How much gravel do I need for a compacted base under a slab?

Work out the compacted volume as area × depth — a 4 m × 3 m base at 100 mm (0.1 m) compacted depth is 4 × 3 × 0.1 = 1.2 m³. Then order more than that: loose material bulks up before it's compacted, so multiply by 1.2–1.3 to get the order quantity — about 1.5 m³ for this base.

Why do I need to order more gravel than the compacted volume I calculated?

Because gravel, crushed rock and road base take up more space loose than they do once a plate compactor or roller packs them down. If you order exactly the compacted volume, you'll run short once it's rolled — the material you tipped out simply won't reach the compacted depth you needed.

What compaction (bulking) factor should I use for crushed rock or road base?

1.2 to 1.3 is the typical working range — order 20–30% more loose material than the compacted volume you calculated. Denser, well-graded crushed rock sits toward the lower end (1.2); looser, more uniform material sits toward the higher end (1.3). If your supplier gives you a product-specific factor, use theirs.

How do I work out the loose-fill volume to order from a compacted depth?

Two steps. First, compacted volume = area × depth, in metres, for an answer in cubic metres. Second, multiply that by your compaction factor (1.2–1.3) to get the loose volume to actually order. Always order the loose figure, never the compacted one.

Is this the same volume method as a concrete slab?

Yes — area × depth is the same rectangular-prism sum used for concrete slab volume. The only extra step for gravel and base material is the compaction factor, because concrete is poured and set at roughly the volume you order, while granular fill compresses under compaction and needs extra loose volume to compensate.

In this section

Gravel & Base Volume: Compacted Depth vs Loose-Fill Order Quantity

Work out how much gravel or road base to order for a compacted sub-base — area × depth for the compacted volume, then a compaction (bulking) factor to get the loose-fill quantity to order.

The requirement

Every slab and most footings sit on a compacted gravel or road-base layer first. The volume method is the same one used for concrete slab volume — area × depth — but gravel has one extra step that concrete doesn’t: compaction.

Compacted volume = Area × Depth

That gives you the volume the base course needs to occupy once it’s spread and rolled or plate-compacted down. But that’s not what you order. Crushed rock and road base are dug, loaded and delivered loose, and loose material takes up more space than the same material packed down. Order the compacted figure and you’ll be short once the compactor’s been over it.

Order (loose-fill) volume = Compacted volume × compaction factor

The compaction factor is typically 1.2–1.3 — order 20–30% more loose material than the compacted volume you need. Where your material sits in that range depends on the product; a supplier’s own figure beats a general one.

Area — length × width of the base you’re covering, in metres.
Depth — the compacted thickness you need, in metres (mm ÷ 1000).
Compaction factor — 1.2 to 1.3, applied after the area × depth sum, not before it.

Work it out by hand

Say you’re prepping the base under a 4 m × 3 m slab, and it needs a 100 mm (0.1 m) compacted depth of road base.

1. Compacted volume. Area × depth = 4 × 3 × 0.1 = 1.2 m³. This is the volume the base occupies once it’s spread and compacted — not what you order.

2. Apply the compaction factor. Loose material bulks by 20–30% before it’s packed down:

At 1.2 (lower end): 1.2 × 1.2 = 1.44 m³
At 1.3 (upper end): 1.2 × 1.3 = 1.56 m³
At 1.25 (a reasonable middle figure): 1.2 × 1.25 = 1.5 m³

3. Order the loose figure. Round up to what you can actually order — call it 1.5 m³ of crushed rock or road base, not the 1.2 m³ compacted volume. That extra 0.3 m³ isn’t wastage — it’s the same material, just accounting for the fact that a cubic metre of loose gravel compresses down to less than a cubic metre once it’s rolled.

If your slab isn’t a plain rectangle, split it into rectangular blocks the same way you would for concrete volume, work out the compacted volume of each, add them together, then apply the compaction factor once to the total.

Do it in 5 seconds

Pillar: Concrete Calculations: The Complete On-Site Guide — every concrete sum in one place.
Next step: How to Calculate Concrete Volume for a Slab — once the base is compacted and ready, this is the pour that goes on top of it.

Sources

National Ready Mixed Concrete Association / Portland Cement Association subgrade-preparation guidance — compacted base course under a slab, and the area × depth volume method.
Engineering ToolBox, “Bulking of Excavated Material” — compaction/bulking factor tables for crushed stone and granular fill, supporting the 1.2–1.3 working range used above.
The exact compaction factor is material- and product-specific — 1.2–1.3 is a typical working range, not a fixed rule. Check your supplier’s figure for the actual product before you order.

Frequently asked questions

How much gravel do I need for a compacted base under a slab?: Work out the compacted volume as area × depth — a 4 m × 3 m base at 100 mm (0.1 m) compacted depth is 4 × 3 × 0.1 = 1.2 m³. Then order more than that: loose material bulks up before it's compacted, so multiply by 1.2–1.3 to get the order quantity — about 1.5 m³ for this base.
Why do I need to order more gravel than the compacted volume I calculated?: Because gravel, crushed rock and road base take up more space loose than they do once a plate compactor or roller packs them down. If you order exactly the compacted volume, you'll run short once it's rolled — the material you tipped out simply won't reach the compacted depth you needed.
What compaction (bulking) factor should I use for crushed rock or road base?: 1.2 to 1.3 is the typical working range — order 20–30% more loose material than the compacted volume you calculated. Denser, well-graded crushed rock sits toward the lower end (1.2); looser, more uniform material sits toward the higher end (1.3). If your supplier gives you a product-specific factor, use theirs.
How do I work out the loose-fill volume to order from a compacted depth?: Two steps. First, compacted volume = area × depth, in metres, for an answer in cubic metres. Second, multiply that by your compaction factor (1.2–1.3) to get the loose volume to actually order. Always order the loose figure, never the compacted one.
Is this the same volume method as a concrete slab?: Yes — area × depth is the same rectangular-prism sum used for concrete slab volume. The only extra step for gravel and base material is the compaction factor, because concrete is poured and set at roughly the volume you order, while granular fill compresses under compaction and needs extra loose volume to compensate.