Concrete Slab Calculator — ft to cu yd, bags, cost
How to use this calculator
Pick a shape, type dimensions in feet + inches (⅛″ fractions). Thickness rule of thumb: 4″ patios and sidewalks, 5″ driveways, 6″ garage slabs, 8″ for heavy or vehicle-loaded pads. The slab is sized for the pour — sub-base, vapor barrier and edge thickening are separate items in a real takeoff, so toggle them on below when they belong in your scope.
Cost — pick ready-mix truck per yd³, 80-lb bags, or 60-lb bags to get the total material cost.
Reinforcement — four welded wire mesh types or four rebar sizes (#3–#6) with spacing 6″–24″ o.c., lap splices per ACI 318-19, and waste allowance.
Labor — rate per ft², per yd³, or a flat price. Grand total sums all active sections.
Saved Calculations
| Time | Shape | Area ft² | Vol yd³ | Concrete $ | Rebar | Qty | Rebar $ | Labor | Total |
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How to Calculate Concrete for a Slab
Volume is area × thickness. Pick the shape, enter dimensions in feet, inches and ⅛″ fractions, then set 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. Order short of the truck minimum (usually 1 yd³) and you pay for the minimum anyway, so round up.
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.
