For rectangular shapes (slabs, footings), volume is calculated as Length Γ Width Γ Depth, converted to cubic feet, then divided by 27 to get cubic yards. For round shapes (columns, post holes), the formula is Ο Γ radiusΒ² Γ height Γ· 27. The waste factor adds a percentage buffer on top of the raw volume. Bag counts are calculated using standard yields: 80 lb bags yield 0.60 cu ft, and 60 lb bags yield 0.45 cu ft each, based on Quikrete and Sakrete manufacturer specifications.
Frequently Asked Questions
Multiply the length Γ width Γ thickness of your slab in feet, then divide by 27 to convert cubic feet to cubic yards. Always add 10% for waste. Our calculator does this automatically for slabs, footings, columns, and post holes.
One cubic yard requires approximately 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix. For projects over 1 cubic yard, ready-mix delivery is usually more cost-effective than mixing bags by hand.
For projects under 1 cubic yard (about 45 bags of 80 lb mix), bagged concrete is practical for DIY. For anything over 1 cubic yard, ordering ready-mix by truck saves time, money, and ensures a more consistent mix. Mixing 50+ bags by hand is extremely labour-intensive.
Walkways and patios: 4 inches. Driveways: 5β6 inches. Garage floors: 6 inches minimum. Structural foundations: 8 inches or more. Always check local building codes for your specific application before pouring.
Ready-mix concrete typically costs $125β$185 per cubic yard in 2026, plus a delivery fee of $75β$150. Bagged concrete (80 lb) runs $5β$7 per bag, which works out to about $225β$315 per cubic yard β more expensive but practical for small jobs.
Concrete reaches about 70% of its final strength in 7 days and over 90% at 28 days. You can walk on it after 24β48 hours and drive on it after 7 days. Keep the surface damp during the first week for best strength.
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