Concrete slab calculator

How do I measure a concrete slab?

1. Measure the concrete footprint in feet. Split an irregular pad into nonoverlapping sections, or draw its outline. Subtract areas that will remain open rather than measuring the bounding rectangle around the entire job.
2. Enter the finished concrete thickness in inches. Use the specified section for your project; the calculator estimates quantity and does not choose slab thickness, reinforcement, soil preparation or a foundation design.
3. Add only the extra volume of thickened edges or separate footings that belongs to this pour. Measure their additional cross section and length, keeping it separate from the uniform slab thickness already included.
4. Choose an allowance based on the measured forms and site. Select ready-mix or the actual bag product, verify its mixed yield, and compare the rounded purchase quantities before requesting a quote.

How is the quantity calculated?

Cubic yards = [area (sq ft) × thickness (in) ÷ 12 + extra edge volume (cu ft)] × (1 + waste % ÷ 100) ÷ 27

• Thickness describes the concrete, not the gravel base beneath it. Calculate base material separately.
• Extra edge volume is additional concrete below or outside the uniform slab volume already counted. Do not enter the entire edge twice.
• The ready-mix order rounds up to a quarter cubic yard in this tool. Confirm the supplier’s actual increment, minimum load and delivery terms.

How does slab thickness affect a concrete order?

How many cubic yards of concrete do I need?

Start with the actual slab volume: area in square feet multiplied by thickness in feet. If thickness is measured in inches, divide it by 12 before multiplying. Divide the resulting cubic feet by 27 to obtain cubic yards. Add only the separate concrete volumes that belong to the same pour.

The net volume describes the geometry. The selected allowance covers the extra quantity you deliberately include, while the rounded order follows the tool’s increment. Review all three rather than calling them the same number. Uneven excavation, forms and changes in section need a field check before delivery.

How many bags of concrete make a cubic yard?

Use the mixed yield printed for the exact product. A bag’s dry weight alone does not establish how much finished concrete it produces. The cited QUIKRETE No. 1101 sheet lists approximate yields for its package sizes; this calculator uses those product-specific values or a custom yield that you enter.

Divide the required cubic feet, including the chosen allowance, by the selected bag yield and round up to whole bags. Do not transfer a yield from ordinary concrete mix to mortar, lightweight concrete or another specialty product. Check regional packaging and the current product sheet before buying.

Should I compare bags with ready-mix concrete?

Both outputs refer to the same adjusted concrete volume. They are alternatives, so adding the ready-mix quantity and the bag quantity would count the pour twice. Enter a current price for each method to compare its material subtotal on the same measured scope.

Include the practical costs outside that subtotal when making the purchase decision: delivery, minimum loads, mixing equipment, access, handling and labor. The calculator does not schedule a pour or determine whether a particular mixing and placement plan can keep up with the work. Ask the supplier or contractor about the actual project.

How do I include thickened edges without double counting?

The uniform slab calculation already includes the slab thickness across its footprint. For a deeper edge, add the volume below that thickness, plus any genuinely additional footprint. Entering the full edge section on top of the slab volume counts their shared concrete twice.

Keep a separate sketch of edge widths, depths and lengths. Intersecting edges and corners also overlap, so check their combined volume. This arithmetic does not size a footing or decide where thickening is needed; use the project’s specified dimensions and seek a detailed takeoff for a complex foundation.

Is gravel base included in the concrete volume?

No. A gravel base occupies a separate layer and has its own depth. The optional base result is a rounded geometric volume for that layer, not a concrete ingredient and not an automatic conversion to tons. Keep it on its own purchase line.

If the supplier quotes loose aggregate while your depth describes a finished compacted layer, obtain the appropriate quantity basis from that supplier. This tool does not invent a density or compaction multiplier. Also measure excavation and disposal separately; neither is established by multiplying the concrete order by a guessed factor.

Does the calculator’s site-mix mode specify a concrete design?

No. That mode is an estimating comparison for its stated ingredient assumptions. It does not establish strength, water content, air entrainment, aggregate grading, exposure suitability or acceptance criteria. A volume result cannot replace a specified concrete mixture.

For work requiring defined concrete performance, use the approved mix or product and its documented yield. When reviewing costs, keep cement, sand and gravel ingredient estimates separate from ready-mix and premixed-bag alternatives. Save the selected method with its assumptions so the shopping list does not combine mutually exclusive ways to buy the same concrete.

How do I estimate the material cost?

Enter current ready-mix and bag prices to compare material quantities on the same scope. The calculated subtotals exclude unentered delivery, equipment and installation. Save the chosen method and retain the volume assumptions for a supplier quote.

Keep the concrete estimate in your list.