How much concrete do you need?
Concrete is estimated by volume: length × width × depth. Multiply your three measurements to get cubic metres, add about 10% for spillage and uneven ground, then divide by the yield of your chosen bag size. For anything larger than a few square metres, ready-mix delivery is usually cheaper than bags.
Getting the depth right
A typical residential slab is 100 mm (10 cm) thick; paths can be 75 mm, and footings vary by load. Depth has the biggest impact on volume, so measure it carefully.
Frequently asked questions
Bags or ready-mix?
Under about 0.5 m³, bags are convenient. Above that, a ready-mix truck is usually cheaper and far less labour.
Why add waste?
Ground is rarely perfectly level and some concrete is always lost to spillage. A 10% buffer avoids running short mid-pour, which can ruin a slab.