Gravel Base Guide: Calculating Depth, Yardage, and Tonnage
A compacted gravel base under a concrete slab does two important jobs: it provides drainage so water doesn’t pool under the concrete, and it creates a stable, evenly-compactable surface so the slab doesn’t settle unevenly and crack. This guide covers how much base depth you need and how to convert that into the cubic yards or tons you’ll order from a supply yard.
How Deep Should the Gravel Base Be?
A 4-inch compacted gravel base is standard for patios, walkways, and sheds — light-duty applications that don’t see vehicle traffic. Driveways and any slab expecting vehicle traffic should use 6 inches or more of compacted base, since the additional depth better distributes the weight of vehicles and reduces the risk of settling.
Calculating Volume
The math uses the same volume formula as a concrete slab: length × width × depth. Add a waste factor — typically around 10% — to account for compaction settling and uneven excavation, both of which mean the raw calculated volume rarely matches exactly what ends up needed on site.
Converting Cubic Yards to Tons
Landscape supply yards often quote bulk gravel in cubic yards, but it’s frequently sold and hauled by the ton, especially for larger deliveries. The tonnage conversion uses a typical density of roughly 2,700 lb per cubic yard for compacted crushed stone or road base — a reasonable planning estimate for most 3/4 inch minus gravel. Exact weight varies with the specific material and how wet it is, so always confirm with your supplier before finalizing an order, especially on larger jobs where the difference adds real cost.
Compacting Before the Pour
Gravel should be compacted in lifts (layers) with a plate compactor before the concrete pour. Uncompacted gravel will settle unevenly under the slab over time and can cause cracking — compaction is not an optional step, even on smaller residential jobs.
Worked Example
A 10 ft × 10 ft patio, 4 in gravel depth, 10% waste factor.
Volume: 10 × 10 × (4/12) = 33.3 cu ft.
With waste: 33.3 × 1.10 = 36.7 cu ft.
Cubic yards: 36.7 ÷ 27 = 1.36 cu yd.
Tonnage (2,700 lb/cu yd): 1.36 × 2,700 = 3,672 lb ≈ 1.84 tons.
Common Mistakes to Avoid
- Using only 4 inches of gravel base under a driveway that needs 6 inches or more for vehicle loads.
- Skipping compaction, which lets the base settle unevenly under the slab after the pour.
- Assuming the standard 2,700 lb/cu yd density applies to every gravel type without checking with the supplier.
- Not adding a waste factor for compaction settling and uneven excavation.
Related Guidence
Frequently Asked Questions
How deep should the gravel base be under a concrete slab?
A 4-inch compacted gravel base is standard for patios, walkways, and sheds. Driveways and any slab expecting vehicle traffic should use 6 inches or more of compacted base.
How much does a cubic yard of gravel weigh?
Compacted crushed stone or road base typically weighs around 2,700 lb (about 1.35 tons) per cubic yard, though this varies by material type and moisture content.
Do I need to compact the gravel base before pouring concrete?
Yes — the gravel should be compacted in lifts (layers) with a plate compactor before the pour. Uncompacted gravel will settle unevenly under the slab and can cause cracking.
Try the Concrete Footing Calculator
Ready to run your own numbers? Use the free Gravel Base Calculator on our site — just enter your measurements and get an instant, accurate estimate.
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