For a 12 × 12-foot patio, the cited QUIKRETE project’s example of a gravel layer extending 6 inches beyond every side makes a 13 × 13-foot base. At its 5-inch loose depth, the calculated quantity is 2.608025 cubic yards before any separate allowance.1 That is one project scenario, not the required base for every slab. Tons need the supplier’s density for the actual material and volume state.
Measure the specified base footprint and depth first. Use the gravel calculator for volume and supplier-density conversion. Concrete is separate: use the concrete calculator for inside form dimensions and concrete thickness.
The volume formula
Base area (sq ft) = (slab length + 2 × extension) × (slab width + 2 × extension). All three lengths must be in feet. For a 6-inch extension on each side, add 1 foot to each overall dimension, not 6 inches.
Base volume (cu yd) = base area × depth (in) / 324. The depth must match the state being estimated: installed compacted depth for installed volume, or a verified loose depth for loose volume.
Loose and compacted volume are different
QUIKRETE’s patio project describes about 5 inches loose tamped to 4 inches and an extension at least 6 inches beyond the project area.1 For that geometry, the loose-to-compacted volume ratio is 5 / 4 = 1.25. This is a ratio calculated from that example, not a universal gravel compaction factor. Material gradation, moisture and placement requirements differ.
If your plan gives only a compacted thickness, calculate that installed volume, then ask the supplier or project designer how much loose material is needed. Do not automatically multiply every base by 1.25. Keep any separately chosen handling allowance distinct from the loose-to-compacted conversion, so you do not add it twice.
Chart: the cited patio scenario
Assumes a 6-inch extension on all sides, 5 inches loose and 4 inches compacted as described in the QUIKRETE project. No separate allowance or supplier order rounding. These are quantity comparisons, not base specifications for every size or use.
| Slab footprint | Base area (sq ft) | 4 in compacted (cu yd) | 5 in loose (cu yd) |
|---|---|---|---|
| 8 × 8 ft | 81 | 1.000000 | 1.250000 |
| 10 × 10 ft | 121 | 1.493827 | 1.867284 |
| 10 × 12 ft | 143 | 1.765432 | 2.206790 |
| 12 × 12 ft | 169 | 2.086420 | 2.608025 |
| 12 × 16 ft | 221 | 2.728395 | 3.410494 |
| 20 × 20 ft | 441 | 5.444444 | 6.805556 |
Depth sensitivity: a 13 × 13-foot base
Same footprint, different selected depths. This table shows geometry only. Label the depth as loose or compacted in your own takeoff; the formula cannot determine that state for you.
| Selected depth | Volume (cu yd) |
|---|---|
| 3 in | 1.564815 |
| 4 in | 2.086420 |
| 5 in | 2.608025 |
| 6 in | 3.129630 |
| 8 in | 4.172840 |
Three checked examples
1. The 12 × 12 patio scenario
Base area = (12 + 1) × (12 + 1) = 169 sq ft. Five-inch loose quantity = 169 × 5 / 324 = 2.608025 cu yd. Four-inch compacted geometry = 2.086420 cu yd. Using only the slab’s 144 sq ft at 5 inches gives 2.222222 cu yd, omitting 0.385802 cu yd of the extended loose layer. The extended gravel footprint does not enlarge the concrete slab.
2. 10 × 12 ft, plus an example allowance
With the same 6-inch extension, base area = 11 × 13 = 143 sq ft. At 5 inches loose: 2.206790 cu yd. An explicitly chosen 10% handling allowance makes 2.427469 cu yd. If, and only if, the supplier accepts quarter-yard increments, this rounds up to 2.5 cu yd. Neither 10% nor quarter-yard ordering is prescribed by the patio source.
3. Convert using a hypothetical supplier density
For the 12 × 12 scenario’s unrounded loose quantity, suppose a supplier quotes 2,800 lb per loose cubic yard for the chosen material. This is an example input, not a sourced gravel density. Weight = (169 × 5 / 324) × 2,800 / 2,000 = 3.651235 US short tons. A density for compacted material must not be applied to loose volume without a valid conversion.
Which material and thickness?
QUIKRETE calls for compactible gravel in its patio project.1 Concrete Network distinguishes subgrade (soil), subbase and base course, and describes different material needs for those layers.2 The terms are not always used consistently. Confirm the intended layer, aggregate specification and compacted thickness rather than buying by a generic stone name alone.
Concrete Network discusses a combined subbase/base thickness of at least 4 inches, with greater thickness where the engineer considers it needed.2 This article is explanatory guidance, not proof that a four-inch layer suits your garage, driveway, shed, soil or loads. The QUIKRETE project also includes a frost-footing exception.1 Resolve those project requirements before ordering or excavating.
Base depth is not excavation depth
The QUIKRETE patio layout uses a particular 4-inch compacted base and 3.5-inch concrete section, giving a 7.5-inch section measured from its slope line.1 Do not treat 7.5 inches as a universal dig depth. Actual excavation depends on finished elevations, specified layers, existing grade and any other required components. Loose material depth is not the finished section depth.
Before ordering
- Confirm the actual base footprint, including each required extension and any irregular regions.
- Label every volume as loose or compacted. Confirm the conversion with the actual material supplier or project designer.
- Use a supplier density matched to that same volume state; confirm US short tons versus metric tonnes.
- Check delivery increments, minimum quantities, access and fees. Round only after the chosen conversion and allowance.
- Keep concrete, gravel, excavation and disposal quantities separate. Check soil, drainage, frost and load requirements before treating any example as a design.
FAQ
How much gravel under a 10 × 10 patio?
For the cited scenario only: an 11 × 11-foot base at 5 inches loose is 1.867284 cu yd; at 4 inches compacted, its installed geometry is 1.493827 cu yd. A different extension or specified section changes the quantity.
Should I order compacted cubic yards?
First establish how the supplier quotes the material. Installed geometry and delivered loose volume are not interchangeable. Ask for the material-specific loose quantity or conversion.
Does all gravel shrink by 20%?
No universal shrinkage is established here. The cited 5-to-4-inch example is a 20% depth reduction from its loose starting depth, or 25% more loose volume than compacted volume. Those percentages have different denominators.
Can this tell me whether concrete can go directly on soil?
No. Concrete Network discusses soil and optional additional layers in general, but that does not establish suitability at your site. Resolve soil preparation and the actual slab section separately.
Related tools
Gravel calculator | Concrete calculator | Slab bag chart | 20 × 20 slab yardage
Sources
- QUIKRETE: Patios (project-specific base, extension and section example)
- Concrete Network: Subgrades and Subbases (layer terminology, materials and support discussion; not a site design)