How Much Gravel Under a Concrete Slab?

How much gravel you need under a concrete slab depends on the slab footprint, base depth, stone type, compaction, waste, drainage, soil, and local practice. For planning, many small slabs use about 4 inches of compacted gravel, but that is not a universal design rule.

Use the Gravel Base Calculator for Concrete for a full estimate, then use this guide to check the formula and supplier scope. If a contractor quote includes base prep, review it in the Concrete Quote Reviewer.

This page focuses specifically on gravel under concrete slabs: how to convert slab area and base depth into cubic yards, when to convert yards to tons, and which quote lines keep base prep from being hidden inside the pour.

Quick answer

For a rectangular slab, multiply slab length by width by gravel depth, divide by 12 and 27, then add waste. A 10 ft x 12 ft slab with 4 inches of gravel uses about 1.48 yd3 before waste, or about 1.63 yd3 with 10% waste. At 1.35 tons per yd3, that is about 2.2 tons.

This is a planning estimate only. Soil, drainage, frost, slab use, compaction, and code requirements should be confirmed by a qualified local professional.

Gravel under slab formula

gravel cubic yards = length ft x width ft x depth in / 12 / 27
order cubic yards = gravel cubic yards x (1 + waste percentage)
tons = order cubic yards x bulk density tons per yd3

If you do not know the stone density, ask the supplier. Many planning estimates use about 1.3 to 1.5 tons per yd3, but real material varies.

What depth should you enter?

Do not enter rough excavation depth as finished compacted gravel depth. A base that starts loose will compact lower.

Quote checklist

Ask whether the quote includes:

• Excavation and disposal.
• Gravel type and compacted depth.
• Compaction equipment and passes.
• Geotextile fabric if needed.
• Drainage correction.
• Final grade before forms.

For cost review, pair this with the Concrete Material Cost Breakdown.

Example ordering path

A practical gravel order starts with the finished slab area, not the rough hole. For a 12 ft x 16 ft shed or patio slab with 4 inches of compacted base:

12 x 16 x 4 / 12 / 27 = 2.37 yd3 before waste
2.37 x 1.10 = 2.61 yd3 ordered volume
2.61 x 1.35 = 3.52 tons at 1.35 tons/yd3

That order assumes the base area matches the slab. Some contractors extend the base beyond the concrete edge for drainage and form support. If the gravel pad is 13 ft x 17 ft instead of 12 ft x 16 ft, the same 4 inch depth becomes about 3.00 yd3 with waste, or about 4.05 tons at the same density.

Base prep items that change the quote

Do not compare a gravel-only material price with a contractor base-prep line that includes excavation, spreading, compaction, fine grading, and cleanup. Those are different scopes.

How to avoid ordering short

Confirm whether the supplier sells by cubic yard, ton, or full truckload. If the quote is by ton, ask for the density assumption used to convert from yards. If the quote is by yard, ask whether it is loose cubic yards or compacted-in place depth. Keep a small buffer for uneven subgrade, edges, and low spots, but avoid using gravel to hide major grade problems that should be excavated or repaired before forms are set.

Delivery and compaction handoff

Before ordering, decide where the truck can dump and how the base will be moved, spread, and compacted. A supplier may price only loose material dropped near the street, while a contractor may price delivery, machine spreading, laser grading, plate compaction, and cleanup. Those are different quotes.

Ask whether compaction happens in one lift or multiple thinner lifts. A deep base placed all at once can bridge over soft spots and settle later. Also confirm whether fabric, drainage stone, or a thicker edge base is needed at doors, driveways, ramps, or shed entries. If the base extends beyond the slab, calculate that wider rectangle separately so gravel, forms, and finish grades all match the same plan.

FAQ

How much gravel do I need under a concrete slab?

Multiply length by width by gravel depth, convert inches to feet, divide by 27, then add waste. For 10 ft x 12 ft at 4 inches, plan about 1.63 yd3 with 10% waste.

Is 4 inches of gravel enough under concrete?

Four inches is a common planning starting point for many small slabs, but soil, drainage, frost, load, and local code can change the correct base.

Should gravel be compacted before concrete?

Yes. The base should be compacted and graded before forms and concrete. Loose stone can settle and change slab support.

How many tons of gravel are in a cubic yard?

It depends on stone type and moisture. A planning range is often 1.3 to 1.5 tons per yd3. Ask the supplier for the material you are buying.

Does gravel cost include delivery?

Not always. Delivery, minimum load, fuel, and placement may be separate.

Next step

Estimate the base in the Gravel Base Calculator for Concrete, then compare base prep scope in the Concrete Quote Reviewer. Contractors can add base prep as a line item in the Concrete Proposal Kit.