Cubic Feet for Concrete Calculator
To calculate cubic feet for concrete slabs, multiply Length (ft) × Width (ft) × Thickness (in feet: inches ÷ 12). Divide cubic feet by 27 to get cubic yards for ready-mix trucks. Always add 10% extra for subbase variance.
Reference & Benchmark Chart
| Input | Volume | Application |
|---|---|---|
| 10 × 10 ft slab at 4 in | 33.33 cu ft | 1.23 cu yds (56 bags of 80-lb) |
| 12 × 20 ft patio at 4 in | 80.00 cu ft | 2.96 cu yds (134 bags of 80-lb) |
| 20 × 20 ft garage at 5 in | 166.67 cu ft | 6.17 cu yds (ready-mix truck recommended) |
| 40 × 16 ft driveway at 5 in | 266.67 cu ft | 9.88 cu yds (~10 cubic yards ready-mix) |
Cubic Feet for Concrete Calculator
Concrete is the world's most utilized building material, composed of Portland cement, fine aggregate (sand), coarse aggregate (gravel), and water. Estimating concrete volume in cubic feet is the essential first step before converting to commercial cubic yards (for ready-mix concrete trucks) or individual pre-mixed bags (such as Quikrete 60-lb or 80-lb sacks). Under-ordering concrete during a pour causes cold joints (structural failure zones where wet concrete cures against already-hardened concrete), while massive over-ordering incurs expensive disposal fees. Structural civil standards dictate minimum slab thicknesses based on anticipated traffic loads.
Formula
Concrete Volume (ft³) = Length (ft) × Width (ft) × (Thickness in inches ÷ 12) × 1.10 (Waste Buffer) Example
For a residential two-car driveway measuring 40 feet long, 16 feet wide, and 5 inches thick (5 ÷ 12 = 0.417 ft): Raw volume = 40 × 16 × 0.417 = 266.88 cubic feet. Add 10% safety buffer: 266.88 × 1.10 = 293.57 ft³. Convert to cubic yards: 293.57 ÷ 27 = 10.87 cubic yards. Contractor orders 11.0 cubic yards from the concrete batch plant.
Applications
- Driveways & Slabs: Sizing 4-inch sidewalk slabs, 5-to-6-inch vehicle driveways, and 6-inch garage floors.
- Post Footings: Calculating circular or rectangular fence post and deck pier foundations.
- Retaining Wall Footers: Computing structural continuous grade beam footings.
Common Pitfalls
- Ignoring formwork bowing: Hydrostatic pressure from wet concrete pushes wood forms outward, consuming 5% to 8% extra volume.
- Ordering without 10% buffer: Subgrades are rarely laser-flat; uncompacted dirt pockets absorb extra concrete.
💡 Soalan Lazim & Petua
- Rule of thumb bag yields: An 80-lb bag yields 0.60 cu ft; a 60-lb bag yields 0.45 cu ft; a 50-lb bag yields 0.375 cu ft.
- If your project requires more than 1 cubic yard (45 bags of 80-lb mix), ordering a ready-mix truck is significantly faster, stronger, and more cost-effective.
Soalan Lazim
How many 80 lb bags of concrete make a cubic foot?
An 80-lb bag yields roughly 0.60 cubic feet. Therefore, you need 1 ÷ 0.60 = 1.67 bags (almost two bags) to make 1 cubic foot of mixed concrete.
How many cubic feet is a yard of concrete?
One cubic yard of concrete equals exactly 27 cubic feet (3 ft × 3 ft × 3 ft).
How thick should a concrete driveway be?
Standard residential car driveways should be at least 4 inches thick (5 inches recommended). For heavy SUVs, delivery vans, or RV parking, 6 inches reinforced with rebar is required.