How much gravel should go under a concrete slab?

Four inches of compacted aggregate is a common residential starting point, but soil, drainage, frost, loading, and local specifications can require a different section.

Why does the calculator add a compaction allowance?

Loose aggregate occupies more volume than the same material after compaction. The allowance increases the compacted design volume to a practical delivered quantity.

How many tons are in a cubic yard of gravel?

A planning figure of about 1.3–1.5 tons per cubic yard is common for crushed aggregate, but the actual value varies with stone type, gradation, and moisture. Use the quarry's conversion for ordering.

Gravel Base Calculator for Concrete Slab

Calculate compacted gravel base for a concrete slab from length, width, and finished depth. Estimate loose cubic yards, tons, compaction allowance, coverage, and material cost.

Compacted Base Dimensions

Enter the finished compacted depth; the allowance converts it to a loose order quantity.

Length

Width

Compacted depth

Compaction allowance

Material density

tons/yd³

Confirm with the quarry; aggregate density varies.

Price

$/ton

Order quantity

2.84yd³ loose

Estimated weight

3.98tons

Compacted volume

2.47yd³

Compacted volume

66.7ft³

Coverage area

200ft²

Material estimate

$179before delivery and tax

Gravel Base Volume Formula

Calculate the compacted volume first, then increase it for settlement and compaction to obtain the loose delivered quantity. Convert yards to tons with the supplier's density.

Loose order quantity

loose yd³ = length × width × (compacted depth ÷ 12) ÷ 27 × (1 + compaction allowance)

Aggregate moisture, gradation, source, and delivery method affect density and compaction. Confirm the tons-per-yard figure with the supplier.

Preparing a Base for a Concrete Slab

Remove organic soil, mud, and uncontrolled fill before placing aggregate.
Use a compactable, well-graded material specified for slab base—not decorative rounded gravel unless approved.
Place and compact in lifts appropriate for the equipment and material.
Maintain drainage and verify final elevation before setting reinforcement and forms.
Use geotextile or stabilization where the soil and design require it.