How Much Gravel Under a Concrete Slab?
Calculate how much gravel goes under a concrete slab by area, depth, cubic yards, tons, compaction, waste, and base prep scope.
How much gravel you need under a concrete slab depends on the slab footprint, base depth, stone type, compaction, waste, drainage, soil, and local practice. For planning, many small slabs use about 4 inches of compacted gravel, but that is not a universal design rule.
Use the Gravel Base Calculator for Concrete for a full estimate, then use this guide to check the formula and supplier scope. If a contractor quote includes base prep, review it in the Concrete Quote Reviewer.
Competitor pages such as ConcreteCalculatorMax's gravel calculator hub and ConcreteCalculatorMax's gravel cost calculator cover gravel volume and cost intent. This page focuses specifically on gravel under concrete slabs.
Quick answer
For a rectangular slab, multiply slab length by width by gravel depth, divide by 12 and 27, then add waste. A 10 ft x 12 ft slab with 4 inches of gravel uses about 1.48 yd3 before waste, or about 1.63 yd3 with 10% waste. At 1.35 tons per yd3, that is about 2.2 tons.
| Slab size | Gravel depth | Gravel with 10% waste | Tons at 1.35 tons/yd3 |
|---|---|---|---|
| 8 ft x 10 ft | 4 in | 1.09 yd3 | 1.47 tons |
| 10 ft x 12 ft | 4 in | 1.63 yd3 | 2.20 tons |
| 12 ft x 16 ft | 4 in | 2.61 yd3 | 3.52 tons |
| 20 ft x 20 ft | 4 in | 5.43 yd3 | 7.33 tons |
This is a planning estimate only. Soil, drainage, frost, slab use, compaction, and code requirements should be confirmed by a qualified local professional.
Gravel under slab formula
gravel cubic yards = length ft x width ft x depth in / 12 / 27
order cubic yards = gravel cubic yards x (1 + waste percentage)
tons = order cubic yards x bulk density tons per yd3
If you do not know the stone density, ask the supplier. Many planning estimates use about 1.3 to 1.5 tons per yd3, but real material varies.
What depth should you enter?
| Situation | Planning depth to discuss |
|---|---|
| Small patio or shed pad | Often around 4 in compacted base. |
| Driveway or garage slab | May need a stronger base design. |
| Poor drainage or soft soil | May need excavation, geotextile, or thicker base. |
| Frost-sensitive area | Local practice and code matter. |
| Replacement slab | Existing base may need removal or correction. |
Do not enter rough excavation depth as finished compacted gravel depth. A base that starts loose will compact lower.
Quote checklist
Ask whether the quote includes:
- Excavation and disposal.
- Gravel type and compacted depth.
- Compaction equipment and passes.
- Geotextile fabric if needed.
- Drainage correction.
- Final grade before forms.
For cost review, pair this with the Concrete Material Cost Breakdown.
FAQ
How much gravel do I need under a concrete slab?
Multiply length by width by gravel depth, convert inches to feet, divide by 27, then add waste. For 10 ft x 12 ft at 4 inches, plan about 1.63 yd3 with 10% waste.
Is 4 inches of gravel enough under concrete?
Four inches is a common planning starting point for many small slabs, but soil, drainage, frost, load, and local code can change the correct base.
Should gravel be compacted before concrete?
Yes. The base should be compacted and graded before forms and concrete. Loose stone can settle and change slab support.
How many tons of gravel are in a cubic yard?
It depends on stone type and moisture. A planning range is often 1.3 to 1.5 tons per yd3. Ask the supplier for the material you are buying.
Does gravel cost include delivery?
Not always. Delivery, minimum load, fuel, and placement may be separate.
Next step
Estimate the base in the Gravel Base Calculator for Concrete, then compare base prep scope in the Concrete Quote Reviewer. Contractors can add base prep as a line item in the Concrete Proposal Kit.
Quote planning next step
Turn this guide into a concrete buying check
Run the matching calculator, then compare ready-mix, bagged concrete, delivery fees, access needs, and quote gaps before you buy materials or approve a contractor number.