Concrete Slab and Pad Calculator
For a rectangular slab, patio or shed pad. Add a thickened edge if your plans call for one, and an optional price per yard from your supplier.
How it works
Slab volume = length × width × thickness (in) ÷ 12. Thickened edge volume = (2 × (length + width) × edge width − 4 × edge width²) × extra depth, a rectangular band around the perimeter with corners counted once. Total × number of slabs × (1 + allowance) = concrete to order. Cubic yards = cu ft ÷ 27; bags = cu ft ÷ bag yield, rounded up.
Worked example: A 20 × 12 ft slab, 4 in thick, is 80 cu ft. A 12 in wide edge 8 in deeper adds (2 × 32 × 1 − 4 × 1) × 8 ÷ 12 = 40 cu ft. Total 120 cu ft = 4.44 cu yd before allowance.
Assumptions
- Rectangular slab of even thickness on a level, compacted base. Base material (gravel) is separate; see the gravel calculator.
- Thickened edges are modeled as a rectangular band; sloped or tapered edges hold less. Use the sizes in your plans.
- Bag yields are QUIKRETE's published Concrete Mix figures (80 lb ≈ 0.60 cu ft). QUIKRETE describes the mix for pours 2 in thick or more.
- The price per yard is your own estimate from a supplier; it is not filled in by default and excludes delivery, short-load and pumping fees.
- The waste allowance is an editable JobsiteCalc default, not an industry standard. Ask your supplier what extra they suggest for your job.
- This is a quantity estimate only. It does not cover structural design, thickness, reinforcement or permits; check local codes and product instructions.
Frequently asked questions
How much concrete do I need for a 12 × 20 slab?
At 4 in thick: 12 × 20 × 4 ÷ 12 = 80 cu ft = 2.96 cubic yards before any allowance. With a 12 in wide edge thickened 8 in deeper, add 40 cu ft, for 4.44 cubic yards.
How many bags of concrete for a 10 × 10 slab?
At 4 in thick, 33.3 cu ft. QUIKRETE's 80 lb bag yields about 0.60 cu ft, so about 56 bags, or 62 with a 10% allowance.
Should I use bags or ready-mix?
That depends on volume, access and your supplier. This calculator shows both the bag count and the cubic yards so you can compare quotes.
Related calculators
- Concrete calculator (bags and cubic yards)
- Cubic yards and cubic feet calculator
- Paver and patio calculator
- Gravel calculator (cubic yards and tons)
- Asphalt calculator (tons)
Sources
Formulas and reference values are from these sources. Page last reviewed 2026-10-19.