Gravel Base for Concrete Slab Calculator

How the gravel base calculator works

The gravel base calculator takes the slab's length and width in feet, the gravel depth in inches, and produces both cubic yards and tons of crushed stone. The formula is straightforward: volume in cubic feet = length × width × (depth ÷ 12). Cubic yards = cubic feet ÷ 27. Tons = cubic yards × 1.35 (the standard conversion for crushed stone, which weighs about 2,700 lb per cubic yard).

A worked example: a 10×10 slab with a 4-inch gravel base. Volume = 10 × 10 × (4/12) = 33.3 cubic feet = 1.23 cubic yards. Tons = 1.23 × 1.35 = 1.66 tons. Order 2 tons to account for compaction — gravel settles 15–20% when tamped, so a 4-inch loose base becomes about 3.3 inches after compaction. Plan for 5 inches of loose gravel to end up with 4 inches compacted.

The 1.35 tons-per-yard conversion factor assumes "3/4-inch minus" crushed stone — the standard base material. Other materials have different densities: pea gravel is about 1.3 tons/yd³, clean crushed stone (no fines) is 1.4 tons/yd³, recycled concrete is 1.4 tons/yd³. The differences are small enough that 1.35 is a reasonable default for any crushed stone product.

Choosing the right gravel for a sub-base

The right material is "3/4-inch minus crushed stone" — also called Class 5, 21A, road base, or 3/4-inch crusher run, depending on your region. The "minus" means the stone includes fines (rock dust smaller than 3/8 inch) that compact tightly when tamped. The fines fill the voids between the larger stones, locking them into a rigid matrix that doesn't settle after compaction.

Avoid these materials for a sub-base: pea gravel (round, won't compact, settles under load), river rock (smooth surface, same problem), clean crushed stone (no fines, settles 10–15% over time), recycled asphalt (asphalt binder breaks down in concrete slabs), topsoil (organic, decomposes and settles). Any of these will cause your slab to crack within 5 years.

For poorly-drained soil (clay, hardpan), add a geotextile fabric between the subgrade and the gravel. The fabric prevents the gravel from sinking into the clay over time, which is a common cause of slab failure. Use a non-woven geotextile (Mirafi 180N or equivalent) at $0.25–0.50/sf. The fabric pays for itself by extending slab life from 10 years to 30+.

Compaction is critical. Loose gravel settles 15–20% when tamped — meaning a 4-inch loose base becomes 3.3 inches after compaction. To end up with 4 inches of compacted base, start with 5 inches of loose material. Compact in 2-inch lifts (layers) with a plate compactor ($40/day rental) — never try to compact 6+ inches of loose gravel in one pass, the bottom won't get tamped. Each lift takes 4–6 passes with the compactor.

Subgrade preparation and what the gravel estimate excludes

The gravel base sits on the subgrade — the natural soil at the bottom of your excavation. Good subgrade preparation is the difference between a slab that lasts 5 years and one that lasts 50. Remove all topsoil, organic material, and debris down to solid bearing soil (clay, sand, or gravel). If you encounter soft spots (peat, fill dirt, buried organic material), excavate deeper and backfill with compacted gravel.

Slope the subgrade away from any structures at 1/4 inch per foot (2% grade) so water drains away from the slab, not toward it. For patios and walkways, the slab should match the subgrade slope so the finished surface drains properly. For driveways and garage floors, slope toward a drain or toward the street.

What the gravel calculator excludes: geotextile fabric ($0.25–0.50/sf), plate compactor rental ($40/day), excavation of the subgrade ($200–800 depending on access), disposal of excavated soil ($200–500 per load), and a vapor barrier over the gravel for interior slabs ($0.10/sf for 6-mil poly). A 10×10 slab with 4-inch gravel base needs about $50–100 of fabric, $40 compactor rental, and $40–80 of gravel — materials only, before any labour.

For driveways, add 6 inches of gravel base under 5–6 inch concrete — about 3 tons per 100 square feet. The extra depth distributes vehicle loads over a wider area, preventing settling and cracking. For garage floors, 6 inches of gravel under 4-inch concrete is standard. For interior slabs (basements, shops), add a 6-mil vapor barrier over the gravel before pouring.

Delivery: bulk gravel is delivered by dump truck in 1–10 ton loads. Most quarries charge a delivery fee based on distance from the quarry — $50–150 typical. For small jobs (under 2 tons), bagged gravel at big-box stores is more convenient but expensive (4–5× the bulk price). For any job over 2 tons, order bulk delivery — the savings easily cover the delivery fee.

Common mistakes and how to avoid them

The most common gravel base mistake is using the wrong material. Pea gravel, river rock, and clean crushed stone (without fines) do not compact — they shift under load and settle over time. A slab on pea gravel will crack within 5 years. The correct material is 3/4-inch minus crushed stone (also called Class 5, 21A, or road base), which includes fines that lock the larger stones into a rigid matrix. The fines are not optional; they are what makes the base compactable.

A second mistake is inadequate compaction. Loose gravel settles 15-20% when tamped. A 4-inch loose base becomes 3.3 inches after compaction. To end up with 4 inches of compacted base, you must start with 5 inches of loose material and compact it in 2-inch lifts with a plate compactor. Each lift takes 4-6 passes. Skipping compaction to save 30 minutes means your 4-inch base is actually 3.3 inches, and the slab above it is more likely to crack.

Subgrade preparation is the third common error. The soil beneath the gravel must be solid bearing material (clay, sand, or gravel), not topsoil or fill dirt. Organic material decomposes and settles; a base on topsoil will drop and crack the slab. Excavate to solid soil, slope the subgrade 1/4 inch per foot for drainage, and remove all soft spots. On clay or poorly-drained soil, install a geotextile fabric between the subgrade and the gravel to prevent the gravel from sinking into the clay over time.

Vapor barrier placement is frequently wrong. For interior slabs (garages, basements, shops), a 6-mil or 10-mil polyethylene vapor barrier goes OVER the gravel, directly under the concrete. This prevents ground moisture from wicking up through the slab and causing efflorescence, mold, or flooring adhesive failure. The vapor barrier is not optional for interior slabs. For exterior slabs (patios, driveways), the vapor barrier is typically omitted — exterior slabs can breathe, and trapped moisture causes more problems than it solves.

Finally, do not underestimate the gravel depth. Four inches is the minimum for patios and walkways on well-drained soil; 6 inches for driveways; 8-12 inches on clay or poorly-drained soil. The extra depth distributes the load over a wider area of subgrade and prevents settling. On a 600-sf driveway, upgrading from 4 to 6 inches adds about $300-400 in material cost — a small fraction of the total and the difference between a 30-year driveway and a 10-year driveway.