Aggregates Specification Guide: Calculating Quantities, Compaction Factors and Drainage Properties

Getting an aggregate specification right sits quietly behind almost every successful stone project we deliver, yet it rarely gets the attention it deserves compared with the finished paving, coping or fabricated stone sitting on top of it. A poorly specified sub-base or bedding layer will eventually show itself through settlement, poor drainage or premature paving failure, regardless of how good the stone above it is. We work with contractors, landscapers and specifiers on projects where the aggregate build-up genuinely determines whether the finished stonework performs for decades or needs remedial work within a few years, so this guide sets out the practical detail we would want any specifier to have before ordering material for a paving, landscaping or fabrication-adjacent groundworks package.

This is not a guide to installation itself, since groundworks and construction activity sit outside our remit as a stone supplier and fabricator, but understanding how aggregate quantities, compaction and drainage properties interact is essential knowledge for anyone specifying or ordering the sub-base that our stone will ultimately sit on. Getting the detail right at this stage protects the investment made in the stone above it, and it is a conversation we have constantly with contractors working through project take-offs. Where the numbers below relate to standard UK aggregate products such as MOT Type 1 and Type 3, we have referenced the density and compaction figures used across the industry so this guide can be used directly for quantity calculations.

We have structured this guide around the three questions that come up most often on a live project: how much material to order, how much allowance to build in for compaction, and how the aggregate’s drainage properties should inform which product gets specified. Whether you are working out quantities for a driveway sub-base, a bedding layer beneath large-format stone paving, or a permeable detail required under SUDS regulations, the calculations below follow the same underlying logic.

Understanding aggregate types and their uses

MOT Type 1 is a graded, crushed limestone or granite aggregate, typically up to 40mm particle size, used almost universally as a sub-base layer beneath driveways, paths and patios because it locks together tightly once compacted, forming a stable, load-bearing foundation. Type 3 sub-base follows a similar grading principle but is designed to be more open and permeable, allowing water to pass through the layer rather than running off the surface, which is a requirement under SUDS regulations on many new-build and commercial developments. For most standard paving and patio sub-base applications, Type 1 remains the default specification, with Type 3 reserved for locations where permeability is a specific design requirement.

Beyond these two workhorse products, aggregate specifications will often call for scalpings, 6F2 capping layer, or drainage-specific gravels depending on ground conditions and the loading the finished surface needs to carry. The correct particle grading matters more than most specifiers appreciate, since a well-graded aggregate with a controlled spread from dust through to the maximum particle size compacts far more predictably than a poorly graded, single-size material, and this directly affects both the load-bearing performance and the drainage characteristics of the finished layer.

Calculating aggregate quantities

The starting calculation for any aggregate order is straightforward volume, calculated as area in square metres multiplied by the finished, compacted depth in metres, giving a volume in cubic metres. For example, an area of 100 square metres at a finished depth of 150mm requires a finished volume of 15 cubic metres. This finished volume figure, however, is not what should be ordered, since it describes the compacted, in-place volume rather than the loose material needed to achieve it.

Converting finished volume into an order quantity means applying a compaction allowance, since aggregate delivered loose occupies significantly more volume than the same material once it has been laid and compacted. Once the loose volume has been established, it can be converted to a tonnage using the material’s bulk density, and a wastage allowance should then be added on top to account for handling loss, trimming and material lost during spreading and levelling.

As a practical reference point, one tonne of compacted MOT Type 1 covers approximately 4.76m² at 100mm depth, or around 3.17m² at 150mm depth, based on a compacted density of roughly 2.1 tonnes per cubic metre. Most specifications for driveways and other vehicular-trafficked areas call for a minimum of 150mm depth, which is worth confirming against the specific loading requirement for the project rather than assuming a single depth applies universally across pedestrian and vehicular areas.

Compaction factors explained

Compaction factor is the percentage increase in loose material volume needed to achieve a given finished, compacted depth, and getting this figure right is one of the most common points of under-ordering on site. Typical compaction factors across common UK sub-base materials run from around 10% for well-graded sub-base (GSB) up to 25% for MOT Type 1, with backfill materials sitting slightly lower at 8-12%. This variation exists because different materials have different particle grading and void ratios in their loose state, which determines how much they consolidate under rolling or plate compaction.

Compacted MOT Type 1 typically reaches a density of around 1,810kg per cubic metre compared with roughly 1,510kg per cubic metre in its loose state, which illustrates why a 15-20% additional material allowance is standard practice rather than an arbitrary buffer. Under-ordering against the compaction factor is one of the most frequent causes of delayed programmes on groundworks packages, since a return delivery to make up a shortfall can hold up the following trade, including the stone fixing team, by a full day or more. Building the correct compaction allowance into the original order is a small piece of arithmetic that protects the entire programme downstream of it.

Drainage properties and material selection

The drainage performance of an aggregate layer is governed primarily by its particle grading and the proportion of fines within the mix, since coarser particles create the void spaces that allow water to move through the layer rather than sitting on or within it. A well-graded Type 1 sub-base, while excellent for load-bearing, is not a free-draining material in the way that a permeable Type 3 or a dedicated drainage gravel is, and specifying the wrong product for a location with a known drainage requirement is a common and avoidable error.

Where SUDS compliance or a known high water table demands genuine permeability, the aggregate specification needs to reflect that from the outset, since retrofitting drainage into a compacted, fines-heavy sub-base after installation is rarely practical. Coarse, open-graded aggregates create clear drainage paths that prevent saturation and reduce the risk of frost damage within the layer during winter, which matters particularly beneath stone paving and coping details where trapped water and subsequent freeze-thaw cycling is one of the most common causes of long-term surface failure. This is precisely the reason our own approach to specification, whether for aggregates beneath a driveway or the drainage detailing beneath a pool surround, always starts with the site’s actual water management requirement rather than defaulting to a standard build-up regardless of context.

Specifying to the correct standard

Any aggregate specification should reference the relevant British Standard for the material in question, such as BS EN 13242 for MOT Type 1, rather than a generic description like “hardcore,” since this ensures the delivered material has been tested and graded to a known, consistent specification. This matters as much for the stone trade as it does for groundworks contractors, because the long-term performance of any fabricated stone element, whether that is a coping, a step or a large-format paving installation, depends directly on the quality and consistency of the layer beneath it. Where a project combines a technical aggregate specification with bespoke or large-format stonework, it is worth involving a technical partner early, since coordinating the sub-base build-up with the final stone thickness, jointing and drainage detail from the outset avoids costly adjustments once the stone itself is on site.