Concrete & masonry
Concrete Slab Calculator
Estimate slab cubic yards, ready-mix order quantity, whole concrete bags, waste and optional material cost.
Estimated order quantity
0.00 yd³ ready-mix order
- Waste and order rounding
- Bag yield
- Supplier minimums, short-load fees, delivery, taxes, reinforcement and labor are not included.
Planning estimate only. Verify product specifications, site conditions, and supplier purchase increments before ordering.
How to calculate concrete for a slab
Multiply the inside form length by width and thickness in feet. Convert cubic feet to cubic yards, add the selected allowance, then round up to the ready-mix increment or to whole bags.
Concrete with waste: net volume × (1 + waste)
Ready-mix order: round up to the selected yd³ increment
Bags: round up(volume with waste ÷ published bag yield)
Worked example
A 20 × 20 ft slab at 4 inches thick contains 133.33 ft³, or 4.938 yd³. With 10% waste, the estimate is 5.432 yd³. Rounded to quarter-yard ordering, plan for 5.50 yd³; the 80 lb bag alternative is 245 whole bags at 0.60 ft³ each.
Ready-mix or concrete bags?
Ready-mix is normally the practical comparison for a slab this size, but supplier minimum loads, short-load fees, access and placement time vary. Bag counts use the selected manufacturer yield and are provided as a purchase-unit comparison, not as a recommendation for a large placement.
Before placing a slab
- Use the actual inside dimensions of the forms and check excavation depth at several points.
- Verify slab thickness, base, reinforcement, joints, slope, drainage and curing against the plans and local requirements.
- Confirm the mix specification, order increment, minimum load and delivery conditions with the ready-mix supplier.
- Use the exact yield and water instructions printed on bagged concrete.
Current US references: QUIKRETE Concrete Mix technical data sheet, Sakrete High-Strength Concrete Mix, and National Ready Mixed Concrete Association consumer resources. Project specifications and supplier documentation control.
Concrete slab calculator FAQ
How much extra concrete should I order?
The calculator exposes 0%, 5%, 10% and 15% allowances. Choose from measurement confidence, subgrade variation, form tolerances and supplier guidance rather than applying an unexplained margin.
Does the calculator include footings or thickened edges?
No. This page estimates a uniform rectangular slab only. Calculate thickened edges, grade beams or separate footings as additional volumes.
Does this select slab thickness or concrete strength?
No. Thickness, strength, reinforcement and exposure requirements must come from the plans, code and qualified project guidance.
Why does the order exceed the calculated volume?
The selected waste allowance is added first, then the result is rounded up to a supplier-style purchase increment so the displayed order is buyable.