Concrete Slab Calculator

How the concrete slab calculator works

This calculator takes your slab's length, width, and thickness and estimates the concrete quantity, bag counts, ready-mix order, and material cost in seconds. Internally, it converts everything to feet and cubic feet, applies a 10% waste allowance by default (adjustable with the slider), and converts the result to cubic yards for ready-mix ordering. The bag counts — at 40, 60, and 80 pound weights — are rounded up because partial bags are not returnable once mixed.

The core formula is straightforward: net volume in cubic feet equals length × width × (thickness ÷ 12). The thickness division by 12 converts inches to feet. Multiply by quantity for multiple identical slabs, then multiply by (1 + wastePercent) to get the gross volume. Divide by 27 (cubic feet per cubic yard) to convert to yards for ready-mix ordering. Bag counts use the manufacturer-stated yield per bag — 0.60 ft³ for 80 lb bags, 0.45 ft³ for 60 lb, 0.30 ft³ for 40 lb.

A worked example: a 10×10 slab at 4 inches thick. Net volume = 10 × 10 × (4/12) = 33.33 ft³. With 10% waste, gross = 36.67 ft³. That's 1.36 cubic yards of ready-mix (rounded up to 1.5 yards in quarter-yard increments). For bags: 80 lb needs ceil(36.67 / 0.60) = 62 bags, 60 lb needs ceil(36.67 / 0.45) = 82 bags, 40 lb needs ceil(36.67 / 0.30) = 123 bags. The cost band shows you which option is cheaper for your local prices.

Decision guidance — choosing the right thickness

Thickness is the single most important decision you'll make about a slab, and it's almost always dictated by what the slab carries. Four inches is the standard for patios, walkways, and shed bases on prepared subgrade. It's thick enough to resist cracking under foot traffic and light storage, and thin enough to keep material costs reasonable. Most residential patios built to code use 4 inches of concrete over 4 inches of compacted gravel.

Move to 5 or 6 inches for any slab that will see vehicles. Driveways, RV pads, and garage floors need at least 5 inches to handle the point load of a 4,000-pound vehicle resting on four tire contact patches. Six inches is better for heavy vehicles (trucks, SUVs) and is required by code in many jurisdictions for garage floors. The extra inch adds about 25% to material cost but dramatically reduces cracking risk over the slab's 30–50 year life.

For anything beyond residential — commercial slabs, equipment pads, or foundations for permanent structures — consult a structural engineer. These slabs often need engineered reinforcement (rebar cages, fiber reinforcement, or post-tensioning), specific concrete mix designs (4000+ psi), and engineered subgrade preparation. Don't guess on engineered slabs; the cost of a failure is too high.

If your soil is expansive clay, peat, or otherwise unstable, the slab thickness matters less than the subgrade. You can pour a 6-inch slab on bad soil and watch it crack within a year. Geotech testing runs $500–1500 and tells you exactly what you're working with. On poor soil, plan to excavate to solid bearing or use a structural engineer's recommendations for the base.

What the slab estimate excludes

The materials cost shown here is the tip of the iceberg for a finished slab. A realistic all-in budget is typically 2–3× the materials cost. The biggest excluded items are labour (concrete finishing alone runs $4–8 per square foot for a small job), forms (lumber, stakes, bracing — $1–3/sf), and excavation ($200–800 depending on access). If you're pouring a 10×10 shed base yourself with bagged concrete, materials are 70% of cost; if you're hiring a crew to pour a 24×24 garage, materials are 25–30% of cost.

Reinforcement is the next-largest exclusion. For a 4-inch slab, welded wire mesh (6×6 W1.4) at $0.15/sf is the standard — $15 for a 10×10 slab. For driveways and garage floors, upgrade to #4 rebar at 24-inch centers both ways — $0.50/sf or $50 for a 10×10. Add rebar chairs ($0.10 each), tie wire, and a tying tool. The rebar calculator on this site estimates linear feet of #4 bar for any slab dimension and spacing.

Other exclusions: vapor barrier (6-mil polyethylene, $0.10/sf, required under interior slabs), gravel base (4 inches minimum, $35/ton delivered), sealer ($0.25/sf for acrylic sealer, applied 30 days after pour), expansion joint material ($5 per 10-foot piece), and finishing tools (mag float, edger, broom — $50 for a basic kit). Permits run $50–300 depending on jurisdiction. Disposal of excavated soil runs $40–80 per ton at landfills or a $200–500 dump-truck load from a roll-off service.

Delivery and pumping can dwarf the concrete cost on small jobs. Ready-mix trucks carry 8–10 yards and charge a short-load fee ($150–300) for orders under 3 yards. For jobs under 1.5 yards, bagged concrete mixed on-site is almost always cheaper than ready-mix plus short-load fee. For jobs with poor access (backyard pours, hillside sites), a line pump adds $400–800 to the bill.