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.

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 · Published · Last reviewed Aug 26, 2026.

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 mistakes to avoid

Confirm the measurement basis and units before using the result.
Keep waste or allowance visible and adjust it for the actual job conditions.
Confirm supplier packaging, delivery increments, specifications, and final site requirements before ordering.

Calculator disclaimer

This planning calculator does not determine structural design, code compliance, installation requirements, or guaranteed pricing. Confirm final requirements with the appropriate responsible professional, authority, manufacturer, and supplier.

Formula review and sources

Internal QA complete · independent external review pending · Formula assumptions documented.

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