Volume = length × width × depth; one 50 kg bag ≈ 0.035 m³.
A concrete calculator tells you how much concrete to order for a slab, footing, column, or any other shape — before the mixer truck arrives. Ordering too little leaves you with a cold joint (a weak seam where fresh concrete meets concrete that has already started to set); ordering far too much wastes money, since ready-mix suppliers charge for the full truck. The calculator converts your dimensions into cubic yards or cubic meters, the units concrete is sold in.
It all starts with volume:
Volume = length × width × thickness (all in the same units)
Then convert to ordering units:
Finally, add a waste factor of 5–10% for spillage, uneven subgrade, and formwork that isn't perfectly square. For bagged concrete, a typical 80 lb (36 kg) bag yields about 0.6 cubic feet (0.017 m³) — divide your total cubic feet by 0.6 to estimate bags, and round up.
You are pouring a patio slab 10 ft × 12 ft × 4 inches thick:
Using bags instead: 40 ft³ ÷ 0.6 ft³ per bag ≈ 67 bags — round up to 68–70 to be safe. (Bag yields vary by brand; treat 0.6 ft³ as a typical figure and check your bag's label.)
Spillage, uneven ground and formwork gaps always waste some concrete — the +10% figure covers it.
About 0.035 m³ of mixed concrete, so roughly 29 bags make one cubic metre.
