Patio Sub-Base Calculator
Calculate the specified compacted sub-base volume first, then convert it to tonnes or whole supplier bags using the product information you actually have.
Measure the layer, then choose the supplier conversion
Enter the real patio area and the compacted depth specified for the job. Then choose a purchasing conversion: supplier density for tonnes, or stated bag coverage for whole bags.
Why the calculator no longer assumes 100 mm, 2.0 t/m³ or 10%
Those values can all be reasonable in a particular job, but none is universal. Current UK manufacturer guidance varies sub-base depth by use and ground conditions, and suppliers publish different coverage/weight information for the products they sell. The calculator therefore makes the project depth and supplier conversion explicit inputs.
Area × specified compacted depth gives the design volume in cubic metres.
Use supplier density for tonnes, or stated coverage at a stated depth for bag purchasing.
The ordering allowance starts at 0%; it never changes the specified compacted depth.
Compacted design volume = area × (specified compacted depth ÷ 1000). Density mode: tonnes = volume × supplier density. Coverage mode: equivalent volume per bag = stated coverage × stated depth.
Worked examples
Density example: a 25 m² layer at a specified 100 mm compacted depth is 2.50 m³. If the actual supplier conversion for the chosen aggregate were 1.60 t/m³, the base estimate would be 4.00 tonnes before any optional allowance.
Coverage example: a supplier product stating 7.7 m² coverage at 50 mm represents 0.385 m³ of stated coverage per bag. A 15.4 m² layer at 50 mm is 0.77 m³, so the theoretical requirement is exactly 2.00 bags at 0% extra.
Compacted volume is not automatically loose delivery volume
Some guidance applies a separate factor when converting a compacted design layer into loose aggregate to order. That factor is not universal enough to hide inside a calculator. If your quarry, merchant or specification gives a project/product-specific loose-to-compacted conversion, use that information when placing the final order.
Depth must come from the actual build-up
Manufacturer guidance commonly gives patio sub-base depths as examples or ranges, while also warning that ground conditions, loading and the installation system matter. This calculator therefore asks for the specified compacted depth rather than deciding the build-up for you.
Where current UK product data differs
As one current merchant example, Wickes lists a jumbo MOT Type 1 bag with a stated weight of 850 kg and approximate coverage of 7.7 m² at 50 mm. Smaller bags have different weights and coverage. That is why the purchasing conversion belongs to the exact product, not a generic “one bag equals…” rule.
Material quantity estimate only. This calculator does not design the ground build-up, assess soil, drainage or loading, choose the aggregate specification, set excavation depth or confirm structural suitability. Verify the project build-up and the supplier’s current product conversion before ordering.
Continue the patio estimating workflow
Frequently asked questions
Should I enter 100 mm for every patio?
No. Enter the compacted depth specified for the real job. Published guidance often gives example ranges, but ground conditions, loading and construction details can change the required build-up.
What density should I enter for MOT Type 1?
Use the current supplier, quarry or product conversion for the aggregate you intend to buy. The tool deliberately leaves this blank rather than assuming one universal tonnes-per-cubic-metre figure.
Is the extra allowance a compaction factor?
No. It is an optional ordering allowance and starts at 0%. Any loose-to-compacted conversion supplied for the actual material should be handled separately and explicitly.
When is supplier coverage better than density?
Use coverage mode when the exact bag or bulk-bag product states how many square metres it covers at a stated depth. The calculator converts that published coverage into an equivalent volume per bag and rounds the final purchase to whole bags.
Current Marshalls guidance on choosing/calculating sub-base and patio installation, plus current Wickes MOT Type 1 product coverage/weight information. Product data and installation guidance can change, so verify the exact material before ordering.