PracticalCalcs

Home improvement calculator

Concrete Slab Calculator

Estimate how much ready-mix concrete you need for a rectangular slab using length, width, thickness, waste allowance, and price per cubic yard.

Estimate concrete needed

Order estimate

3.26yd³

Base volume
2.963 yd³
Waste allowance
0.296 yd³
Metric volume
2.49 m³
Estimated concrete cost
$538

Formula and methodology

  1. Thickness in feet = thickness in inches / 12.
  2. Cubic feet = length in feet x width in feet x thickness in feet.
  3. Cubic yards = cubic feet / 27.
  4. Order quantity = cubic yards x (1 + waste percentage).
  5. Estimated cost = order quantity x price per cubic yard.

Worked example

20 ft x 12 ft patio at 4 in thick

  • Length: 20 ft
  • Width: 12 ft
  • Thickness: 4 in
  • Waste: 10%
  • Concrete price: $165/yd³

Base volume is 2.96 yd³. With 10% waste, order about 3.26 yd³, estimated at $538.

Assumptions

  • The slab is rectangular and has a consistent thickness.
  • Waste allowance defaults to 10 percent.
  • Concrete price defaults to $165 per cubic yard and should be replaced with a local quote.
  • The calculator estimates material volume only; labor, rebar, gravel base, forms, permits, and delivery fees are not included.

Last updated: 2026-06-06

FAQ

Should I round up concrete orders?

Yes. Ready-mix concrete is commonly ordered with a small waste allowance because under-ordering can be expensive and disruptive.

Does this include labor or reinforcement?

No. This calculator estimates concrete volume and ready-mix material cost only.

This estimate is for planning only. Confirm slab design, site conditions, and order quantities with a qualified contractor or local supplier.