How the concrete calculator works
For a rectangular slab, volume is length × width × thickness. Thickness is converted from inches to feet, then cubic feet are converted to cubic yards by dividing by 27.
Concrete formula
Cubic feet = length × width × (thickness ÷ 12). The calculator then applies your extra allowance for uneven subgrade, spillage and measurement error.
Bag estimates
Bag yields vary by manufacturer. This calculator uses approximate planning yields of 0.60 ft³ for an 80-lb bag, 0.45 ft³ for a 60-lb bag and 0.30 ft³ for a 40-lb bag. Always use the yield printed on the product you intend to buy.
When to order ready-mix
For larger projects, a ready-mix delivery may be more practical than handling dozens of bags. Suppliers may also have minimum-load or short-load charges, so compare the total project cost.