How concrete volume is calculated
Slab: volume (cu ft) = length × width × thickness(in) ÷ 12
Round: volume (cu ft) = π × (diameter(in) ÷ 12)² ÷ 4 × depth(in) ÷ 12
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag, rounded up
Bag yields come from the QUIKRETE Concrete Mix (No. 1101) product data sheet: a 40 lb bag yields about 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 and 90 lb 0.675 cu ft. Other brands are similar but check the bag.
Worked example
A 10 × 12 ft patio slab, 4 in thick, plus 10%:
| Volume (10 × 12 × 4 ÷ 12) | 40.0 cu ft |
|---|---|
| With 10% extra | 44.0 cu ft = 1.63 cu yd |
| 80 lb bags (÷ 0.60) | 74 |
| 60 lb bags (÷ 0.45) | 98 |
At 74 bags of 80 lb, that is over 3,500 lb of material to mix by hand — ready-mix delivery is usually easier for anything this size.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
At QUIKRETE’s published yield of 0.60 cu ft per 80 lb bag, a cubic yard (27 cu ft) takes 45 bags.
How thick should a concrete slab be?
Patios and walkways are commonly 4 inches; driveways and heavier loads are often thicker. Local building codes and your engineer or contractor set the requirement.