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Concrete calculator: how much concrete do I need?
Work out the volume of concrete for a slab, base or footing, and the cement and aggregate to mix it yourself or the amount to order ready-mixed.
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How the calculation works
Volume is length × width × depth, all in metres. A 3 × 3 m slab 100 mm deep is 0.9 m³. The calculator then adds your wastage allowance and, if you mix on site, converts the volume into bags of cement and aggregate for the strength class you choose.
As a rough guide, one cubic metre of concrete weighs about 2.4 tonnes, which helps when planning deliveries and access.
Mix on site or ready-mixed?
Mixing on site with a mixer suits small bases, paths and footings. For bigger slabs, ready-mixed concrete is usually quicker and gives a more consistent result, especially if the pour must be done in one go.
Pick the strength class for the job: lower classes for blinding and fill, higher ones for slabs, garage floors and driveways.
Worked example
A 3 × 3 m slab, 100 mm deep, mixed on site:
| Material | Quantity | Price (ex VAT) |
|---|---|---|
| Blue Circle Mastercrete Grey Cement in Plastic Bag 25kg | 12 | £92.04 |
| Bulk Bag 20mm Gravel | 2 | £457.38 |
| Materials total | £549.42 | |
Today's prices, excluding VAT and delivery. Labour, plant hire and some items are priced when you request a quote. Use the calculator above for your own measurements.
Frequently asked questions
How do I work out concrete volume?
Multiply length by width by depth in metres. For example, 3 m × 3 m × 0.1 m = 0.9 m³. Add a little extra for wastage and uneven ground.
How thick should a concrete slab be?
Around 100 mm is common for paths, patios and light bases; garage floors and driveways are usually thicker and may need reinforcement mesh.
What is a good concrete mix ratio?
A general-purpose mix is about one part cement to five or six parts all-in ballast. The calculator uses the strength class you choose to set the quantities.