Show the math
- Convert to base units: Length 12 ft; Width 12 ft; Base thickness 4 in = 0.3333 ft
- Loose volume = 12 × 12 × 0.3333 = 48 ft³ = 1.7778 yd³
- With 15% compaction allowance = 2.0444 yd³
- Weight = 2.0444 yd³ × 1.4 tons per yd³ = 2.86 US tons
How to measure
The base is wider than the slab. Plan on extending it a few inches beyond the form on each side so the edge has support, and add that to the length and width you enter. Base thickness is the compacted depth, commonly 4 in for patios and walks and more for driveways or soft soil.
Gravel is sold by the ton. The tons-per-yard figure changes with the stone type, size and moisture, so ask the supplier. The default 1.4 tons per cubic yard is a typical figure for loose crushed stone and is editable.
The gravel formula
The base is a slab-shaped volume: length × width × thickness. Convert to cubic yards by dividing by 27. Because loose gravel compacts, you order more than the compacted volume. The calculator adds a compaction allowance (15% by default) to cover this. Finally, convert to tons by multiplying cubic yards by the tons-per-yard figure.
Formula: yd³ = L × W × (T ÷ 12) ÷ 27 × (1 + compaction%). Tons = yd³ × tons per yd³.
Worked example: a 12 × 12 ft patio base, 4 in thick
- Convert to base units: Length 12 ft; Width 12 ft; Base thickness 4 in = 0.3333 ft
- Loose volume = 12 × 12 × 0.3333 = 48 ft³ = 1.7778 yd³
- With 15% compaction allowance = 2.0444 yd³
- Weight = 2.0444 yd³ × 1.4 tons per yd³ = 2.86 US tons
That is 2.86 tons (2.04 yd³ · 2.60 metric tonnes). Suppliers often add a delivery fee, so order the full amount in one load.
Base quantities for common slabs
| Slab | Cubic yards | US tons |
|---|---|---|
| 10 × 10 ft | 1.42 | 1.99 |
| 12 × 12 ft | 2.04 | 2.86 |
| 12 × 16 ft | 2.73 | 3.82 |
| 20 × 20 ft | 5.68 | 7.95 |
| 24 × 24 ft | 8.18 | 11.45 |
How thick should the base be?
Four inches of compacted crushed stone is a common base under patios, sidewalks and many slabs on grade. Driveways, garages and poor soil often call for 6 in or more. Frost-prone areas rely on a drained base to reduce heave. In cold regions a thicker base may be recommended. Follow the plans or your local practice. The base should be compacted in lifts of no more than about 4 in, using a plate compactor.
Stone type
Crushed stone with fines compacts well and locks together; round pea gravel does not compact as tightly. Base courses are usually graded crushed stone, sometimes called road base, dense-graded aggregate or crusher run. Name them to your supplier, not just “gravel”. The weight per yard is different for each.
Mistakes to avoid
- Ordering the compacted volume with no allowance.
- Making the base the same size as the slab.
- Using pea gravel and expecting it to compact.
- Skipping compaction; a loose base settles and cracks the slab.
- Not checking the delivery truck’s access and where the pile will go.
Delivery, spreading and compaction
Gravel arrives as a pile, usually by dump truck or bulk bag. A 20×20 ft base 6 in thick needs about 11.93 tons, which is 8.52 yd³ · 10.82 metric tonnes. Ask the supplier how the material is measured and how the truck will deliver. Plan where the pile will go, since moving a ton of gravel by wheelbarrow is hard work, and cover the pile if rain is expected.
Spread the stone in layers no thicker than about 4 in, wet it lightly and compact each layer with a plate compactor until it feels firm and no longer settles. Check the surface with a straightedge and string. Fix low spots with more stone, not with sand.
- Crushed stone with fines compacts best under slabs.
- Washed stone drains well but compacts less. It is used for drainage layers.
- Fabric over soft soils keeps fines from mixing with the base.
- Do not compact when the sub-grade is a mud pit. Fix drainage first.
Frequently asked questions
How many tons of gravel for a 10 × 10 slab base?
About 1.99 tons for a 4 in base with 15% compaction at 1.4 tons per yd³.
How many tons of gravel in a yard?
Around 1.4 tons for loose crushed stone, but it varies by material and moisture. Ask your supplier.
Why add a compaction allowance?
Loose stone compacts to a smaller volume. Ordering the final compacted volume would leave the base short.
How much gravel under a concrete slab?
Often 4 in of compacted crushed stone for a patio or sidewalk, and more for driveways or poor soil.
Should the base extend past the slab?
Yes. A few inches on each side supports the slab edge.
Can I use sand instead of gravel?
Sand alone does not drain well and can erode. A crushed stone base is usually the better choice.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.