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Concrete Calculator
Calculate cubic yards, bag count, and ready-mix cost for slabs, footings, columns, and stairs.
Concrete Needed (with 10% waste)
1.36 CY
33.33 cubic feet net
| Volume (net) | 33.33 ft³ |
| Cubic yards (net) | 1.23 CY |
| With 10% waste | 1.36 CY |
| 80 lb bags needed | 62 bags |
| 60 lb bags needed | 82 bags |
| Rebar suggestion | #4 rebar at 18" OC |
Methodology
Volume = length × width × depth. Cubic yards = cubic feet ÷ 27. Ready-mix price based on national BLS PPI average of $165/CY. Always add 5–10% for waste, spillage, and form seating. Bag counts use 80 lb = 0.60 cf and 60 lb = 0.45 cf of finished concrete. Column formula uses V = πr²h. Stair volume uses a triangular prism approximation: each step occupies half a rectangular block.
Frequently Asked Questions
How many cubic yards do I need for a 10×10 slab at 4 inches?
A 10×10 slab at 4 inches thick is 10 × 10 × (4/12) = 33.33 cubic feet, or about 1.23 cubic yards. With a 10% waste factor, you should order 1.36 CY. Most ready-mix suppliers have a minimum order of 1–2 CY, so this is close to the threshold for bagged concrete versus truck delivery.
What is the formula for concrete volume?
For a slab: Volume (CY) = Length (ft) × Width (ft) × Depth (ft) ÷ 27. For example, a 20×30 slab at 6 inches = 20 × 30 × 0.5 ÷ 27 = 11.11 CY. Always convert thickness to feet before multiplying (divide inches by 12).
How much does ready-mix concrete cost?
Ready-mix concrete ranges from $130 to $200 per cubic yard in 2026 depending on region, mix strength, and delivery distance. The national average is approximately $165/CY for standard 3,000–4,000 PSI mix. Short-load fees apply for orders under 5 CY. Fiber reinforcement, air entrainment, and accelerator additives each add $5–15/CY.
80 lb vs 60 lb bags — which should I buy?
80 lb bags yield 0.60 cubic feet per bag and are the most economical choice per cubic yard. 60 lb bags yield 0.45 cubic feet and are easier for a single person to handle — ideal for smaller jobs or when mixing by hand. For very small repairs, 50 lb bags are also available. If you need more than 2 CY, ordering ready-mix is almost always cheaper and faster than mixing bags.