Concrete calculator - volume, cement bags, sand and gravel

• Slab: length × width × thickness. 5 × 4 m at 10 cm: 5 × 4 × 0.10 = 2 m³.
• Strip footing: total length × width × depth. 40 m × 0.6 × 0.4 m = 9.6 m³.
• Square column: a × b × height × quantity. Four 25 × 25 cm columns, 3 m tall: 0.75 m³.
• Round column: π × r² × height. 30 cm diameter, 3 m: 3.1416 × 0.15² × 3 = 0.212 m³.
• Stairs on a waist slab: width × [steps × tread × riser / 2 + slab thickness × pitch length]. 14 steps of 28 × 17.5 cm, 1 m wide, 15 cm slab: about 1.04 m³.

Add 5-10% for spillage, uneven excavation and formwork deflection.

Values are approximate, for 32.5 grade cement and dry aggregate. Example: 2 m³ of C16/20 with 5% extra (2.1 m³) needs 672 kg of cement - 27 bags of 25 kg - 1.37 t of sand, 2.52 t of gravel and about 370 l of water.

For more than a couple of cubic metres, ready-mix concrete is usually cheaper and more reliable: the strength class is guaranteed and the whole slab is poured in one go. Cost = m³ with extra × price per m³ (plus delivery and pump hire if needed).

Mixing by hand or in a small mixer makes sense for fence posts, deck footings, blinding and small repairs. Structural elements should use the concrete class specified by the engineer.

Measure each dimension at several points when an excavation or form is uneven, then use a realistic average and calculate separate sections before adding them together. Check that lengths are in metres and thicknesses entered in centimetres are converted correctly. Confirm access for the delivery truck, the unloading point and whether a pump is needed. Concrete suppliers may also have a minimum load or charge for waiting time. The estimate is a planning aid: the structural design and supplier mix specification determine the required concrete class and reinforcement.