Concrete Slab Calculator

Methodology

How every calculator on this site computes its results — the formulas, the constants, and the conventions baked into the math.

Volume formulas

  • Slab / footing / wall: V = length × width × thickness, with thickness in inches converted to feet.
  • Round column / sonotube: V = π × (diameter / 24)² × height (feet).
  • Round slab: V = π × (diameter / 2)² × thickness.
  • Post hole: cylinder volume minus optional rectangular post cross-section.

Constants & conventions

  • 1 cubic yard = 27 cubic feet (3 ft × 3 ft × 3 ft).
  • Concrete density = 150 lb/ft³ for normal-weight ready-mix — the standard NRMCA figure, or about 4,050 lb per cubic yard.
  • Bag yields are the manufacturer-published figures (Quikrete and Sakrete are identical for these sizes):
    • 40 lb bag → 0.30 ft³
    • 50 lb bag → 0.375 ft³
    • 60 lb bag → 0.45 ft³
    • 80 lb bag → 0.60 ft³
  • Default waste factor: 10% for slabs and footings, 5% for sonotubes and round columns. Waste covers spillage, over-excavation, and form leakage. The factor is overridable on every calculator.

Pricing model

Ready-mix pricing defaults to a $160 / yd³ midpoint based on 2024–2026 US national averages ($140–$180 / yd³ delivered). Orders under 3 yd³ trigger a default $60 short-load fee — a reasonable midpoint within the typical $50–$120 range. Both numbers are user-overridable and should be confirmed with your local supplier.

Bagged retail pricing defaults to $5.50 per 60 lb bag (Quikrete big-box average). The cost calculator scales other bag sizes proportionally to the entered 60 lb price.

Slab thickness recommendations

The thickness presets in the slab calculator are industry rules of thumb for residential pours, not engineered values. Verify with your local building department:

  • 4″ — sidewalks, patios, standard passenger-vehicle driveways
  • 5–6″ — heavy-duty / RV driveways
  • 4″ — garage floors, shed foundations
  • 6″ — light commercial slabs (verify per ACI 360 / engineered)

Where we're less precise

  • We assume normal-weight concrete (150 lb/ft³). Lightweight mixes (90–115 lb/ft³) and high-density mixes (180+ lb/ft³) need a custom density.
  • We don't account for sub-grade settling — a soft sub-base can consume an extra 5–10% of volume that the 10% waste factor only partially covers.
  • Cost figures exclude labor, forms, rebar, gravel base, and finishing. Use the cost calculator output as material-only.
  • Bag yields assume properly mixed concrete at recommended water content. Over-watering reduces yield and strength.

Disclaimer

Estimates only. Consult a licensed concrete contractor, structural engineer, or your local building department for final pour quantities, thickness, reinforcement, and code compliance.