Concrete Slab Calculator
Estimate cubic yards of concrete for a rectangular slab, patio, driveway, or footing. Enter length, width, and thickness with an optional waste allowance for spillage and over-excavation.
Assumptions
- Slab is a uniform rectangular prism with no steps or thickened edges.
- Thickness is consistent across the entire pour area.
- Default waste factor is 10% for typical residential pours.
- 1 cubic yard = 27 cubic feet.
- Does not include gravel base, rebar, or pump truck fees.
Volume = length × width × thickness (converted to cubic yards). The result adds your waste percentage: ordered volume = base volume × (1 + waste% ÷ 100).10 ft × 10 ft patio, 4 in thick
- Area = 10 × 10 = 100 ft². Thickness 4 in = 0.333 ft. Volume = 100 × 0.333 = 33.3 ft³ = 1.23 yd³. With 10% waste: 1.23 × 1.10 ≈ 1.36 yd³ to order.
How to measure
Measure the longest straight length and width of the pour area at grade. For thickness, use the specified slab depth from your plan — typical patios are 3.5–4 in and driveways 4–6 in. Deduct only areas that will not receive concrete.
Common mistakes
- Measuring thickness in feet when the field is in inches (4 in ≠ 4 ft).
- Forgetting to add waste for spillage, uneven subgrade, or partial loads.
- Using interior room dimensions instead of the actual form or edge dimensions outdoors.
Frequently asked questions
How many 80 lb bags equal a cubic yard?
Roughly 45 bags of 80 lb premix per cubic yard, but ready-mix by the truck is usually cheaper for slabs over about 1 yd³.
Should I add extra for a thickened edge?
Yes. This calculator assumes uniform thickness. Add a separate volume for footings or thickened perimeter beams.
What waste percentage should I use?
10% is common for flatwork. Use 5% for simple rectangles with good access, or 15% for irregular shapes.
Does this include the gravel base?
No. Use the gravel or topsoil calculators for base material volume separately.