Gravel base calculator for under a concrete slab
Enter the slab size, the base depth from your plans and the stone to see how many tons to order for a compacted base.
Enter your measurements
More options: Waste
- Compacted volume in place
- 1.23 cu yd (33.3 cu ft)
- Loose volume (as delivered)
- 1.62 cu yd
- Base area
- 100 sq ft
10 ft × 10 ft = 100 sq ft × 4 in = 1.23 cu yd compacted
× 1.9 tons per compacted cu yd + 5% = 2.46 tons
Loose, as delivered: 2.46 tons ÷ 1.52 tons/cu yd = 1.62 cu yd
- Compact in layers of a few inches with a plate compactor, dampening crusher run so it packs. A soft or wet subgrade under the stone needs fixing first — gravel won’t bridge it.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much gravel under a concrete slab | About 2.98 tons of crusher run, 4 in compacted, for a 10×10 slab |
Your base at other depths
| Base depth (compacted) | Stone to order | Compacted volume in place | Loose volume (as delivered) |
|---|---|---|---|
| 2 in | 1.23 tons | 0.62 cu yd | 0.81 cu yd |
| 3 in | 1.85 tons | 0.93 cu yd | 1.22 cu yd |
| 4 in | 2.46 tons | 1.23 cu yd | 1.62 cu yd |
| 6 in | 3.69 tons | 1.85 cu yd | 2.43 cu yd |
| 8 in | 4.93 tons | 2.47 cu yd | 3.24 cu yd |
Most slabs on grade sit on a layer of compacted stone. It gives the concrete a firm, even bed, lets water drain away underneath, and makes it easier to get a slab of consistent thickness. How deep it should be depends on your soil, climate and plans; once you know, the stone is a volume and weight calculation.
How the base is calculated
compacted cu yd = (length + 2 × overhang) × (width + 2 × overhang) × depth (in) ÷ 12 ÷ 27 tons = compacted cu yd × tons per compacted yard × (1 + waste) loose cu yd = tons ÷ loose tons per yard
| Material | Tons per compacted cu yd | Tons per loose cu yd | Source |
|---|---|---|---|
| Crusher run / dense-graded base | ≈ 1.9 (1.75–2.1) | ≈ 1.5 | WisDOT conversion factors |
| #57 stone (vibrated / loose) | ≈ 1.47 | ≈ 1.29 | FHWA-HRT-15-034 unit weights |
Crusher run (also sold as DGA, road base or 21A depending on region) compacts into a hard, flat bed and is the usual choice under patios and sheds. Clean #57 stone drains better and is often used where water is a concern; it doesn’t pack as tight, so screed it carefully. The worked example is on how much gravel under a concrete slab.
Tips
- Strip sod and topsoil first — organic soil under a slab keeps settling.
- Compact in lifts of a few inches; a single thick layer packs only at the top.
- Set the top of the compacted base exactly one slab-thickness below the top of the forms. A base that is half an inch high anywhere makes a thin spot in the slab.
- Indoors and under heated buildings, plans often call for a vapor barrier or insulation over the base — follow them.
What you’ll need
- Hand tamperBudget price
Compacts gravel base in small areas and post holes. For a full slab base, rent a plate compactor.
See options on Amazon (opens in a new tab) - Square-point shovelBudget price
Pushes and places concrete, and loads sand and gravel. The flat blade spreads a pour better than a round-point shovel.
See options on Amazon (opens in a new tab) - Poly vapor barrier sheetingBudget price
Plastic sheeting under an indoor slab, and over a fresh slab to hold moisture in while it cures.
See options on Amazon (opens in a new tab) - Contractor wheelbarrowMid-range price
A 6 cu ft steel or poly tray with a pneumatic tire moves concrete from the mixer to the forms. Never load it to the brim with wet concrete.
See options on Amazon (opens in a new tab)
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Frequently asked questions
How much gravel do I need under a 12×12 slab?
At 4 in compacted, a 12 × 12 ft base is 1.78 cubic yards in place — about 3.5 tons of crusher run with 5% extra. Enter your own depth above.
Should I use crusher run or #57 stone under concrete?
Crusher run packs into a firm, flat bed and is the usual base for patios and sheds. Clean #57 drains better but doesn’t lock together as tightly. Follow your plans or ask your supplier what is used locally.
Why order more tons than the compacted volume suggests?
Loose stone takes up more room in the truck than it does once packed. Crusher run can lose 20–30% of its loose volume when compacted, so you order by weight for the compacted volume, not by loose yards.
Related guides
Sources
- Wisconsin DOT research report 0092-22-05 — Weight-volume relationships and conversion factors for soils and aggregates (2024) (opens in a new tab) — Compacted dense-graded base: 1.75–2.1 tons per cubic yard (Table 1.2).
- FHWA-HRT-15-034 — Strength characterization of open-graded aggregates for structural backfills (opens in a new tab) — No. 57 stone: 95.4 pcf loosest, 108.7 pcf vibrated dry unit weight (Table 7).
- Quikrete Concrete Mix (No. 1101) data sheet (opens in a new tab) — Yields: 40 lb ≈ 0.30, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675 cu ft; unit weight ≈ 140 lb/cu ft; for pours 2 in thick or more; 6–9 pints of water per 80 lb bag; control joints every 6–8 ft; cure 5 days at 70 °F or above, 7 days at 50–70 °F; protect from freezing for 48 hours.