Landscaping + Concrete planning guide
Gravel Base Under a Concrete Slab
Measure and order the gravel base layer that sits under a concrete slab. Understand base depth, compaction, and drainage requirements.
Published 2026-07-16 · Updated 2026-07-16 · BuildMeasure Editorial Team
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Use the Gravel CalculatorA gravel base layer sits under the concrete slab
Before concrete is poured, a prepared subgrade is often topped with a compacted gravel or crushed-stone layer. This base layer provides drainage, a stable bearing surface, and a level working platform for concrete placement.
The base layer is not optional structural concrete—it is a bed of stones compacted to support the slab. The thickness, stone size, and compaction method are set by the slab design and local building code.
Typical base layer depth is 4 to 6 inches compacted
Most residential concrete slabs use a 4-inch compacted base layer. Some soil conditions or local codes require 6 inches. A structural engineer or local building official will specify the depth for your project.
Do not assume or reduce the base depth on your own. Insufficient base can lead to slab settlement, cracking, or frost heave in cold climates.
Calculate gravel base volume using the same slab dimensions
The gravel base covers the same footprint as the slab itself. Measure the slab length and width in feet, then the base depth in inches (4 or 6 inches, typically).
Gravel base volume = slab length × slab width × (base depth ÷ 12), then convert to cubic yards by dividing by 27.
For a 20 ft × 15 ft slab with a 4-inch base: 20 × 15 × (4 ÷ 12) = 100 cubic feet = 3.7 cubic yards.
Account for settling when ordering loose gravel
Loose gravel settles 10–20% when compacted, depending on stone size and compaction force. Order 10–15% extra to account for settling. A 3.7 cu yd base with settling allowance becomes 3.7 × 1.15 ≈ 4.25 cu yd (order 4 or 4.5 cu yd).
Confirm the compaction method with your contractor or concrete supplier: hand-tamped, walk-behind compactor, or vibratory plate. Each method results in different settling.
Stone size and drainage
Base-layer stone is typically ¾-inch crushed stone (also called compacted aggregate base or CA base). Larger stones do not compact well and leave voids. Smaller, dusty material compacts too densely and can trap water.
Confirm the stone type with your concrete supplier or building code. Using the wrong material can compromise drainage and slab performance.
Cross-cluster planning: base layer feeds the concrete slab project
Gravel base is step 1 of a concrete slab project. Once the base is prepared and compacted, you order concrete (measured from the completed slab size and thickness). Measure and order gravel first, prepare the base, then proceed to the concrete calculation.
Use the concrete-slab calculator to estimate concrete after the gravel base is in place.
Field preparation checklist
Before ordering gravel, confirm the slab footprint and base depth with your plans or contractor.
- Slab location and size (length and width in feet).
- Subgrade condition: firm soil, compacted fill, or disturbed ground that needs additional preparation.
- Base depth required by the local code or structural design (typically 4 or 6 inches).
- Confirmation of stone type (crushed stone, recycled asphalt, pea gravel, etc.).
- Drainage direction and any low spots that need grading before base placement.
- Site access for delivery truck and compaction equipment.
- Schedule: base must be prepared and compacted before the concrete supplier arrives.
Worked example: 12 × 10 ft patio with 4-inch base
Patio size: 12 ft × 10 ft. Base depth: 4 inches (typical for residential). Loose gravel volume: 12 × 10 × (4 ÷ 12) = 40 cubic feet = 1.48 cubic yards. With 15% settling allowance: 1.48 × 1.15 = 1.70 cubic yards. Order 1.75 or 2 cubic yards depending on the supplier's increment.
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Sources and limits
Check the project-specific details
- International Building Code (IBC) concrete slab requirements — Code requirements for subbase preparation, compaction, and drainage.
- Portland Cement Association (PCA) slab design guide — Guidance on subgrade preparation and base-layer specifications for residential slabs.
- American Society for Testing and Materials (ASTM) compaction standards — Standards for gravel and crushed-stone base preparation, compaction, and verification.
Review status: Formulas and conversions covered by automated tests; measurement practice pending human trade review.
This guide supports planning only. It does not specify structural design, code compliance, or a supplier quotation.