How Much Gravel Do I Need Under a Concrete Slab? (Base Guide)
Quick answer: most slabs need a 4-inch compacted gravel base, which takes about 1.42 cubic yards of crushed stone per 100 sq ft (1.99 tons), including 15% extra for compaction. A 20x20 slab needs about 5.7 yd³ (8.0 tons) at 4 inches, or 8.5 yd³ at 6 inches. At $35 to $55 per yard, the 4-inch base costs roughly $199 to $312 in stone, plus delivery.
The gravel base is the cheapest layer of a slab and one of the most important. It gives the concrete a flat, even surface of uniform support, it drains water away from the underside of the slab, and it keeps fine soil from pumping up through cracks. This guide shows how much to order for common slab sizes, which stone to ask for, how to compact it, and when you can use sand or skip the base.
Gravel Needed for Common Slab Sizes
Quantities below are the amount to order: compacted depth, plus 15% because loose stone settles when it is compacted. Tons use about 1.4 tons per cubic yard for crushed stone; ask your supplier for their own figure, since it changes with the stone and how wet it is.
| Slab size | Area | 4 in base: yd³ | 4 in base: tons | 6 in base: yd³ | 6 in base: tons |
|---|---|---|---|---|---|
| 10 × 10 (shed, AC pad) | 100 sq ft | 1.4 | 2.0 | 2.1 | 3.0 |
| 12 × 12 (patio, shed) | 144 sq ft | 2.0 | 2.9 | 3.1 | 4.3 |
| 16 × 24 (large patio) | 384 sq ft | 5.5 | 7.6 | 8.2 | 11.4 |
| 20 × 20 (patio, 2-car pad) | 400 sq ft | 5.7 | 8.0 | 8.5 | 11.9 |
| 24 × 24 (2-car garage) | 576 sq ft | 8.2 | 11.4 | 12.3 | 17.2 |
| 20 × 40 (driveway) | 800 sq ft | 11.4 | 15.9 | 17.0 | 23.9 |
| 30 × 40 (shop) | 1,200 sq ft | 17.0 | 23.9 | 25.6 | 35.8 |
Have a different size or depth? The gravel base calculator does the same math for any shape, and the slab cost calculator adds the base to the full project price.
How to Calculate It Yourself
The 1.15 factor matters. Order only the geometric volume and a 4-inch base will come out closer to 3.5 inches after compaction, which is thinner than the IRC's 4-inch base course and leaves your slab thicker (and more expensive) in the low spots.
What It Costs
Crushed stone is sold by the cubic yard or by the ton. Local quarry prices vary a lot, and delivery is often a separate charge with a minimum load, so get a delivered price. Using $35 to $55 per yard or $25 to $40 per ton:
| Slab size | 4 in base (priced by yard) | 4 in base (priced by ton) | 6 in base (priced by yard) |
|---|---|---|---|
| 10 × 10 | $50 to $78 | $50 to $80 | $75 to $117 |
| 12 × 12 | $72 to $112 | $72 to $114 | $107 to $169 |
| 16 × 24 | $191 to $300 | $191 to $305 | $286 to $450 |
| 20 × 20 | $199 to $312 | $199 to $318 | $298 to $469 |
| 24 × 24 | $286 to $450 | $286 to $458 | $429 to $675 |
| 20 × 40 | $398 to $625 | $398 to $636 | $596 to $937 |
| 30 × 40 | $596 to $937 | $596 to $954 | $894 to $1,406 |
Digging out sod and soil to make room for the base usually costs more than the stone. Our calculator uses $1.00 to $2.50 per sq ft for excavation, which is $400 to $1,000 on a 20x20. Excavation plus a 4-inch base together come to about $1.40 to $3.30 per sq ft.
Which Gravel to Use
| Material | What it is | Good for | Watch out for |
|---|---|---|---|
| Crusher run / dense-graded base (also called 3/4-minus, road base, ABC, or DGA) | Crushed stone from about 3/4 in down to dust | The most common slab base. The fines fill the gaps, so it compacts into a hard, flat layer. | Drains slower than clean stone. Do not leave it soaked before the pour. |
| #57 clean stone | Washed, angular stone about 1/2 to 1 in, no fines | Wet sites and under interior floors where you want a drainage layer. | Hard to screed perfectly flat. Many crews top it with a thin layer of fines or use a dense-graded base instead. |
| Clean sand | Coarse, graded sand | Allowed by the IRC as a base course. Easy to grade flat. | Washes out if water runs under the slab edge. Needs to be kept damp and compacted. |
| Pea gravel | Small rounded stone | Drainage around foundations, not under slabs. | The round stones roll instead of locking together, so it never compacts firmly. Avoid it as a slab base. |
Names vary by region, so describe what you want: "crushed, angular stone that compacts" for a dense base, or "washed, crushed stone with no fines" for a drainage layer. The IRC limits base course material to stone that passes a 2-inch sieve.
