This is a volume estimate only. Mix design, reinforcement, compaction, formwork and site conditions are not included.
How to calculate concrete volume
For a rectangular slab, multiply length × width × depth using the same unit for all three measurements. If measurements are in metres, the result is cubic metres (m³). The optional wastage allowance is applied after the basic volume.
Worked example
A slab 5 m long, 4 m wide and 0.10 m thick needs 5 × 4 × 0.10 = 2.00 m³ of concrete. With a 5% allowance, the estimate is 2.00 × 1.05 = 2.10 m³.
Before ordering
- Measure the actual dimensions and check whether the thickness varies.
- Split irregular shapes into rectangular sections and add their volumes.
- Confirm the appropriate mix, strength grade, reinforcement and site preparation with a qualified professional.
- Ask the supplier how it rounds delivery quantities and whether minimum-load or delivery charges apply.
This tool estimates volume only. It does not design structural members, specify a concrete mix, or account for every site condition. Verify dimensions and engineering requirements before construction.
Practical slab estimate and unit check
For a slab 5 m long, 4 m wide and 100 mm thick, convert the thickness to metres: 100 mm = 0.10 m. The volume is 5 × 4 × 0.10 = 2.00 m³. A 5% planning allowance gives 2.00 × 1.05 = 2.10 m³. This is a volume estimate, not a mix design or a structural specification.
Before ordering concrete
- Make sure all three dimensions use the same unit before multiplying.
- Measure the prepared formwork and check for varying slab thickness or irregular edges.
- Do not assume that a volume estimate specifies cement, sand, aggregate or water quantities; mix proportions depend on the required grade and design.
- Confirm the mix grade, reinforcement, sub-base and placement requirements with the responsible qualified professional.
- Ask the supplier about minimum delivery loads, rounding and site access before confirming the order.
For multiple rectangular sections, calculate each section's volume and add them. Apply any allowance only after the total base volume is calculated. The entered wastage percentage is a user-selected estimate and may not reflect site conditions.
For related measurements, use the Square Feet & Area Guide and Tile Calculator.