Concrete calculator
Concrete slab
Volume, ready-mix order quantity, and 80 lb bag equivalent.
Calculator result and formula
Ready-mix order: 3.25 yd³
Formula: L × W × D × quantity ÷ 27 × (1 + waste) → ready-mix order
Formula ID: CONC-SLAB-001 · Version 1.0.0.
Inputs
- Length: Finished slab length Default: 20.
- Width: Finished slab width Default: 12.
- Depth: Slab thickness Default: 4.
- Number of slabs: Count identical rectangular slabs Default: 1.
- Waste factor: Allowance for spill and site conditions Default: 8.
How this calculator works
Use finished slab dimensions and depth to estimate net volume, then apply a practical waste allowance before ready-mix purchase rounding.
- Measure finished length, width, and slab thickness.
- Select a waste allowance that reflects access, placement, and spill risk.
- Use the rounded ready-mix quantity as the starting point for a supplier quote.
Worked example: Example: a 20 ft × 12 ft slab at 4 in depth has 80 ft³ of net concrete. With an 8% waste allowance, this tool rounds the ready-mix order to 3.25 yd³.
Assumptions and limitations
- Ready-mix quantity is rounded up to the next 0.25 yd³.
- 60 lb and 80 lb bag yields are planning assumptions of 0.45 ft³ and 0.60 ft³ respectively; confirm the selected product label.
- Truck-load equivalent assumes a 10 yd³ planning load, not supplier delivery capacity.
- Waste is applied to net volume before purchase rounding.
Common questions
- Why is ready-mix rounded up?
- Concrete is ordered in supplier increments; rounding protects the pour from a small material shortfall.
- Does this replace a structural mix design?
- No. Confirm strength, reinforcement, thickness, local code, and mix requirements with the responsible professional.
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