How to use it
- Choose a slab, footing or wall, or a round column.
- Enter the length and width in feet or metres, and the thickness in inches or millimetres.
- Set the quantity if you are pouring several identical footings or columns.
- Keep a waste allowance of 5 to 10%.
Worked example
A 10 × 10 ft patio slab, 4 in thick, is 10 × 10 × 4 ÷ 12 = 33.3 ft³, which is 1.23 yd³. With 5% waste that becomes 35 ft³, or 1.30 yd³. At 0.60 ft³ per 80 lb bag, you need 59 bags. For a slab this size, ready-mix delivery is usually easier than mixing 59 bags by hand.
In metric, a 3 × 3 m slab, 100 mm thick, is 0.90 m³, or about 0.95 m³ with 5% waste.
Choosing a thickness
Patios and walkways are commonly 4 in (100 mm) thick. Driveways that carry cars are commonly 5 to 6 in (125 to 150 mm). Your local code, the soil and the loads decide the final figure, so treat these as starting points.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
A cubic yard is 27 ft³. At 0.60 ft³ per bag, that is 45 bags of 80 lb, or 60 bags of 60 lb.
How much extra concrete should I order?
Order 5% extra for a formed slab on a level, compacted base, and up to 10% if the ground is uneven or the shape is irregular. Running short mid-pour leaves a weak joint, so it is better to have a little left over.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. For example, 54 ft³ is 2 yd³.
How much does concrete weigh?
Normal-weight concrete is about 2,400 kg per cubic metre, which is roughly 4,000 lb per cubic yard.