Enter dimensions to see bag estimates.
Formula: yards = (L × W × T in feet) ÷ 27, then × (1 + waste%). Example: 20 ft × 10 ft × 4 in (0.333 ft) = 66.7 cu ft = 2.47 yd.
Stop typing dimensions. Trace the slab on the plan.
This calculator assumes a rectangle. Real slabs follow the drawing — odd corners, steps, and cutouts. Upload the PDF, set scale, trace the pour, and get the area without retyping every dimension.
Trace it on a PDF — free guest modeHow to use this calculator
- Measure the pour as a rectangle — length and width on the ground or from the plan.
- Enter thickness. Slabs are usually 4 in; footings might be 8–12 in deep and 16–24 in wide.
- Leave waste at 10% unless you know the subgrade is rough or access is tight.
- Order ready-mix by cubic yards with waste. Use bag counts for small pads and post holes.
Not a stamped engineering calculation
This is a kitchen-table estimate for ordering concrete on a simple rectangular pour. It does not account for rebar chairs, pump line loss, or irregular shapes. When the slab isn't a rectangle — or you have the plan open — trace the real outline instead of guessing dimensions.
FAQ
How many 80 lb bags of concrete do I need?
Divide your total cubic feet (including waste) by 0.6. An 80 lb bag yields about 0.6 cubic feet. Round up — running short mid-pour is worse than one extra bag.
How much waste should I add for a concrete slab?
10% is a common default for flatwork: spillage, uneven subgrade, and a little left in the truck chute. Tight access or complex forms might need 15%. This calculator defaults to 10% — adjust if your job site is messy.
Is this the same for a slab and a footing?
The math is the same for any rectangular pour: length × width × thickness, divided by 27 for cubic yards. Footings are usually thinner in one direction but deeper — measure each run as its own rectangle if the dimensions change. This tool handles one rectangle at a time.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. Ready-mix is ordered in cubic yards; bags are sold by cubic feet. Formula: yards = (length × width × thickness in feet) ÷ 27.