How does the Concrete Calculator work?
This calculator estimates the volume of a rectangular concrete pour from slab length, slab width and thickness. Length and width are entered in feet, while thickness is entered in inches. The tool converts thickness to feet, calculates cubic feet, applies the waste or overage percentage you choose, and divides by 27 to show cubic yards—the unit commonly used when ordering ready-mix concrete in the U.S.
The result is most useful for simple slabs, patios, pads and driveway sections with a reasonably uniform thickness. Separate footings, thickened edges, piers, steps or sloped sections should be measured as additional volumes instead of being hidden inside one average thickness. If the project also needs a stone base, use the Gravel Calculator separately.
How to measure a concrete pour
Measure the inside dimensions of the forms or the finished slab dimensions shown on the plan. Confirm thickness in several locations if the subgrade is already prepared, because an uneven excavation can increase actual volume. For a pour made of several rectangles, calculate each section and add the cubic-yard amounts.
Do not use the waste field to compensate for major geometry that should be measured directly. A thickened perimeter, footing or grade beam can contain a significant amount of concrete and deserves its own calculation. For the footprint of a slab before volume is considered, the Square Footage Calculator can be useful.
Why a concrete overage allowance matters
The theoretical form volume assumes perfectly shaped excavation and no loss. Real pours can require extra material because of subgrade variation, form movement, spillage, pump or chute loss and small measurement differences. The optional overage field lets you add a project-specific percentage to the mathematical volume. If the pour includes post footings, the Fence Calculator can help establish the fence layout before footing volumes are calculated separately.
Ready-mix suppliers may have minimum-order increments, short-load fees or ordering practices that differ from the exact calculated number. Before placing an order, confirm the required cubic yards, mix specification and delivery constraints with the supplier or contractor rather than treating the calculator as an order ticket.
What this concrete estimate does not determine
The calculator does not design reinforcement, concrete strength, joints, footing dimensions, frost protection, drainage or load capacity. Those details can depend on plans, soil conditions and local building requirements. Use the volume for budgeting and material planning only; structural or code-critical work should follow approved drawings and qualified professional guidance. Browse the Home & Construction Calculators category for other project quantities.
Frequently asked questions about Concrete
It multiplies length by width by thickness after converting thickness from inches to feet, applies the selected overage, and divides cubic feet by 27.
Ready-mix concrete in the U.S. is commonly sold by volume in cubic yards, so the calculator converts the geometric slab volume to that ordering unit.
Use an allowance that reflects the project and supplier guidance. Uneven subgrade, form tolerances and placement losses can make the real quantity higher than the perfect geometric volume.
Yes for simple rectangular sections, but calculate each footing, beam or thickened edge as a separate volume and add them rather than averaging everything into one slab thickness.
No. It estimates volume only. Mix design, compressive strength, air content and other specifications should come from the plans, engineer, contractor or supplier.
Break the project into sections with different thicknesses and calculate them separately. One average thickness can hide a large volume difference.
Treat it as a planning estimate. Confirm order increments, minimums and project conditions with the ready-mix supplier before ordering.
Yes. After a valid calculation, the result panel enables Copy Result, Download and Print for the displayed volume and supporting values.
Concrete measurement and purchasing references
The conversion used here is 27 cubic feet per cubic yard. For standardized U.S./SI measurement relationships, see NIST Office of Weights and Measures SI Units guidance. For an actual pour, the project drawings, concrete specification and ready-mix supplier’s ordering requirements are the controlling references.