under a slab is a volume of stone. Length and width are the footprint. Thickness is the depth of the base. Cubic yards come last, and the delivery usually steps in quarter yards. The concrete on top of that base is a second volume. The Concrete Calculator will do the arithmetic. It has one thickness field and no switch that labels the result as gravel, so the yards below stay marked as stone.
Measure the footprint and the base you will place
Measure length and width in feet, to the inside of where the stone will stop. That is usually the inside of the forms, the same rectangle the concrete will sit on. A base that runs past the forms is a larger footprint. Measure the stone you will actually place.
Write the gravel depth in inches, then divide by 12 before you multiply. Four inches of compacted base is a common patio depth. Four inches is 4 ÷ 12 of a foot. The number is the stone, even when the concrete on top of it is also 4 inches. Those are two layers.
The same rectangle, labeled as stone
The volume steps are the ones a slab uses. Only the material changes.
- Cubic feet = length (ft) × width (ft) × base depth (ft).
- Cubic yards = cubic feet ÷ 27.
There are 27 cubic feet in a cubic yard. After waste, round the order up to the next 0.25 cubic yard, the same step the calculator uses for a ready-mix ticket. On this page that ticket is gravel or crushed stone. It is not concrete.
A 10 by 12 foot footprint with 4 inches of gravel
One rectangle, 10 feet by 12 feet, 4 inches of base. That is the footprint in How Much Concrete Do I Need for a Slab?, with the thickness read as stone.
- Depth in feet: 4 ÷ 12.
- Cubic feet: 10 × 12 × (4 ÷ 12) = 40 cu ft.
- Cubic yards: 40 ÷ 27 = 1.48 cu yd of gravel.
1.48 cubic yards is the stone in that box, with nothing extra. Enter 10 feet, 12 feet, and 4 inches in the concrete calculator, quantity 1, and the exact volume line is this 1.48. Call it gravel when the 4 inches are the base.
Add waste, then round the stone to the next quarter yard
Waste is extra stone so the base can finish at the depth you measured after compaction. The calculator accepts 0, 5, 10, 15, or 20 percent. It applies that percent to the cubic yards. Use the same percent here.
- 5% — a rectangle you have already graded, and stone that arrives close to the yield you ordered.
- 10% — the usual base. On this example, 10 percent waste on the full 40 ÷ 27 is 1.63 cubic yards of gravel.
- 15% or 20% — soft spots you will fill, or a footprint that is still out of square.
1.63 cubic yards rounds up to 1.75 cubic yards. Deliveries commonly take quarter-yard steps, and 1.50 would put you under the volume you just allowed. Order 1.75 cubic yards of gravel. Give that number to the stone yard. The concrete plant gets its own number from the slab guide.
Concrete thickness is a different order
The slab guide’s 4 inch concrete pour on this same 10 by 12 footprint is also 1.48 cubic yards exact, 1.63 with 10 percent waste, and a 1.75 cubic yard order. That second 1.75 is concrete. The two layers match only because both happen to be 4 inches. A thicker slab changes the concrete yards. It does not change this gravel depth. A thinner slab does the same in the other direction. Run each thickness on its own.
Adding the depths and entering 8 inches once is a single run: 10 × 12 × (8 ÷ 12) = 80 cubic feet, 2.96 cubic yards exact, 3.26 with 10 percent waste, and a 3.50 cubic yard order. That order is one material. The calculator cannot split it into stone plus concrete. Keep the 1.75 cubic yards of gravel on this page, and keep the concrete yards on the slab guide.
What the 1.75 yards of gravel leave out
The 1.75 yards are base stone only. They do not include the concrete, rebar or mesh, form boards, or fabric under the stone. Compaction can lower a loose pile, which is part of why the waste percent is on the order. A price per cubic yard typed into the calculator prices the 1.75 yard line as if it were concrete. It does not know a gravel yard price, a delivery minimum, or tax.
FAQ
How much gravel for a 10 by 12 foot slab with 4 inches of base?
10 times 12 times (4 divided by 12) is 40 cubic feet, which is 1.48 cubic yards. At 10 percent waste that is 1.63 cubic yards, and the delivery rounds up to 1.75 cubic yards of gravel.
Is 1.75 cubic yards also the concrete order?
Only when the concrete is also 4 inches thick on that footprint. The slab guide prints the same 1.48, 1.63, and 1.75 for a 4 inch pour, and that ticket is concrete. One 1.75 yard order covers one material. Tell the plant the concrete yards, and order the 1.75 yards of gravel as stone.
Can I add the gravel depth to the slab thickness and run the calculator once?
No. Four inches of gravel plus 4 inches of concrete is not an 8 inch concrete slab. The calculator has one thickness field and no gravel switch. An 8 inch entry is a single volume. Run the gravel depth on this page, and the concrete thickness on the slab guide.
Use the calculator
The Concrete Calculator turns length, width, and one thickness into cubic yards and rounds that volume up to the next 0.25 yard. Enter the gravel depth as the thickness and read the result as stone. Concrete yards are a second run, on the slab guide. The result is a planning quantity, not a contractor bid or a store quote.
If a step here does not match the result you get, email [email protected] with the measurements you entered.