Concrete Calculator

A worked example

With the calculator’s starting values (length: 5 m; width: 4 m; thickness: 100 mm; concrete grade: 25 MPa (general)), it returns:

• Concrete volume: 2.20 m³ (2.00 m³ + 10% waste)
• Cement (50 kg bags): 16 bags (~7 bags/m³)
• Sand & stone: 1.1 m³ sand · 1.7 m³ stone (1 : 2 : 3 mix approx)
• Estimated ready-mix cost: R 5 335 (R 2 425/m³ (25 MPa, incl. placing))

How the calculator works

It multiplies length by width by thickness to get the volume in cubic metres, then adds 10% for spillage, over-excavation and uneven ground. From that it estimates cement at about seven 50 kg bags per cubic metre, with sand and stone in roughly a 1:2:3 proportion, and prices the volume at a ready-mix benchmark rate that includes placing.

Measuring different elements

• Surface bed or slab: length × width × thickness. A 100 mm house surface bed is common; garage and driveway slabs are often thicker.
• Strip footing: total length of the footings × width × depth. Measure the centre line around the building and add the internal walls that carry footings.
• Pad footing: length × width × depth for one pad, multiplied by the number of pads.
• Column: cross-section area × height. A 230 × 230 mm column 3 m high holds about 0.16 m³.
• Steps and thickenings: measure them separately and add them, rather than averaging the slab thickness.

Choosing the strength

Concrete is specified by its compressive strength in megapascals. For structural elements, the engineer's drawings set the strength, the cover to reinforcement and often the maximum aggregate size, and those take precedence over any rule of thumb. On a house without an engineer, the NHBRC Home Building Manual sets the minimum requirements for foundations and floor slabs. Higher strengths use more cement, so they cost more per cubic metre.

Ordering ready-mix

Ask for a delivered price to your site. Check the truck can reach the pour; if it cannot, you will need a concrete pump, which is a separate hire charge. Small orders often attract a part-load surcharge, so it can pay to combine pours. Have the formwork, reinforcement and labour ready before the truck arrives, because ready-mix has to be placed within the time the supplier specifies.

After the pour, keep the concrete moist for at least the first week so it gains strength and does not crack from drying too quickly. On structural work, the engineer may ask for cube tests to confirm the strength achieved.