Concrete Slab Calculator
Calculate concrete volume, rebar needs, and cost estimate for a slab.
Enter dimensions and reinforcement type for complete material estimates.
Concrete slab volume and bag estimation prevents the common mistake of ordering too little (visible cold joints) or too much (expensive waste). The formula is straightforward but requires careful unit conversion.
Volume formula: Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
Converting to cubic yards (standard ordering unit for ready-mix): Cubic Yards = Cubic Feet ÷ 27
Quick formula directly to cubic yards: Cubic Yards = (Length × Width × Thickness in inches) ÷ 324
Always add 10% overage for uneven subgrades, spillage, and slight thickness variations.
Thickness standards by slab type:
- Patio / sidewalk: 3.5–4 inches (minimum 3.5")
- Driveway (passenger cars): 4–5 inches
- Driveway (heavy trucks): 6 inches
- Garage floor: 4–6 inches
- Structural floor slab: 4–8+ inches (engineer-specified)
- Concrete countertop: 1.5–2 inches
Concrete mix strengths:
- 2,500 psi: sidewalks, patios (light use)
- 3,000 psi: driveways, garage floors (standard)
- 3,500–4,000 psi: heavy loads, commercial
- 4,500+ psi: structural, high-traffic areas
Bags of concrete (pre-mix) vs. ready-mix threshold:
- Under 0.5 cubic yards: bags are practical
- 0.5–1.0 cubic yards: borderline — bags or mixing service
- Over 1.0 cubic yard: order ready-mix — far more economical
Bags of pre-mix concrete per cubic yard:
- 60 lb bag = 0.017 cubic yards → ~45 bags per cubic yard (very labor intensive)
- 80 lb bag = 0.022 cubic yards → ~45 bags per cubic yard
- 90 lb bag = 0.025 cubic yards → ~40 bags per cubic yard
Ready-mix concrete cost (US average):
- $120–$200 per cubic yard (includes delivery for typical 7-8 yard min load)
- Short load fee if less than minimum: $100–$200 extra
Worked example: Backyard patio: 16 ft × 20 ft × 4 inches thick.
- Volume: (16 × 20 × 4) ÷ 324 = 1,280 ÷ 324 = 3.95 cubic yards
- Add 10%: 3.95 × 1.10 = 4.35 cubic yards → order 4.5 yards
- Cost at $150/yard: 4.5 × $150 = $675 for ready-mix concrete
- Mix specification: 3,000 psi (standard for outdoor patio)
- Reinforcement: #3 rebar at 18″ on center each way, or 6×6 wire mesh