Groundworks calculators

What this cluster is for

Groundworks is the layer that is buried before anyone takes a photograph. It covers three related jobs: the sub-base under a paved surface, the concrete in a slab, footing or base, and the loose aggregate that goes around a drainage run. The two calculators here answer the two questions that actually block a job: how many cubic metres of concrete, and how many tonnes of aggregate.

Both are volume questions wearing an area costume. A patio is measured in square metres, but MOT Type 1 is sold in tonnes and the figure that matters is area × compacted depth × bulk density. Concrete is worse: the wet volume you pour is not the dry volume you mix, because sand, cement and aggregate bulk up when they are loose. Get either wrong and the error is not small — it is several bulk bags or half a cubic metre.

What usually goes wrong

• Ordering sub-base by area. 24 m² at 100 mm is 2.4 m³, and at 1.8 t/m³ that is 4.32 tonnes — six 850 kg bulk bags. Ordered as "a couple of bags" it arrives three tonnes short.
• Mixing concrete by the wet volume. Dry ingredients bulk by roughly 1.54×, so 1 m³ of finished concrete needs about 1.54 m³ of loose sand, cement and aggregate. A mix worked out on wet volume alone leaves you about a third short.
• Compacting one thick layer. Sub-base is compacted in layers of roughly 75–100 mm. A 200 mm layer compacted in a single pass never reaches the density the design assumes.
• Under-specifying the depth for vehicles. 100 mm of compacted Type 1 carries people and a wheelbarrow. A car needs 150 mm, and a van at the front of a drive wants 200 mm or more.
• No fall. Concrete and paved surfaces both have to shed water. A fall is built into the sub-base and the formwork; it cannot be corrected later.

What to order together

• MOT Type 1 sub-base and a geotextile membrane — the membrane stops the sub-base pumping into soft ground
• Cement, sharp sand and aggregate, or a ready-mix delivery once the pour is over about 1 m³
• Timber for formwork, plus a polythene membrane under a slab
• Hire: a plate compactor for the sub-base, and a mixer for anything over a barrow-mix

Related clusters

Cross-links here are contextual, not a second home: each calculator belongs to exactly one cluster above.