Gravel Base Guide: Calculating Depth, Yardage, and Tonnage

Introduction

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 may need and how to convert that into the cubic yards or tons you’ll order from a supply yard.

For related project planning, see our concrete calculators page, slab guide , and driveway cost guide .

How Deep Should the Gravel Base Be?

A compacted aggregate base is commonly used beneath concrete slabs, but the required material and thickness depend on soil conditions, drainage, climate, expected loads, local practice, and project specifications.

Important: These are common ranges, not universal requirements. Always verify the required base depth with your project specifications or a qualified professional.

Calculating Volume

The math uses the same volume formula as a concrete slab: length × width × depth. Add a waste allowance — 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.

Formula:

Length × Width × Depth = Cubic feet
Cubic feet ÷ 27 = Cubic yards
Cubic yards × allowance = With allowance

For help with this calculation, use the Gravel Base Calculator .

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.

Illustrative density: Compacted crushed stone or road base typically weighs around 2,700 lb (about 1.35 tons) per cubic yard — a reasonable planning estimate for most 3/4 inch minus gravel. Exact weight varies with the specific material, moisture content, and compaction. Always confirm with your supplier before finalizing an order.

Formula:

Cubic yards × 2,700 lb/cu yd = Total pounds
Total pounds ÷ 2,000 = Tons

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.

Compaction tips:

• Compact in 4-inch lifts

• Use a plate compactor for most slabs

• Dampen the gravel slightly for better compaction

• Check for a stable, level surface before pouring

Worked Example

The project:

• 10 ft × 10 ft patio

• 4 inches gravel depth

• 10% allowance

Step 1: Volume

10 × 10 × (4/12) = 33.3 cu ft

Step 2: Add allowance

33.3 × 1.10 = 36.7 cu ft

Step 3: Cubic yards

36.7 ÷ 27 = 1.36 cu yd

Step 4: Tonnage (illustrative density)

1.36 × 2,700 = 3,672 lb ≈ 1.84 tons

Confirm density with your supplier.

Common Mistakes to Avoid

• Using insufficient base depth for the expected 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 allowance for compaction settling and uneven excavation

Important Limitations

Estimates Are Not Construction Specifications

This guide provides estimating methodology. Actual base requirements depend on soil conditions, loads, and local requirements.

For soil conditions:

Poor soil conditions may require additional base depth or geotextile fabric. Consult a qualified professional for site-specific recommendations.

Frequently Asked Questions

How deep should the gravel base be under a concrete slab?
A compacted aggregate base is commonly used, but required thickness depends on soil conditions, loads, and project specifications. Common ranges are 4–6 inches for many residential applications. Always verify with your project requirements.

How much does a cubic yard of gravel weigh?
Compacted crushed stone or road base typically weighs around 2,700 lb (1.35 tons) per cubic yard, though this varies by material type and moisture content. Confirm with your supplier.

Do I need to compact the gravel base before pouring concrete?
Yes — the gravel should be compacted in lifts with a plate compactor before the pour. Uncompacted gravel will settle unevenly and can cause cracking.

What type of gravel is best for a base?
3/4 inch minus crushed stone or road base (class 5) is commonly used. The appropriate material depends on local availability and project requirements.

Can I use sand instead of gravel?
Sand is sometimes used, but gravel provides better drainage and load distribution. The appropriate base material depends on soil conditions and project requirements.

How much gravel do I need for my project?
Use the Gravel Base Calculator for an instant estimate.

Ready to calculate your gravel base? Use the free Gravel Base Calculator — just enter your measurements and get an instant estimate.

Plan Every Layer of Your Concrete Project

A proper gravel base is essential for slab longevity, but there’s more to concrete estimating than just the base layer. Our concrete calculators hub includes 8 free tools for every aspect of concrete work — slabs, footings, columns, driveways, rebar, mix ratios, and stairs. Use them together to build a complete material list for your next project.