Concrete volumes
Slab or base volume
- where
- plan area (m²)
- slab length (m)
- slab width (m)
- concrete volume (m³)
- slab thickness (mm)
Explain this equation
Works out the concrete needed for a slab, base or oversite. Use it before ordering ready-mix or bagged materials.
Length times width gives the area in square metres. The thickness is in millimetres, so dividing by 1000 turns it into metres; area times thickness is then the volume in cubic metres.
Volume of a rectangular block.
Sense check: a 6 × 4 m slab 100 mm thick is 2.4 m³. Every extra 25 mm of thickness on that slab adds 0.6 m³.
Watch out: ground is never perfectly level. Check the depth at several points and add a waste allowance when ordering.
Works out the volume of concrete in a rectangular slab, base or oversite from its plan size and thickness.
Enter your values to see the result.
Good to know
Measure the thickness you will actually pour: if the sub-base is uneven or you are laying over a membrane with a thickened edge, take an average depth from several dips, or add thickenings separately with the strip footing calculator. Feed the answer into the order quantity calculator to add waste and round up to a load.
Common mistake
Entering the thickness in metres as if it were millimetres, or forgetting that 100 mm is 0.1 m, so the answer is ten or a thousand times out.
Limits
Rectangular, uniform-thickness slabs. Split L-shapes and other outlines into rectangles and add them. Thickness and grade come from your design or the Building Regulations guidance, not from this calculator.
Sources
- Geometry: prism volume = plan area × depth; cylinder volume = π d²/4 × h
- Heidelberg Materials (Hanson), Ready-mix concrete volume calculator FAQ (add 5–10% to the calculated volume for spillage and uneven surfaces)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.