Gravel depth, type, volume, and delivery planning
Concrete Gravel Base Calculator
How much gravel under a concrete slab? Most residential slabs need 4–6 inches of compacted crushed stone. Driveways and sidewalks can skip it on stable soil; interior slabs and garage floors should not. Free calculator: enter area and gravel depth to get cubic yards, tons, and material cost.
12x20 ft garage at 4-in depth
2.96 yd3 — about 4.0 to 4.3 tons
Standard 4-inch crushed stone base for a residential garage slab. Add 10% compaction buffer: order 3.26 yd3 or ~4.4 to 4.7 tons.
20x20 ft driveway at 6-in depth
7.41 yd3 — about 10 to 10.7 tons
Deeper base for vehicle traffic. Most suppliers deliver in 10-ton loads — a 20x20 at 6 inches is right at one full truck.
Gravel base volume and tons examples
Use these examples as a supplier-call screen. The ton range is a planning range, not a ticket guarantee; ask the supplier for the density used for the exact clear stone, crushed stone, or road base product.
| Base scenario | Loose volume screen | Ton range screen | Supplier question |
|---|---|---|---|
| 3.33 yd3 clear stone | 3.33 cubic yards | about 4.3 to 4.8 tons | What tons-per-yard conversion does this quarry use for this clear stone? |
| 100 sq ft slab, 4 in base | 1.23 yd3 before buffer; 1.36 yd3 with 10% | about 1.8 to 2.0 tons with buffer | Is the base depth measured loose, compacted, or after final grading? |
| 10x12 shed pad, 4 in base | 1.48 yd3 before buffer; 1.63 yd3 with 10% | about 2.1 to 2.4 tons with buffer | Does the pad quote include excavation, compaction, geotextile, and haul-off? |
| 12x20 driveway apron, 6 in base | 4.44 yd3 before buffer; 4.89 yd3 with 10% | about 6.4 to 7.1 tons with buffer | Can the truck dump close to the work area without driveway or turf damage? |
Interactive workflow
Run the numbers, then save or export the project
What cubic-yard calculators usually miss
Generic cubic-yard calculators answer volume. The useful concrete base decision also needs material density, compaction, access, drainage, delivery, and whether the contractor quote includes excavation and base repair.
| Benchmark says | Replace with local input | Next check |
|---|---|---|
| 3.33 cubic yards = one fixed ton number | Ask the supplier for tons per cubic yard for that product. A practical loose range is often about 1.3 to 1.45 tons per yd3, so 3.33 yd3 screens near 4.3 to 4.8 tons. | Confirm the product density, moisture, and whether the quote is sold by ton, yard, or truckload. |
| Base depth is separate from slab estimate | Tie base depth to the finished slab or pad footprint, then add waste, compaction, and edge overbuild where needed. | Open the slab components estimator when base, forms, rebar, and vapor barrier must be checked together. |
| Installed quote says base included | Separate excavation, unsuitable soil removal, geotextile, aggregate type, lift thickness, compaction, drainage, haul-off, and access. | Use the quote reviewer when the base line is vague or two bids differ on prep scope. |
Concrete base material decision checklist
Use this table before ordering stone or accepting a bid that bundles concrete, excavation, and base prep into one line.
| Signal to check | What it means | Next step |
|---|---|---|
| Area and base depth | Square feet or square meters times base depth gives the starting volume before waste or compaction. | Confirm whether the footprint includes over-excavation beyond the slab or pad edge. |
| Material type | Clear stone, crushed stone, road base, crusher run, and recycled aggregate can have different density and compaction behavior. | Ask the supplier for the product name, gradation, and tons-per-yard conversion used on the ticket. |
| Compaction and drainage | Base thickness after compaction may differ from loose delivered volume, especially when placed in lifts. | Clarify lift thickness, compaction responsibility, drainage correction, and whether geotextile is included. |
| Delivery access | Dump location, driveway protection, wheelbarrow distance, skid steer access, and cleanup can change the real cost. | Use the wheelbarrow or delivery cost calculators when base or concrete must be moved far from the truck. |
| Quote wording | A base line that says gravel included may hide material type, depth, compaction, haul-off, soil repair, and warranty limits. | Require the base scope in writing before comparing the lowest concrete bid. |
Related concrete planning tools
Move from a single calculation into a purchase, proposal, or pour-day decision with connected calculators and checklists.
How many inches of gravel do you need under a concrete slab?
The standard is 4 inches of compacted gravel for residential slabs (patios, garage floors, driveways). Use 6 inches in areas with poor drainage, expansive clay soil, or heavy vehicle traffic. Some codes require a minimum of 4 inches; check your local building department. The gravel base provides drainage, prevents frost heave, and creates a stable, uniform surface for the concrete.
Do you always need gravel under a concrete slab?
Not always. A driveway or sidewalk poured on stable, well-drained native soil can often skip the gravel base, especially in warm climates without hard freezes. However, interior slabs (garage floors, basement floors, concrete patios over clay) should almost always have a gravel base to prevent cracking from soil movement and to allow drainage under the vapor barrier. Consult your local building code — many jurisdictions require a minimum 4-inch base for permitted work.
What type of gravel is best under a concrete slab?
Crushed stone (3/4-inch angular crushed limestone or granite) is the most common choice. It compacts well and drains water effectively. ABC stone (Aggregate Base Course) or road base also works well. Avoid pea gravel and round river rock — they do not interlock and compact poorly, which can lead to uneven settling. In some regions, clean sand over compacted gravel is used, but gravel alone is typically sufficient for residential work.
How much gravel do I need for a 20x20 concrete slab?
For a 20x20 ft (400 sq ft) slab with a 4-inch gravel base: 400 × (4/12) = 133 ft³ ÷ 27 = 4.94 yd³, plus 10% compaction buffer = about 5.4 yd³ or 7–8 tons of crushed stone. At 6 inches depth: 7.41 yd³ + buffer = about 8.2 yd³ or 10–11 tons. Most gravel suppliers deliver in 10-ton loads, so a 20x20 at 6 inches is right at one full truck.
Do you put a vapor barrier over the gravel under a concrete slab?
Yes — for interior slabs (garage floors, basement floors, any slab inside a conditioned space), a 6-mil or 10-mil polyethylene vapor barrier goes on top of the gravel, directly under the concrete. The vapor barrier prevents ground moisture from wicking through the concrete and causing efflorescence, mold, or flooring adhesive failure. For exterior slabs (driveways, patios exposed to rain), a vapor barrier is typically not used.
Planning disclaimer
This concrete gravel base calculator is for planning and quote review only. It does not replace supplier tickets, geotechnical advice, drainage design, compaction testing, engineering, permits, inspections, or local building-code requirements.