How Much Gravel Under a Concrete Slab? Easy Base Guide

A slab, shed or paver patio is only as good as what is underneath it. Working out how much gravel under a concrete slab you need is simple once you know the depth and how much the stone compacts. This guide covers the depths most builders use, the best base gravel, and real examples you can copy.

How Thick Should the Gravel Base Be?

Project	Base depth	Best gravel
Concrete patio or walkway slab	4 in	Crusher run, compacted
Garage or driveway slab	4 to 6 in	Crusher run, compacted
Shed pad (gravel only)	4 to 6 in	Crusher run, or #57 for drainage
Paver patio or walkway	4 to 6 in + 1 in sand	Crusher run / paver base
Paver driveway	8 to 12 in + 1 in sand	Crusher run / paver base
Hot tub pad	4 to 6 in	Crusher run, compacted and level

The gravel layer is called the base course. It spreads the load, stops the ground underneath from moving, and lets water drain away instead of freezing and lifting the slab. On clay or wet ground, go to the deeper end of each range. Getting how much gravel under a concrete slab right is what keeps it from cracking.

How to Calculate Gravel Under a Slab

• Measure the slab and add 6 to 12 inches on every side, because the base should extend past the edges.
• Multiply length x width x depth (in feet) to get cubic feet.
• Divide by 27 for cubic yards.
• Multiply by 1.5 for tons of crusher run (or 1.35 for #57 stone).
• Add about 25% for compaction and waste.

Gravel per 100 Square Feet

Base depth	Cubic yards	Tons (crusher run)	Order with 25% extra
4 in	1.23	1.85	about 2.3 tons
6 in	1.85	2.78	about 3.5 tons
8 in	2.47	3.70	about 4.7 tons
12 in	3.70	5.56	about 6.9 tons

5 Worked Examples

Project	Base area and depth	Cubic yards	Tons with 25% extra
10 x 10 ft patio slab	10 x 10 ft, 4 in	1.23	about 2.3
20 x 20 ft garage slab	20 x 20 ft, 4 in	4.94	about 9.4
8 x 10 ft shed	10 x 12 ft pad, 4 in	1.48	about 2.8
12 x 16 ft paver patio	12 x 16 ft, 4 in base	2.37	about 4.5 (+0.6 yd sand)
20 x 40 ft paver driveway	20 x 40 ft, 8 in base	19.75	about 37

Whatever the project, the question of how much gravel under a concrete slab or pad you need always comes back to area x depth, plus extra for compaction.

Crusher Run or #57: Which Base Gravel?

• Crusher run (crush and run, road base, #411, MOT Type 1 in the UK) has stone dust mixed in. It compacts into a hard, flat surface and is the standard choice under slabs, pavers and sheds. It is the usual answer when people ask how much gravel under a concrete slab and which kind.
• #57 stone is clean, 3/4 inch stone with no dust. It drains much better but does not lock together as tightly. Use it under slabs on wet ground or as a drainage layer under a shed.
• Avoid pea gravel and river rock under anything that must stay level. Round stones roll and shift.

Once you know how much gravel under a concrete slab you need, choose the stone. For more on picking stone, see which gravel is best. To convert between yards and tons for a quote, see how many tons in a yard of gravel.

Tips for a Solid Base

• Remove topsoil and roots first, down to firm ground.
• Compact in lifts of 2 to 3 inches with a plate compactor, wetting the gravel lightly.
• Check the level with a long straightedge before the concrete or pavers go down.
• Order extra. Running short with the compactor hired is expensive. Most of the shortage comes from compaction, not measuring. When in doubt about how much gravel under a concrete slab to buy, round up.

Calculate Your Base

Now you know how much gravel under a concrete slab, shed or paver patio to plan for. For your exact size, the calculator works out yards and tons for crusher run or #57, with a compaction allowance built in.