Calculate concrete volume for slabs, columns, footings, and steps in cubic metres or cubic yards. Instantly shows number of pre-mix bags needed with cost estimate.
X is what % of Y: (X ÷ Y) × 100
X% of Y: (X ÷ 100) × Y
% change from A to B: ((B − A) ÷ A) × 100
Original value before X% added: Final ÷ (1 + X/100)
Original value before X% removed: Final ÷ (1 − X/100)
Worked examples: 45 is what % of 180? (45 ÷ 180) × 100 = 25%. 15% of 240? 0.15 × 240 = 36. Price rose from 80 to 96 — % increase: ((96−80) ÷ 80) × 100 = 20%. Price was 120 after 20% tax added — original pre-tax: 120 ÷ 1.20 = 100.
Multiply the length × width × depth of the area to get volume. For a slab 4m × 3m × 0.1m thick: volume = 1.2 m³. Always add 10% for wastage.
A standard 25kg bag of pre-mix concrete yields approximately 0.012 m³ when mixed. So roughly 83 bags are needed per cubic metre.
Cement is an ingredient in concrete. Concrete is a mixture of cement, sand (fine aggregate), gravel (coarse aggregate), and water. Cement acts as the binder.
A standard residential patio or driveway slab should be 100mm (4 inches) thick. Garage floors and heavier-use areas typically need 125–150mm (5–6 inches).
PSI (pounds per square inch) is the compressive strength of cured concrete. Standard residential concrete is 3000 PSI (≈20 MPa). Structural applications may require 4000–5000 PSI.