Concrete Slab Calculator
Calculate concrete yards and bag estimates for a rectangular slab, patio, pad, or walkway.
Enter your numbers
Which concrete tool fits your question?
- Concrete calculator: rectangular or circular placements from dimensions.
- Slab calculator: a quick rectangular slab with a thickness check.
- Bag calculator: known volume or rectangular slab dimensions, using your product’s listed yield.
- Cubic yard calculator: generic bulk volume, not a concrete mix recommendation.
Patio purchase planner: compare bagged mix with a complete ready-mix quote, order increments and delivery fees.
How this calculator works
Cubic feet = length × width × (thickness ÷ 12). Cubic yards = cubic feet ÷ 27. The selected waste allowance is added last.
Worked example
A 20 × 12 ft slab at 4 in thick with 10% extra requires about 3.26 cubic yards.
Use the structural slab thickness
Length and width establish the footprint, but thickness controls a large part of the volume. Enter the specified thickness and calculate thickened edges, grade beams, ramps, and steps separately. Do not use a common residential thickness without confirming loads, soil, reinforcement, and local requirements.
Measure forms after they are set when possible. Small differences across a large slab can change the order by a meaningful fraction of a cubic yard.
Keep volume and finish planning separate
The result estimates concrete volume, not reinforcement, vapor barrier, base stone, joints, finishing labor, pump service, or curing materials. Coordinate crew capacity and delivery spacing so the ordered volume can be placed and finished safely.
Sources & assumptions
Volume is length × width × thickness with an editable waste allowance. Bag yields follow QUIKRETE Concrete Mix product data; structural thickness and reinforcement require project-specific review.
Frequently asked questions
Is four inches thick enough?
Four inches is common for many light-duty slabs, but soil, reinforcement, loads, frost, and local code can require a different design.
Should I round ready-mix up?
Concrete is normally ordered in practical increments. Confirm the supplier’s minimum order and rounding policy.