Concrete Calculator

Estimate concrete slab volume in cubic feet and cubic yards from length, width and thickness, with patio examples and ordering guidance. Calculate it clearly with Figur.

Calculate concrete slab volume in cubic yards

For a rectangular slab, convert thickness from inches to feet by dividing by 12. Multiply length × width × thickness in feet to find cubic feet, then divide by 27 to find cubic yards. Enter length and width in feet and thickness in inches; mixing these units without converting gives the wrong volume. The result is the geometric slab volume, not an order that includes waste.

Method and assumptions

Cubic feet = length in feet × width in feet × (thickness in inches ÷ 12). Divide that volume by 27 to get cubic yards. One yard contains three feet, so a cubic yard contains 3 × 3 × 3 = 27 cubic feet.

  • The slab is rectangular and has the same thickness throughout.
  • Waste, uneven ground and minimum supplier order increments are not included.
  • Calculate other shapes or sections with different thicknesses separately and add their volumes.
  • The calculation does not determine strength, reinforcement, foundations, structural design or cost.

Sources and references

Volume uses geometry and the exact relationships of 12 inches per foot and 27 cubic feet per cubic yard; it does not use price data or assumed bag yields.

Worked example

A rectangular patio is 20 feet long and 10 feet wide, with a specified thickness of 4 inches.

20 × 10 × (4 ÷ 12) = 66.67 cubic feet. Divide by 27 for 2.47 cubic yards before an ordering allowance.

Frequently asked questions

How much concrete do I need for a 20 × 10 slab?

At 4 inches thick, a 20 × 10 foot slab is 66.67 cubic feet, or about 2.47 cubic yards. A thicker slab needs more concrete; enter the thickness your project requires.

How much concrete does a sidewalk need?

A rectangular sidewalk 30 feet long, 3 feet wide and 4 inches thick needs 30 cubic feet, or about 1.11 cubic yards before any extra allowance. Calculate separate sections and add their volumes if widths or thicknesses differ.

Does the answer include waste or extra concrete?

No. Discuss uneven ground, spillage and the supplier's ordering increments before placing an order. If you choose a 10% allowance, multiply the unrounded volume by 1.10; the 20 × 10 foot example becomes about 2.72 cubic yards.

Can I convert cubic yards to concrete bags?

Yes, but bag weight alone does not determine yield. Convert cubic yards to cubic feet by multiplying by 27, divide by the finished-volume yield printed on your chosen product, and round up to whole bags. This calculator does not assume a universal bag yield.

Does this tell me how thick a slab should be?

No. Thickness is an input, not a structural recommendation. Loading, soil, reinforcement, local requirements and the slab's purpose need a project-specific design.

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