Key Takeaways
- The depth you plan for a base is a compacted depth
- Aggregate is delivered and spread loose, before it is rolled or plated down
- Loose material has more air between particles, so it takes up more volume
- Ordering only the compacted volume leaves the base short once it is compacted
- The compaction allowance and the material’s density both vary by product
A four inch gravel base does not arrive as a four inch layer of loose stone that you compact in place and call it done. It arrives as a pile larger than four inches of coverage, gets spread, and only becomes four inches once it has been rolled or plate-compacted. Order the compacted volume by itself and the base comes up short the moment the compactor runs over it.
Why loose and compacted are different numbers
Loose aggregate has air pockets between the individual stones. Compacting it, whether by hand tamper, plate compactor, or roller, drives the particles closer together and forces some of that air out, which reduces the volume the material occupies. The stone itself has not shrunk, the spaces between the pieces of stone have. That is the entire mechanism: compacted volume is smaller than loose volume for the same weight of material, because compaction removes void space, not substance.
Scaling the order up, not just measuring the finished layer
The practical fix is to calculate the compacted volume you actually want, the length times width times the compacted depth you are designing for, and then scale it up by a compaction allowance to get the loose volume to buy. A common working figure for that allowance is around 20 percent, meaning you order roughly a fifth more loose material than the finished compacted layer will occupy. That allowance is a working assumption, not a fixed rule: it depends on the material’s gradation, how well graded the mix is, and how thoroughly it gets compacted on site. A supplier who handles the specific base material you are ordering can usually give you a better number than any general default, especially for a load-bearing base under a driveway or a shed pad.
Weight follows the same loose-volume number
Once the loose volume is known, weight is just that volume times a density figure in pounds per cubic foot, the same way any bulk material converts from volume to tonnage. That density is a separate assumption from the compaction allowance, and it also varies by gradation, so both figures are worth confirming with a supplier before a large order rather than trusting a single default for the whole job.
Work out your own order quantity with the aggregate calculator, which takes the compacted depth you want and returns the loose volume and weight to actually buy. If the base sits under a layer of bedding sand, such as under pavers, the sand calculator handles that layer with its own volume-to-weight math. And if the aggregate is part of a concrete pour rather than a standalone base, the concrete calculator works out the concrete volume separately, since concrete mix design is a different calculation from a compacted stone base.