4 Inches or 6 Inches?
- 4 inches is the IRC base course for floors and the normal choice for patios, sheds and garage floors on reasonably firm soil.
- 6 inches makes sense for driveways and RV pads, on clay or soft soil, and where you need to raise the slab or level a sloping site. The extra 2 inches costs about $99 to $156 in stone on a 20x20.
- More than 6 inches of fill under a slab should be placed and compacted in layers, and the IRC limits how deep loose fill can go (R506.2.1: no more than 24 inches of clean sand or gravel, or 8 inches of earth, unless it is compacted to engineered standards).
Your slab thickness and base depth add up to the dig depth. A 4-inch slab on a 4-inch base needs about 8 inches of excavation below the finished top. See how thick a slab should be to set the first number.
Compacting the Base in Lifts
Gravel only works if it is compacted. Loose stone settles under the weight of wet concrete and later under use, and the slab cracks where it loses support.
- Compact the subgrade first. Run a plate compactor over the bare soil until it stops leaving a mark. Dig out and replace any soft, spongy spots.
- Spread in lifts. A walk-behind plate compactor works through about 4 to 6 inches of loose stone at a time. A 4-inch base can usually go down in one lift; a 6-inch or deeper base should go down in two.
- Dampen dense-graded stone before compacting. Slightly moist stone packs much tighter than bone-dry stone. Do not flood it.
- Make several passes in overlapping rows, then cross the area at a right angle. When you can walk on it without leaving footprints, it is ready.
- Check the grade with a string line or a screed board on the forms. High spots make the slab thin; low spots waste concrete.
Our yard prep checklist covers the full order of work, from the 811 call to setting forms.
For a slab under a house, garage or heated shop, the IRC calls for a 6-mil polyethylene vapor retarder between the base and the concrete (R506.2.3), with joints lapped at least 6 inches. Lay it over the compacted base so stone does not puncture it, and patch any tears before the pour. Outdoor patios and driveways usually do not need one.
When Sand or No Base Is OK
The IRC requires a 4-inch base course of clean graded sand, gravel or crushed stone under floor slabs, with one exception: no base is required where the slab sits on well-drained or sand-gravel soils (Group I soils in the code's soil classification table, R405.1). In practice:
- Sandy, gravelly native soil that drains well and has not been disturbed can take a slab directly once it is compacted. Many patios and walks in sandy regions are poured this way.
- Clay, silt, or wet ground needs a base. These soils hold water, swell, and can heave when they freeze.
- Sand as the base is acceptable under the code and grades flat easily, but it can wash out at the edges. It works best inside a perimeter footing or on a slab with good drainage away from the edges.
- Driveways and garage floors almost always benefit from a gravel base, even on decent soil, because vehicle loads and freeze-thaw cycles are hard on any weak spot.
Local code can be stricter than the IRC, and an inspector may check base depth before the pour. See slab permits and costs.
DIY or Hire It Out?
Base work is the most DIY-friendly part of a slab project. A plate compactor rents by the day, the stone can be dumped close to the site, and mistakes are easy to fix before the concrete arrives. The hard part is moving the stone: 8.0 tons for a 20x20 is a lot of wheelbarrow trips. Many homeowners do the excavation and base themselves and hire a crew only for the pour and finish, which the DIY vs. contractor guide discusses.
Sources and Method
Base requirements are from the International Residential Code: R506.2.1 (fill), R506.2.2 (base course and its exception for well-drained soils), R506.2.3 (vapor retarder) and R405.1 (soil classification). Base preparation guidance follows ACI 302.1R and the Portland Cement Association's guidance on subgrades and subbases. Quantities use the standard volume formula with a 1.15 compaction factor and about 1.4 tons per cubic yard; prices are this site's working benchmarks (see our methodology) and vary by region.
Frequently Asked Questions
For a 4-inch compacted base, about 1.42 cubic yards (1.99 tons) per 100 sq ft, including 15% for compaction. A 20x20 slab needs about 5.7 yd³ and a 24x24 about 8.2 yd³.
About 8.0 tons for a 4-inch compacted base, or 11.9 tons for 6 inches, using about 1.4 tons per cubic yard of crushed stone.
A dense-graded crushed stone (crusher run, 3/4-minus or road base) is the most common choice because the fines help it compact into a firm, flat layer. Washed #57 stone drains better and suits wet sites. Avoid rounded pea gravel, which does not compact.
Sometimes. The IRC requires a 4-inch base course under floor slabs but makes an exception for slabs on well-drained or sand-gravel soils. On clay, silt or wet ground, or under a driveway, use a compacted gravel base.
Crushed stone runs about $35 to $55 per cubic yard or $25 to $40 per ton. A 4-inch base for a 20x20 slab costs roughly $199 to $312 in stone, plus delivery.
Calculate Your Concrete Project Budget
Use our comprehensive concrete slab cost calculator to get a complete line-item breakdown of materials, labor, reinforcement, and base preparation.
Launch Concrete Slab Calculator →