Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate how much concrete you need for a slab, patio, or pad in cubic yards and cubic feet.
This calculator works out how much concrete you need for a flat, rectangular pour -- a patio, sidewalk, garage pad, or any similar slab -- by turning three simple measurements (length, width, and thickness) into a volume you can order from a ready-mix supplier or buy as bagged concrete. It's the general-purpose version of the concrete calculators on this site; if you're pouring a specific shape like a footing, a wall, or a set of steps, the Home Improvement Calculators category has a dedicated tool built around that shape's actual geometry.
Enter the length and width of your pour in feet, and the thickness in inches (concrete slabs are almost always specified in inches, even though the footprint is measured in feet). A 10% waste factor is included by default to cover spillage, uneven subgrade, and minor over-excavation -- a commonly used contractor allowance -- but you can adjust it to 0% if you want the theoretical exact volume instead.
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
cu yd = cu ft ÷ 27
Dividing the thickness by 12 converts it from inches to feet so all three dimensions share the same unit before multiplying. Dividing the resulting cubic feet by 27 converts to cubic yards, since a cubic yard is 3ft × 3ft × 3ft = 27 cubic feet -- the unit ready-mix concrete is actually sold in.
This is the calculator most people reach for when pouring a patio, a small shed pad, a sidewalk section, or any flat rectangular slab under a few inches thick. If you're ordering from a ready-mix truck rather than mixing bags, the Concrete Slab Calculator takes the same inputs and rounds your order up to the nearest quarter cubic yard, which is how ready-mix is actually sold. If you'd rather work directly in bag counts instead of cubic yards, use the Concrete Bag Calculator.
This assumes a flat, level pour of uniform thickness. It doesn't account for a sloped subgrade, a thickened edge, or embedded features like a drain -- for those, add extra volume manually or consult a contractor. The default 10% waste factor is a common estimating allowance, not a guarantee; very rough or uneven ground may need more.
Once you have your volume in cubic feet from this calculator, use the Concrete Bag Calculator to convert it into 40, 60, or 80 lb bag counts based on each bag's published yield.
4 inches is standard for patios, sidewalks, and shed pads. Garage floors are commonly 5 inches, and driveways that need to support vehicle weight are typically 6 inches.
Real pours rarely match the exact theoretical volume -- subgrade isn't perfectly flat, some concrete spills during placement, and forms aren't perfectly precise. A 10% allowance is a widely used contractor rule of thumb to avoid running short mid-pour.
No -- reinforcement doesn't meaningfully change the concrete volume needed and isn't part of this calculation.
Only approximately -- split an irregular shape into rectangular sections, calculate each separately, and add the volumes together.