Gravel Under Concrete Slab Calculator: How Much and What Type
A 20×20 ft slab needs 5.43 cubic yards of gravel for a 4 in base with 10% extra; the 2021 IRC calls for a 4 in base course under slabs below grade.
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5.43 yd³ of gravel
- Gravel volume
- 133.33 ft³ = 4.94 yd³
- With 10% extra
- 5.43 yd³
- Tons
- ask your supplier for tons per yd³
Key takeaways
- 4 in base course below grade (IRC R506.2.2).
- Base: sand, gravel, crushed stone or slag passing a 2 in sieve.
- 10-mil vapor retarder under house slabs (R506.2.3).
- A 20×20 slab needs 5.43 yd³ of gravel at 4 in with extra.
Do You Need Gravel Under a Concrete Slab?
Yes for most house slabs below grade: the 2021 IRC requires a 4 in (102 mm) base course on the prepared subgrade (R506.2.2). The code makes an exception where the slab sits on well-drained or sand-gravel mixture soils classified as Group I, so a base is not always required.
How Much Gravel Under a Concrete Slab?
Gravel needed equals slab length × width × base depth, divided by 27 for cubic yards. A 20×20 ft slab with a 4 in base takes 133.33 ft³, or 4.94 yd³, and 5.43 yd³ with 10% extra. Ask your supplier for the tons per cubic yard of their material.
Gravel Needed for Common Slab Sizes
| Slab | 4 in base (yd³ + 10%) | 6 in base (yd³ + 10%) |
|---|---|---|
| 10×10 ft | 1.36 | 2.04 |
| 12×12 ft | 1.96 | 2.93 |
| 12×16 ft | 2.61 | 3.91 |
| 20×20 ft | 5.43 | 8.15 |
| 24×24 ft | 7.82 | 11.73 |
What Type of Gravel Goes Under a Slab?
The 2021 IRC base course is clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag that passes a 2 in (51 mm) sieve (R506.2.2). Compact it in layers before the pour.
Is 2 Inches of Gravel Enough?
No for slabs below grade: the 2021 IRC base course is 4 in thick. A thinner layer may work on firm, well-drained Group I soil, where the code does not require a base at all.
Vapor Barrier Over the Gravel
Under house slabs, the 2021 IRC requires a vapor retarder of at least 10 mil meeting ASTM E1745 Class A, with joints lapped at least 6 in (R506.2.3). It keeps ground moisture out of floor finishes.
Can You Pour Concrete Directly on Dirt?
Yes, on firm, well-drained Group I soils the 2021 IRC does not require a base course (R506.2.2 exception); on other soils, remove vegetation and topsoil, compact the subgrade and add the 4 in base.
Worked Examples: Gravel for Three Slabs
10×12 shed at 4 in base. 1.63 yd³ of gravel.
24×24 garage at 4 in base. 7.82 yd³ of gravel.
12×16 patio at 6 in base. 3.91 yd³ of gravel.
How to Prepare the Base Under a Slab
- Remove vegetation, topsoil and foreign material inside the slab area (IRC R506.2).
- Compact the subgrade.
- Spread the base in layers and compact each layer.
- Lay the vapor retarder over the base for house slabs (R506.2.3).
Gravel Under a Driveway or Patio
Driveways and patios are outside the IRC floor rules, but the same 4 in compacted base is common practice for drainage and support; frost and soil conditions set the depth locally.
Gravel Under a 4 Inch vs 6 Inch Slab
The base depth does not change with slab thickness under the IRC: a 4 in base course applies under any slab below grade (R506.2.2). Thicker bases are a design choice for poor soils or heavy loads.
Gravel Under Concrete Slab Calculator FAQ
What happens if you don't use gravel under concrete?
On soils that drain poorly, a slab without a base can settle or crack; the IRC skips the base only on well-drained Group I soils.
How thick should a gravel base be for a concrete slab?
4 in is the IRC base course for slabs below grade (R506.2.2).
Sand or gravel under a concrete slab?
Either: the IRC base course may be clean graded sand, gravel, crushed stone, crushed concrete or slag passing a 2 in sieve.
Should I put gravel under my concrete slab?
Yes for slabs below grade: the 2021 IRC requires a 4 in base course unless the slab sits on well-drained Group I soil.
What type of gravel is best for under a concrete slab?
The IRC base course is clean graded sand, gravel, crushed stone, crushed concrete or slag passing a 2 in sieve; compact it in layers.
Next step
Calculate the slab itself with the slab concrete calculator and add rebar only if needed with the slab rebar calculator.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.