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4 in compacted for floors, garages and patios, 6 in for driveways or clay. Length × width × depth (in) ÷ 324 = compacted cubic yards; add 20% for compaction and multiply by about 1.5 tons per cubic yard of crusher run. A 10 × 12 ft slab takes 1.78 cu yd, about 2.7 tons.
Put 4 inches of compacted gravel under a concrete slab — house floor, garage, patio, walkway or shed pad — and 6 inches under a driveway or any slab on clay. The 2021 IRC (R506.2.2) calls for a 4 in base course of clean graded sand, gravel, crushed stone, crushed concrete or slag passing a 2 in sieve under floor slabs, except on well-drained Group I soils, and R506.2.3 adds a 6-mil polyethylene vapor retarder lapped 6 in under heated space. To size it, multiply length × width × depth in inches and divide by 324 for compacted cubic yards, add about 20% for compaction, and multiply by the stone's weight: about 1.5 tons per cubic yard for crusher run, 1.4 for #57 stone. A 10 × 12 ft slab on a 4 in base is 1.48 cu yd compacted, 1.78 cu yd to order, about 2.7 tons; a 24 × 24 ft garage takes about 12.8 tons. Crusher run compacts into the firmest pad; #57 stone drains better; pea gravel rolls and makes a poor compacted base. At $22–$40 a ton delivered, a 20 × 20 ft slab base costs about $196–$356 in stone.
Short answer: 4 in of compacted crusher run under house floors, garages, patios, walks and shed pads, 6 in under driveways or on clay. Cubic yards = length × width × depth (in) ÷ 324; order about 20% more for compaction and multiply by about 1.5 tons per cubic yard. A 10 × 12 ft slab on a 4 in base takes about 2.7 tons.
| Slab | 4 in base: order | Tons | 6 in base: order | Tons |
|---|---|---|---|---|
| 10 × 10 ft (100 sq ft) | 1.48 cu yd | 2.2 | 2.22 cu yd | 3.3 |
| 10 × 12 ft (120 sq ft) | 1.78 cu yd | 2.7 | 2.67 cu yd | 4.0 |
| 12 × 16 ft (192 sq ft) | 2.84 cu yd | 4.3 | 4.27 cu yd | 6.4 |
| 16 × 20 ft (320 sq ft) | 4.74 cu yd | 7.1 | 7.11 cu yd | 10.7 |
| 20 × 20 ft (400 sq ft) | 5.93 cu yd | 8.9 | 8.89 cu yd | 13.3 |
| 24 × 24 ft (576 sq ft) | 8.53 cu yd | 12.8 | 12.80 cu yd | 19.2 |
| 30 × 40 ft (1,200 sq ft) | 17.78 cu yd | 26.7 | 26.67 cu yd | 40.0 |
| Slab | Concrete | Compacted base | Why |
|---|---|---|---|
| House floor slab (heated space) | 3½ in minimum (IRC R506.1), 4 in typical | 4 in | IRC R506.2.2 base course, plus a 6-mil vapor retarder (R506.2.3) |
| Patio, walkway, shed floor | 4 in | 4 in | Even support and drainage under light loads |
| Garage floor | 4 in (5–6 in for heavy trucks) | 4 in | Same as a floor slab; vapor retarder optional in an unheated garage |
| Driveway | 5–6 in | 4–6 in | Car and delivery-truck wheel loads |
| Any slab on clay or wet ground | as above | 6 in | Clay holds water and heaves; a deeper drained base spreads the load |
The IRC drops the base course only on well-drained or sand-gravel soils, Group I in Table R405.1 (GW, GP, SW, SP, GM, SM). On clay or silt, put the 4 in base in.
Length × width × depth in inches ÷ 324 gives compacted cubic yards; order about 20% more for compaction, then multiply by about 1.5 tons per cubic yard for crusher run. A 10 × 12 ft slab on a 4 in base takes 1.48 cu yd compacted, 1.78 cu yd to order — about 2.7 tons. A 24 × 24 ft garage takes about 12.8 tons at 4 in.
4 inches compacted for house floors, garages, patios, walks and shed pads — the 2021 IRC (R506.2.2) calls for a 4 in base course under floor slabs. Use 6 inches under driveways that see trucks and under any slab on clay or poorly drained soil.
Crusher run (also sold as #411, dense-grade aggregate or road base) is the usual choice because its fines compact into a hard, flat pad. Clean #57 stone drains better and breaks capillary moisture, but it does not lock up the same way and needs a vapor retarder or a thin choke layer on top. The IRC allows clean graded sand, gravel, crushed stone, crushed concrete or slag passing a 2 in sieve.
Sometimes. The IRC drops the base course where the slab sits on well-drained, sand-gravel soils (Group I in Table R405.1: GW, GP, SW, SP, GM, SM). On clay, silt or organic soil, skipping the base leaves the slab on ground that holds water and moves, which is how slabs crack and settle.
Both are allowed as the IRC base course. Gravel or crusher run gives a stiffer, better-draining base and is the default for driveways and garages; clean sand is easier to fine-grade to a flat surface and is common under house slabs over a vapor retarder. On wet sites use gravel.
It is not a good compacted base: the stones are rounded, so they roll under a plate compactor and under the screed boots instead of locking together. Pea gravel can serve as a drainage layer, but most builders put crusher run or #57 stone under a slab.
Under a heated floor, yes: IRC R506.2.3 asks for a 6-mil polyethylene (or approved) vapor retarder with joints lapped at least 6 in between the slab and the base. Garages, unheated sheds and accessory buildings, driveways, walks and patios are exempt.
Crusher run runs about $22–$40 a ton delivered and #57 stone $30–$55. A 4 in base for a 20 × 20 ft slab is about 8.9 tons of crusher run, roughly $196–$356 before any delivery minimum.
Converting a supplier's quote? The cubic yards to tons gravel calculator uses each stone's weight.
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