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 as a conservative planning value, or about 4,050 lb per cubic yard. NRMCA describes normal-weight concrete as approximately 140–150 lb/ft³.
  • Bag yields are the manufacturer-published figures from the standard-mix manufacturer data used by this site:
    • 40 lb bag → 0.30 ft³
    • 50 lb bag → 0.375 ft³
    • 60 lb bag → 0.45 ft³
    • 80 lb bag → 0.60 ft³
  • Primary yield references: Quikrete Concrete Mix data sheet and Sakrete High-Strength Concrete Mix. Product-specific yields can differ, so the bag label controls.
  • 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 uses an editable $160 / yd³ planning default and an editable $60 short-load allowance below 3 yd³. These are examples, not a national quote or live market data. Supplier prices, minimum loads, delivery zones, fuel charges, taxes, and short-load policies vary; replace both values with a written local quote before budgeting.

Bagged retail pricing defaults to an editable $5.50 per 60 lb bag for comparison only. 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 through an engineered design)

For design context, review the American Concrete Institute's slabs-on-ground guidance. ACI notes that ACI 360 primarily addresses industrial slabs; it is not a substitute for local residential code or a project-specific design.

Sources and assumptions reviewed September 20, 2026.

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.