A half-tonne short on a driveway sub-base can stop the job just as surely as a missing pallet of blocks. Order too much and you have paid for material, delivery and disposal that the site does not need. This aggregate quantities project guide gives you a practical way to work out volumes and tonnes before the lorry is booked.
The calculation itself is straightforward. Getting it right depends on measuring the finished area accurately, allowing for the compacted depth rather than guessing, and ordering the aggregate in the unit your supplier uses. Most bulk aggregate is sold by the tonne, while your excavation and dimensions are normally measured in cubic metres. The conversion between the two is where many orders go wrong.
Start with the finished depth
Before calculating quantities, establish what the aggregate is doing in the build-up. A decorative gravel layer, a paving bedding layer and a compacted MOT Type 1 sub-base all need different depths and different products.
For a typical domestic path, a compacted Type 1 base may be 100mm deep on firm ground. A driveway carrying cars commonly needs 150mm, while soft ground, poor drainage or regular van traffic may call for a deeper engineered build-up. Do not simply add more stone because the ground looks wet. Check whether drainage, geotextile separation or excavation of unsuitable material is required first.
Measure every area in metres and convert depths from millimetres to metres. For example, 100mm becomes 0.1m, 150mm becomes 0.15m and 200mm becomes 0.2m. This avoids one of the most common errors: multiplying metres by millimetres and producing a figure that is ten times too high.
The basic volume calculation is:
Length (m) × width (m) × depth (m) = volume in cubic metres
A 10m long driveway that is 4m wide and needs a 150mm compacted sub-base is calculated as follows:
10 × 4 × 0.15 = 6m³
That is the compacted volume required in the ground. It is not yet the tonnage to order.
Aggregate quantities project guide: convert m³ to tonnes
Aggregate density varies by material, moisture content, particle size and how tightly it sits in the load. For estimating purposes, a useful working figure for MOT Type 1, crushed concrete and similar sub-base materials is around 2 tonnes per cubic metre.
Using the driveway example:
6m³ × 2 tonnes per m³ = 12 tonnes
A 12-tonne order is therefore a sensible starting point for a 10m × 4m area at 150mm compacted depth. However, aggregates are placed loose then compacted. Depending on the material and ground conditions, allow a sensible margin for compaction, trimming and uneven excavation. Around 5 to 10 per cent is often enough on a well-measured rectangular area. For a 12-tonne calculation, that may mean ordering 12.5 to 13 tonnes.
Do not use one conversion factor for every product. Decorative gravel, sharp sand, ballast and lightweight products can all differ materially in density. If the project specification or product page provides a coverage rate, use that figure over a generic rule of thumb. When ordering from a merchant, give the finished dimensions, required depth and intended material so the quantity can be checked against the product being supplied.
Measure the site, not the estate agent’s plan
Drawings are useful, but completed gardens and driveways rarely match a simple rectangle. Fence lines drift, kerbs flare out, and existing walls may not run square. Measure the actual site with a tape or laser, then split awkward spaces into basic shapes.
For a rectangular section, use length × width. For a triangular section, use:
Length × width ÷ 2
For a circular patio or tree surround, calculate:
3.142 × radius²
Then multiply each area by the planned depth and add the volumes together. A driveway with a 30m² main rectangle and a 6m² triangular return is not a 36m² rectangle if the depths differ. Calculate the sections separately. It takes a few more minutes but prevents over-ordering material that cannot be returned once tipped.
Slopes need particular care. Measure the plan area, but establish the average depth of the layer. If one end of a 12m run needs 100mm and the other needs 200mm, use an average depth of 150mm only if the slope is consistent. If the dig varies sharply around drainage runs, thresholds or existing services, break it into shorter sections and calculate each one.
Choose the aggregate for the job
Quantity is only useful if the product is right. A compacted sub-base under paving or tarmac normally requires MOT Type 1 or an approved equivalent. Its range of particle sizes and fines helps it bind and compact. Clean drainage stone is not a direct substitute because it does not lock together in the same way.
For concrete mixes, use the specified ballast, sharp sand and cement proportions. For block paving, use the correct bedding sand and jointing product for the system being installed. Decorative gravel needs a suitable base and edging if it is being laid over a trafficked area, otherwise it will migrate into the lawn and adjacent beds.
Recycled aggregates can offer strong value for suitable applications, particularly general fill and sub-base work, but consistency can vary. For structural, specified or inspected works, confirm the grading and compliance required by the engineer, local authority or project specification. The cheapest tonne is not good value if it fails compaction, causes surface movement or has to be dug back out.
Allow for compaction, waste and access
Compaction allowance is not the same as waste. The loose material needed to achieve a finished 150mm compacted layer will be greater than the compacted volume. Waste is extra material lost through over-excavation, poor level control, contamination or spreading beyond the edge restraint.
On a simple, well-contained job, 5 per cent is a practical allowance. Use 10 per cent where ground levels are uncertain, access makes accurate placement difficult, or the area has several returns and curves. Avoid building in a large contingency simply because the calculation is uncertain. Re-measure instead.
Also think about delivery before placing the order. A large loose load needs a safe tipping point, adequate headroom and firm access. A loaded lorry cannot cross weak paving, soft verges or a steep loose drive just because the aggregate is needed at the rear of the property. If access is restricted, bulk bags may be more practical, although the cost per tonne is usually higher and handling takes longer.
For trade work, sequence matters. Get the dig completed and levels checked before the aggregate arrives where possible. Leaving Type 1 in a heap on a wet pavement or narrow site entrance creates double handling and can hold up other deliveries. If the job is spread over several days, a staged delivery may cost more in transport but save time, labour and site congestion.
Worked examples for common jobs
A garden path measuring 12m by 1.2m with a 100mm compacted Type 1 base has an area of 14.4m². Multiply by 0.1m depth to get 1.44m³. At roughly 2 tonnes per m³, the requirement is 2.88 tonnes. With a modest allowance, order around 3 to 3.25 tonnes.
For a 5m by 5m shed base at 150mm depth, the volume is 3.75m³. At 2 tonnes per m³, that is approximately 7.5 tonnes of sub-base before an allowance. If the base is being finished with concrete, calculate the concrete materials separately. The sub-base quantity does not include ballast, sand or cement for the slab.
A decorative gravel bed is different. A 20m² area at 50mm depth requires 1m³ of gravel. The tonne conversion should be based on the selected gravel’s stated coverage or density, not automatically doubled. Decorative stone sizes and rock types vary, so a product-specific check is worthwhile.
Check the order before you commit
Write down the area, finished depth, material, conversion factor and allowance. This creates a quick record that another person on the job can check. It is particularly useful where the person ordering is not the person excavating.
At Wel Timber & Building Supplies, practical checks like this help keep deliveries aligned with the work on site. If your calculation sits between load sizes or you are unsure whether the depth is suitable for the intended traffic, get the dimensions and application clear before ordering. A short conversation is cheaper than an urgent second load.
The best time to fix an aggregate quantity is before the excavator starts, when you can still adjust depths, drainage and delivery access without wasting labour. Measure carefully, use the correct material density, and leave only the allowance the job genuinely needs.
