Concrete Calculator
Estimate the cubic yards and bags of concrete you need for a slab, footing or patio.
- Concrete needed1.23 cubic yards
- Volume33.33 cubic feet
- 80 lb bags56 bags
- 60 lb bags75 bags
How to use
- Enter the length and width of your slab in feet.
- Enter the thickness in inches (4" is typical for a slab).
- Read the cubic yards and number of bags you need.
About this calculator
This concrete calculator works out the volume of a rectangular slab or footing and converts it to cubic yards (how ready-mix concrete is ordered) and to the number of pre-mixed bags. The math is simple: volume = length x width x thickness, converted to cubic yards by dividing by 27. An 80 lb bag of concrete yields about 0.60 cubic feet, a 60 lb bag about 0.45 cubic feet. For large pours it is almost always cheaper to order ready-mix by the yard than to buy bags. Always add about 5-10% extra for spillage and uneven subgrade.
🧮 How it's calculated
- 1Step 1 — Convert thickness to feet: Thickness (ft) = Thickness (in) ÷ 12
- 2Step 2 — Find volume in cubic feet: Volume (ft³) = Length × Width × Thickness (ft)
- 3Step 3 — Convert to cubic yards: Cubic yards = Volume (ft³) ÷ 27
- 4Step 4 — Count bags: 80 lb bag ≈ 0.60 ft³ → bags = ⌈Volume ÷ 0.60⌉
Cubic yards = (Length ft x Width ft x Thickness ft) / 27, where Thickness ft = inches / 12.
Frequently asked questions
How many 80lb bags of concrete are in a yard?+
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cubic feet, so it takes roughly 45 bags to make one cubic yard.
How thick should a concrete slab be?+
4 inches is standard for patios, walkways and shed floors. Use 5-6 inches for driveways or anything bearing heavy vehicles.
Should I add extra concrete?+
Yes. Add about 5-10% to allow for spillage, over-excavation and uneven ground so you do not run short mid-pour.