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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

Order 1.2 to 1.3 times the compacted volume of your base. First get the compacted volume: area × depth, in metres. Then add 20–30% on top for compaction. Loose gravel and road base pack down under the compactor, so the bare sum will leave you short.

Each slab and most footings sit on a packed layer of gravel or road base first. The sum is the same one used for concrete slab volume. Gravel just has one extra step that concrete does not: compaction.

Compacted volume = Area × Depth.

That is the room the base takes up once it is spread and packed down. It is not what you order. Crushed rock and road base come off the truck loose. Loose stone takes up more room than packed stone. Order the compacted figure and you will be short once the compactor has been over it.

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

The factor is 1.2–1.3 for most base products. That means you order 20–30% more loose stone than the compacted sum. A figure from your supplier beats the rule of thumb. If they quote one for their product, use theirs.

  • Area — length × width of the base, in metres.
  • Depth — the compacted thickness you need, in metres (mm ÷ 1000).
  • Compaction factor — 1.2 to 1.3. It goes on after the area × depth sum, not before.

Work it out by hand

Say you are prepping the base under a 4 m × 3 m slab. 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. That is the room the base fills once packed down. It is not your order.

2. Apply the factor. Loose stone bulks up by 20–30% before it is 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 fair middle pick): 1.2 × 1.25 = 1.5 m³

3. Order the loose figure. Round up to what you can buy. Call it 1.5 m³ of crushed rock or road base, not the 1.2 m³ compacted volume. The spare 0.3 m³ is not wastage. It is the same stone. A loose cubic metre just packs down to less than a cubic metre once it is rolled.

Slab not a plain rectangle? Split it into blocks, the same way you would for concrete volume. Work out the compacted volume of each block. Add them up. Then apply the factor once, to the total.

Do it in 5 seconds

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?
Compacted volume is area × depth. A 4 m × 3 m base at 100 mm (0.1 m) deep is 4 × 3 × 0.1 = 1.2 m³. Then order more. Loose base bulks up, so multiply by 1.2–1.3. That gives 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 packed down. A plate compactor or roller squeezes the pile smaller. Order only the compacted volume and you will run short once it is rolled. The heap you tipped out will not reach the depth you needed.
What compaction (bulking) factor should I use for crushed rock or road base?
Use 1.2 to 1.3. That means order 20–30% more loose stone than the compacted volume. Denser, well-graded crushed rock sits toward the lower end (1.2). Looser, more uniform stone sits toward the higher end (1.3). If your supplier gives a figure for their product, 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. That gives cubic metres. Second, multiply by your compaction factor (1.2–1.3). That is the loose volume to 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 sum used for concrete slab volume. Gravel and base just add one step: the compaction factor. Concrete sets at about the volume you order. Loose fill packs down smaller, so you order extra to make up for it.