Concrete Slab Calculator
Cubic Yards, Bags & Cost

The concrete slab formula

Concrete is ordered in cubic yards in the United States. One cubic yard equals 27 cubic feet. The slab calculation converts your dimensions to cubic feet first, then divides by 27 to get the order quantity.

For 4-inch slabs specifically, there is a field shortcut called the Magic Number 81. Divide your total square footage by 81 to get cubic yards. This works because 4 inches is exactly one-third of a foot, and 3 × 27 = 81. A 20×10 patio is 200 square feet ÷ 81 = 2.47 CY. Use the concrete volume calculator for non-standard thicknesses or mixed pour types.

Always add the ACI-recommended 10% overage before placing your order. Ordering the exact theoretical volume almost always results in running short — see Section 3 in the concrete slab guide for a detailed explanation of why short pours happen and how to avoid cold joints.

Slab thickness by application

Thickness is the single most important specification for a concrete slab. Under-specify it and you risk cracking under load. Over-specify it and you pay for concrete you don't need. ACI 301 and the IRC provide minimum requirements; the table below shows both code minimums and recommended practice for residential work.

The IRC 2024 minimum for any concrete slab on grade in a residential structure is 3.5 inches. In practice, 4 inches is the standard for residential flatwork because it is the practical minimum for acceptable durability and the most cost-effective use of forming materials. For driveways specifically, ACI 330R recommends 5 inches for residential use and 6 inches where occasional medium-truck traffic is expected. Use the concrete driveway calculator for driveway-specific thickness and reinforcement calculations.

PSI specification guide

PSI (pounds per square inch) is the compressive strength of the concrete at 28 days. Higher PSI concrete costs more per cubic yard but is more durable, more resistant to freeze-thaw damage, and less permeable to water and deicers. Specify the right strength for your climate and use case — over-specifying wastes money, under-specifying causes premature failure.

For any slab subject to freeze-thaw cycling and deicing salts, ACI 318-19 Table 19.3.3.1 sets a minimum 4500 PSI and requires air entrainment (typically 6% total air content). In practice, specify 4000 PSI with 5 to 7% air entrainment for any exterior slab in USDA Hardiness Zones 1 through 7. Check with your ready-mix plant — they will confirm air entrainment availability and any minimum order requirements for specialty mixes.

Rebar vs welded wire mesh

Both rebar and welded wire mesh (WWM) are used to control cracking in concrete slabs on grade. They do not add meaningful load-bearing capacity — that comes from the concrete thickness and subgrade preparation. What reinforcement does is hold cracks together when they form, preventing them from widening and becoming structural or waterproofing failures.

The most common field mistake is placing WWM at the bottom of the slab instead of the middle third. Mesh on the ground has zero structural value — it must be supported on concrete chairs or dobies at approximately one-third of the slab depth. ACI 360R-10 notes that improperly placed mesh is a major cause of slab cracking failures in residential construction.

For most residential driveways and garage floors, #3 rebar on 18-inch centers provides meaningfully better crack control than WWM at a modest cost premium. Factor the labour and material cost into your total using the concrete cost calculator. For a full comparison of slab reinforcement options with cost data, see the concrete slab guide.

2025 concrete slab cost guide

Installed concrete slab cost in 2025 ranges from $6 to $12 per square foot for standard residential flatwork, with regional variation of plus or minus 20 to 30%. This price includes forming, pouring, finishing, and curing — but not excavation, base preparation, or hauling. For projects where you are supplying concrete and hiring labour separately, use the table below alongside the concrete bag calculator for smaller pours.

Ready-mix concrete averaged $160 to $195 per cubic yard nationally in 2025, up 8 to 10% from 2023. Short-load fees apply when ordering less than the plant minimum — typically 3 to 5 cubic yards — and range from $50 to $200 per delivery. For small pours under 1.5 cubic yards, bagged concrete is often more economical once short-load fees are included. The calculator above runs this comparison automatically.