Concrete Slab Calculator

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, so weigh bags against a mixing service
• Over 1.0 cubic yard: order ready-mix, which is far more economical

Bags of pre-mix concrete per cubic yard. A cubic yard is 27 cubic feet, so divide 27 by the bag’s yield:

• 60 lb bag = 0.45 cu ft = 0.0167 cubic yards → 60 bags per cubic yard
• 80 lb bag = 0.60 cu ft = 0.0222 cubic yards → 45 bags per cubic yard
• 90 lb bag = 0.675 cu ft = 0.025 cubic yards → 40 bags per cubic yard

Sixty bags is 3,600 lbs of mix to open, carry and stir by hand, which is why the one-yard line above is a real threshold and not a shrug.

Ready-mix concrete cost (US average, 2026, get a local quote):

• $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: ordering 4.5 yards is 4.5 × $150 = $675 for ready-mix concrete. The calculator prices the exact 4.35 instead, at $652, because it does not know what increments your supplier sells in. Most sell in quarter or half yards.
• Mix specification: 3,000 psi (standard for outdoor patio)
• Reinforcement: #3 rebar at 18″ on center each way, or 6×6 wire mesh

The same pour in bags would be 4.35 × 45 = 196 bags of 80 lb mix, close to four tons. That is the comparison worth running before anyone decides to save the delivery fee.