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.
Calculate gravel under a concrete slab from the base footprint and specified compacted depth, then account separately for waste and extra loose material needed for compaction. The examples below use 4 inches of compacted base; that is an example input, not a universal design rule.
Use the Concrete Gravel Base Calculator 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.
This page focuses specifically on gravel under concrete slabs: how to convert slab area and base depth into cubic yards, when to convert yards to tons, and which quote lines keep base prep from being hidden inside the pour.
Quick answer
For a rectangular base, multiply length in feet by width in feet by compacted depth in inches, then divide by 12 and 27. A 10 ft x 12 ft base at 4 inches contains about 1.48 yd3 compacted. Adding 10% waste gives 1.63 yd3, before any additional loose-material allowance for compaction. Using an example conversion of 1.35 US short tons per loose yd3 gives 2.20 tons on that waste-only basis, not a confirmed delivery quantity.
The table assumes the base matches the slab footprint, 4 inches of compacted depth, 10% waste, 0% additional compaction allowance, and an illustrative 1.35 US short tons per loose yd3. Confirm both allowances and the conversion with the supplier before ordering. Values are rounded only for display.
| Base size | Compacted depth | Volume with waste only | 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
compacted yd3 = base length ft x base width ft x compacted depth in / 12 / 27
waste-only yd3 = compacted yd3 x (1 + waste percent / 100)
loose order yd3 = waste-only yd3 x (1 + compaction allowance percent / 100)
US short tons = loose order yd3 x supplier US short tons per loose yd3
Here, 10 means 10%, so the waste multiplier is 1.10. The compaction allowance is extra loose volume added to the waste-adjusted target. It is not a measured shrinkage rate or a laboratory compaction percentage. The calculator's example 8% setting is adjustable, not a specification for every gravel product.
Ask the supplier for the conversion appropriate to the material's loose delivery condition. Do not multiply loose yards by a compacted bulk density. US short tons, metric tonnes, moisture and material grading are not interchangeable. If the supplier already calculated a loose order from your finished dimensions, do not add the same waste or compaction allowance again.
What depth should you enter?
| Situation | What to confirm before entering depth |
|---|---|
| Small patio or shed pad | Use the specified compacted depth; 4 in is only the example here. |
| 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.
NRMCA's CIP 29 guidance on moisture and concrete slabs discusses stable, well-draining support and compacted subgrade/base, and distinguishes the base, capillary break and vapor retarder. Those layers serve different purposes. It does not establish a single gravel depth or ordering allowance for every slab; local soil, loads, drainage and the project design still control the layer selection.
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.
Example ordering path
A practical gravel order starts with the planned base footprint, which can be larger than the concrete slab. For a 12 ft x 16 ft base with 4 inches of compacted depth, keep full precision through the calculation:
compacted volume = 12 x 16 x 4 / 12 / 27 = 2.370370... yd3
waste-only volume = (12 x 16 x 4 / 12 / 27) x 1.10 = 2.607407... yd3
with an additional 8% allowance = (12 x 16 x 4 / 12 / 27) x 1.10 x 1.08 = 2.816 yd3
estimated mass = 2.816 x 1.35 = 3.8016 US short tons
The waste-only result is 2.61 yd3 and 3.52 tons. With the separate example 8% loose-material allowance, it becomes 2.82 yd3 and 3.80 tons. Confirm the allowance with the supplier rather than treating either result as a guaranteed order. Round the final outputs, not each intermediate number.
If the specified base extends 6 inches beyond each of the slab's four sides, the base footprint becomes 13 ft x 17 ft. At the same 4-inch depth, the comparison is:
| Base footprint | Compacted target | Waste only | Waste plus 8% allowance |
|---|---|---|---|
| 12 ft x 16 ft | 2.37 yd3 | 2.61 yd3 / 3.52 tons | 2.82 yd3 / 3.80 tons |
| 13 ft x 17 ft | 2.73 yd3 | 3.00 yd3 / 4.05 tons | 3.24 yd3 / 4.38 tons |
Both rows use 10% waste and the illustrative 1.35 US short tons per loose yd3. In the Concrete Gravel Base Calculator, enter length 12, width 16, compacted depth 4, waste 10, and custom density 1.35. Set compaction buffer to 0 for the waste-only comparison or 8 for the example loose order. Set edge overbuild to 6 inches each side for the second row. Do not also change the dimensions to 13 x 17 when using that overbuild setting.
Base prep items that change the quote
| Item | Why it changes cost | What to ask |
|---|---|---|
| Excavation depth | Removing soil takes labor and disposal. | Is excavation included or only gravel delivery? |
| Stone type | Clear stone, crusher run, and road base behave differently. | What material name and size are quoted? |
| Compaction | Loose gravel settles lower than compacted gravel. | Is compaction equipment included? |
| Fabric | Soft soil may need separation fabric. | Is geotextile included or excluded? |
| Drainage | Water under the slab can shorten service life. | Where does water go after the base is placed? |
| Access | Dump location affects wheelbarrow or machine time. | Can the truck dump close to the forms? |
Do not compare a gravel-only material price with a contractor base-prep line that includes excavation, spreading, compaction, fine grading, and cleanup. Those are different scopes.
How to avoid ordering short
Confirm whether the supplier sells by cubic yard, ton, or full truckload. If the quote is by ton, ask for the density assumption used to convert from yards. If the quote is by yard, ask whether it is loose cubic yards or compacted-in place depth. Keep a small buffer for uneven subgrade, edges, and low spots, but avoid using gravel to hide major grade problems that should be excavated or repaired before forms are set.
Delivery and compaction handoff
Before ordering, decide where the truck can dump and how the base will be moved, spread, and compacted. A supplier may price only loose material dropped near the street, while a contractor may price delivery, machine spreading, laser grading, plate compaction, and cleanup. Those are different quotes.
Ask whether compaction happens in one lift or multiple thinner lifts. A deep base placed all at once can bridge over soft spots and settle later. Also confirm whether fabric, drainage stone, or a thicker edge base is needed at doors, driveways, ramps, or shed entries. If the base extends beyond the slab, calculate that wider rectangle separately so gravel, forms, and finish grades all match the same plan.
FAQ
How much gravel do I need under a concrete slab?
For a 10 ft x 12 ft base at 4 inches compacted depth, the compacted target is 1.48 yd3. With 10% waste it is 1.63 yd3 before any extra loose-material compaction allowance. Confirm the actual base footprint, allowance and supplier delivery units before ordering.
Is 4 inches of gravel enough under concrete?
Four inches is the worked example here, not a design approval. Soil, drainage, frost, load and local code can change the required base. Use the specified compacted depth from the project design.
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, moisture and whether the volume is loose or compacted. These examples use 1.35 US short tons per loose yd3 only to show the conversion. Ask the supplier for the actual material and confirm whether the ticket uses US short tons or metric tonnes.
Does gravel cost include delivery?
Not always. Delivery, minimum load, fuel, and placement may be separate.
Next step
Estimate the base in the Concrete Gravel Base Calculator, 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.