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.
Worked example
A 10 × 10 foot slab at 4 inches thick contains 33.33 cubic feet before waste. With a 10% allowance, plan for 36.67 cubic feet, or about 1.36 cubic yards.
Practical tips and limitations
- Measure the actual excavation and account for thickened edges separately.
- Confirm bag yield or ready-mix order increments with the supplier.
- The estimate assumes a rectangular slab of uniform thickness and does not evaluate structural requirements.
Frequently asked questions
How many cubic feet are in a cubic yard?
One cubic yard contains 27 cubic feet.
Should I order extra concrete?
A 5–10% allowance often helps cover uneven subgrade, spillage, and measurement error, but confirm large orders with the ready-mix supplier.