Updated June 17, 2026
Most residential concrete slabs need at least 4 inches of compacted crushed stone or gravel beneath them. To find the volume, multiply your slab's length x width x 0.333 (for 4 inches) and divide by 27 to get cubic yards, then add 10 to 15 percent for compaction loss.
Gravel coverage by depth
| Depth | 1 cubic yard covers | 1 ton covers (approx) |
|---|---|---|
| 2 in | 162 sq ft | ~100 sq ft |
| 4 in | 81 sq ft | ~50 sq ft |
| 6 in | 54 sq ft | ~33 sq ft |
| 8 in | 40 sq ft | ~25 sq ft |
Gravel weighs roughly 1.4 to 1.6 tons per cubic yard, so a cubic yard and a ton are not interchangeable. Suppliers often sell by the ton; divide your cubic yard estimate by 0.65 to convert to tons, then round up. Use the Concrete Calculator to get the concrete volume, and apply the same formula to your gravel depth.
What type of gravel is best under concrete?
Compactable crushed stone (often called road base, crusher run, or Class II base) is the standard choice. It compacts firmly under a plate compactor, drains well, and resists frost heave better than rounded pea gravel. Angular crushed stone packs tighter and stays put once compacted. Pea gravel is not recommended under slabs because it does not interlock and can shift.
How much gravel under a 10 x 10 slab?
A 10 x 10 ft area at 4 inches deep is 10 x 10 x 0.333 = 33.3 cu ft / 27 = about 1.23 cubic yards. Add 15 percent for compaction: order about 1.4 yards, or roughly 2 tons from a supplier that sells by weight.
What is the 4-2-1 rule for concrete?
The 4:2:1 rule is a sub-base recipe: 4 inches of compacted crushed stone at the bottom, 2 inches of coarse sand in the middle, and 1 inch of fine sand on top before the concrete. It is commonly used in areas with poor drainage or freeze-thaw cycles, because the layered approach provides excellent support and drainage. Standard residential jobs often simplify to just 4 inches of compacted crushed stone without the layered sand.
Sub-base depth guidelines by climate and soil type
Soil type and climate both affect how deep your sub-base needs to be. Sandy, well-drained soils in mild climates can get by with 4 inches. Clay soils in cold climates, where frost heave is a real risk, often require 6 inches of compacted base material and a layer of geotextile fabric to prevent clay migration into the stone. In areas that see hard freezes (frost depth of 24 inches or more), some contractors use 8 inches of base and a vapor barrier to further isolate the slab from ground movement.
| Condition | Recommended base depth | Notes |
|---|---|---|
| Sandy, well-drained soil, mild climate | 4 in compacted stone | Minimum for residential |
| Mixed soil, moderate climate | 4 to 6 in compacted stone | Standard residential spec |
| Clay soil, cold climate (frost depth > 24 in) | 6 to 8 in + geotextile | Prevents frost heave |
| Heavy clay or organic soil | 6 to 8 in + possible over-excavation | May need engineer review |
| Commercial / heavy equipment pads | 8 to 12 in engineered fill | Requires compaction test |
Gravel quantity by common slab size
Here is a quick reference for how much gravel to order for common residential projects at a 4-inch base depth, including the 15 percent compaction allowance:
| Slab size | Base cu yd (4 in) | With 15% compaction | Tons (approx) |
|---|---|---|---|
| 10 x 10 ft | 1.23 | 1.41 | 2.0 |
| 12 x 12 ft | 1.78 | 2.05 | 2.9 |
| 20 x 20 ft | 4.94 | 5.68 | 8.1 |
| 24 x 24 ft | 7.11 | 8.18 | 11.7 |
| 10 x 30 ft driveway | 3.70 | 4.26 | 6.1 |
Compacting the sub-base properly
Loose gravel is not enough. The sub-base must be mechanically compacted with a plate compactor or jumping jack before any concrete is poured. Compact in lifts no deeper than 4 inches at a time; if you need 8 inches of base, compact the first 4 inches, add more material, then compact again. After compaction, the surface should feel solid with minimal deflection when you walk on it. If footprints sink more than a quarter-inch, the base needs more compaction. A poorly compacted base is the leading cause of slab settling and cracking in residential projects.
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Once you have your sub-base sorted, confirm your slab depth in how thick a concrete slab should be before ordering the concrete.