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Concrete Slab Calculator

Pour the right amount for garage floors, basement slabs, patios, and equipment pads. Enter length, width, and thickness and get yardage, bag counts, and a waste factor.

Concrete Slab Calculator

The live calculator embeds here. Enter your dimensions to get cubic yards, bag counts, and cost in seconds.

Open the Calculator

The basics

How the calculator works

  • 1

    Enter slab length and width

    Measure the actual pour area in feet. For L-shaped slabs, save each rectangle as its own area under the job.

  • 2

    Set the thickness

    Pick 4, 5, or 6 inches. The calculator converts to feet and runs the volume for you.

  • 3

    Add thickened edges if needed

    Monolithic slabs with turn-down edges need extra concrete at the perimeter — add those as a separate footing area.

  • 4

    Apply waste and save

    Default 10% covers subgrade dips. Save the slab to the job so it totals with your footings and walls.

Show the math

The formula

Cubic Yards = (L ft × W ft × Thickness ft) ÷ 27

A slab is a simple rectangular prism. Convert thickness from inches to feet (4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft), multiply by area, and divide by 27 for cubic yards.

Worked example

A 20 ft × 20 ft patio slab at 4 inches thick.

  1. 1.4 in ÷ 12 = 0.333 ft
  2. 2.20 × 20 × 0.333 = 133.3 cubic feet
  3. 3.133.3 ÷ 27 = 4.94 cubic yards
  4. 4.4.94 × 1.10 = 5.43 cubic yards

Order 5.5 cubic yards.

20+ years in the field

Real contractor tips

Check your subgrade depth

A 4-inch slab over a rutted base can pull 4.5–5 inches in the low spots. Screed a few test points and average your real depth.

Thickened edges add up fast

A 12-inch turn-down around a 24x24 slab can add nearly a full yard. Never ignore the perimeter on a monolithic pour.

Order for one continuous pour

Slabs should go in one shot for a clean finish. Round up so you are never waiting on a second short-load truck mid-pour.

Account for the crown

Slabs sloped for drainage are thicker on the high side. Use the average thickness, not the thin edge.

Avoid these

Common mistakes contractors make

  • Using the thin drainage edge instead of average thickness.
  • Forgetting thickened edges on monolithic slabs.
  • Skipping the waste factor on an uneven subgrade.
  • Treating an L-shaped slab as one rectangle.
  • Confusing 4-inch (0.333 ft) with 0.4 ft in the math.

Versus typical concrete calculators

  • Save each slab and pad under one named job.
  • Add thickened edges as a linked footing area.
  • Bag counts and ready-mix yardage in one view.
  • Waste factor applied automatically.

Versus heavy takeoff software

  • No install — works on your phone at the site.
  • Free to start with no per-seat pricing.
  • Focused on slabs, not a full PDF markup suite.
  • Start a takeoff in seconds, not after onboarding.

Frequently asked questions

Four inches is standard for patios, sidewalks, and residential garage floors. Step up to 5–6 inches for driveways and slabs that carry heavy vehicles or equipment. See our slab thickness guide for load-based guidance.

Stop guessing yardage on the back of a napkin

Run your next pour through the calculator, save the job, and keep every area in one place. Free to start.

Open the App