How to Calculate Subbase Gravel and Sand for Pavers and Patios

A paver patio or walkway is only as durable as the foundation beneath it. Here is how to accurately calculate excavation depth, crushed stone subbase, and bedding sand with real-world compaction factors.

The most common failure in DIY paving projects—sunken blocks, pooling water, and shifting stones—almost never stems from the pavers themselves. It happens because the subbase was too shallow, poorly compacted, or laid on uneven subsoil. A durable patio requires a structured cross-section: compacted native subgrade, a thick layer of crushed stone aggregate (such as 3/4-inch minus or MOT Type 1), a sharp 1-inch bedding sand layer, and finished pavers locked with polymeric joint sand.

1. Sizing Your Layers and Excavation Depth

Before ordering materials, determine the total excavation depth needed. For standard pedestrian walkways and garden patios, aim for a 4 to 6 inch (100 to 150 mm) compacted gravel subbase. If the area will bear vehicular traffic (such as a driveway), increase the gravel base to 8 to 12 inches (200 to 300 mm). On top of the gravel sits exactly 1 inch (25 mm) of bedding sand, followed by the thickness of your paving stones (typically 2 to 2.5 inches / 50 to 60 mm).

Add these layers together to determine your total dig depth: for a standard patio, that means excavating roughly 7 to 9 inches (180 to 230 mm) below the intended finished surface level. Always excavate 6 inches beyond the patio perimeter on all sides to provide lateral support for edge restraints.

2. The Compaction Factor: Why Pure Math Leaves You Short

Crushed aggregate contains jagged stone fragments and fine rock dust designed to lock together when compacted with a heavy vibratory plate compactor. During compaction, the stone loses volume as air voids collapse. Loose crushed stone compacts by approximately 20% to 25%.

To calculate the base volume accurately, multiply the patio area by the desired compacted depth, then add a 20% compaction allowance: Length (ft) × Width (ft) × (Depth in inches / 12) ÷ 27 × 1.20 = Cubic Yards of Gravel. Because aggregate is sold by weight at quarries, convert cubic yards to tons by multiplying by roughly 1.4 to 1.6 tons per cubic yard (or 1.8 to 2.0 tonnes per cubic meter).

3. Bedding Sand: Coarse vs Fine

A frequent beginner mistake is using fine playground sand, mason sand, or stone dust for the bedding layer. Fine sand holds moisture, washes away through joints during heavy downpours, and turns into an unstable slurry. Always use coarse, angular, washed concrete sand (ASTM C33 specification).

Keep the bedding sand layer at an even 1 inch (25 mm) thickness across the entire patio. Screed it over the compacted gravel base using two 1-inch conduit pipes as guide rails and a straight 2x4 board. Do not compact the bedding sand before laying the pavers; the sand compacts into the paver joints when you run the plate compactor over the finished stones.

Summary

A solid foundation requires 4 to 6 inches of compacted crushed stone, a 20% compaction buffer, and a uniform 1-inch bed of washed concrete sand. Ensure your base slopes gently away from structures at a rate of 1/4 inch per foot (approx. 2%) to shed rainwater. Use our Paver Calculator to estimate brick and slab counts, and our Gravel Calculator to order exact tonnage without costly mid-project shortages.