Concrete calculator

Work out the concrete for a slab, footing, pad or pier, the ready-mix to order, and the cement, sand and gravel if you mix it yourself.

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Worked example

A shed base 4 m × 3 m and 100 mm thick, in C20 with 5 percent extra:

1.26 m³ of concrete with 5% extra

1.20 m³ before waste
Ready-mix: order 1.50 m³ (rounded up to the next quarter)
To mix it yourself at C20 (1:2:4): 17 bags of cement (25 kg), 0.84 t of sharp sand and 1.64 t of 20 mm gravel, or 2.46 t of all-in ballast with the cement
About 3.02 tonnes once placed

How the sum works

Volume in m³ = length × width × depth, all in metres. A round pad or pier is 3.14 × radius × radius × depth. The calculator adds your waste figure and rounds the ready-mix up to the next 0.25 m³, the step most suppliers sell in.

Mixing it yourself

For each m³ of finished concrete, the calculator allows about 320 kg of cement, 0.67 t of sharp sand and 1.3 t of 20 mm gravel for C20, or 1.95 t of all-in ballast with the cement. C10 and C25 use less and more cement. These are rule-of-thumb site mixes: for anything structural, the mix comes from the engineer or the specification.

Over about a cubic metre, mixing by hand is slow and hard work: compare the price of a ready-mix delivery or a mix-on-site lorry.

Before you order

Measure the open trench or the formwork, not just the drawing: over-dig is the usual reason a pour runs short. Check the supplier's minimum load and the access for the lorry, and the forecast for frost.

Guides that go with it

How to calculate how much concrete you needFree concrete calculator for slabs, footings, trench fill and round piers, in m3 with extra for waste and the number of bags. Worked examples and ordering ready-mix.How do I calculate how much screed I need?How to work out screed volume as area x depth, allow for uneven slabs and waste, and order ready-mixed or sand and cement. With worked examples.When does an excavation have to be inspected under CDM 2015?When a competent person must inspect an excavation under CDM 2015 regulation 22, when the report is due, and the main risks and controls for builders.

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