Concrete Slab Calculator — ft to cu yd, bags, cost

Concrete slab calculator in feet — cubic yards, bags, cost

This is a slab calculator in feet: enter dimensions in feet, inches and ⅛″ fractions, set the thickness, and it returns cubic yards, 60/80-lb bags, rebar, mesh and a full cost estimate. To calculate concrete for a slab, volume is area × thickness — 4″ for patios and sidewalks, 5–6″ for driveways, 8″+ for heavy-duty or vehicle-loaded slabs per IRC and ACI 318-19 slab-on-grade practice. The result divides by 27 to convert ft³ to yd³, then adds your waste factor. As a worked example, how many bags of concrete for a 12×12 slab: a 12×12 ft slab at 4″ thick is 4 ft³ × 12 × 12 ÷ 27 ≈ 1.78 yd³ — about 81 sixty-lb bags or 61 eighty-lb bags, plus 10% waste; above ~1 yd³ a ready-mix truck is usually cheaper. Order short of the truck minimum (usually 1 yd³) and you pay for the minimum anyway, so round up. For non-slab members (columns, walls, footings) in the same order, use the concrete volume calculator.

Waste, bags vs. ready-mix

Use 5% waste for clean rectangular pours on level subgrade, 10% as the typical default, and 15–20% for L-shapes, hand-dug edges, or pumped placement where spillage is higher. Bagged mix is only economical below roughly 1 yd³: an 80-lb bag yields about 0.022 yd³ (0.6 ft³) and a 60-lb bag about 0.017 yd³. Above ~1 yd³, ready-mix delivery per cubic yard is cheaper and faster than mixing bags by hand.

Reinforcement per ACI 318-19

Choose welded wire mesh (WWM 6×6 or 4×4) for crack control in non-structural slabs, or #3–#6 rebar on a 6–18″ grid for load-bearing work. Lap splices are taken at 40 bar diameters per ACI 318-19 §25.5, plus sheet overlap and cutting waste. For thickened edges or footings cast with the slab, size them with the spread footing calculator or the strip footing calculator. Isolated supports — deck posts, columns — use the concrete pier calculator, and turned-down beams use the grade beam calculator.

On pour day

Three line items estimators forget. Short-load fee — most plants charge $80–150 if you order under ~3 yd³, so a 12×12 patio (≈1.8 yd³ at 4″) often comes out cheaper in 80 lb bags. Thickened edge — monolithic shed and garage slabs need an 8″×8″ haunch around the perimeter; add it as its own line, not as more "waste". Pump truck — backyard pours past 30 ft of hose, second-floor decks, or anywhere a truck can't back up need a boom or line pump (typically $700–1,200 minimum). Ready-mix in 2025 USA averages $150–200/yd³ delivered with a 1 yd³ minimum, and ACI 318-19 §26 requires control joints at roughly 24–36× slab thickness in each direction. Save runs to History and export text, CSV, A4 JPG, or PDF with the diagram — estimates are planning numbers, not a substitute for a structural drawing.