Concrete Calculator: Cubic Yards and Bags
Enter the size of the pour to get the volume in cubic yards and cubic feet and the number of bags to buy, with an allowance for waste and uneven subgrade.
Your numbers
Result
Enter your numbers to see the result.
Take this to the job site
The same tools, and more, in apps that work offline when there is no signal.
BuildSmith: Construction Calc
NICET/Tools37 trade calculators, code tables and converters for the whole job site, fast and offline.
GableForge: Home Inspection
Utilities/Misc.Write home inspection reports offline: rate items, add photos and send a PDF from the site.
The math
Volume (ft³) = length × width × thickness in feet (or π × r² × depth for a round column). Divide by 27 to get cubic yards. Add the waste allowance (5 to 10% is typical, more for a rough subgrade).
Bag yields: an 80 lb bag makes about 0.6 ft³, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. For more than about a cubic yard it is usually cheaper to order ready-mix; ready-mix suppliers often charge a short-load fee under 3 to 4 yards.
Worked example
A 20 × 12 ft slab, 4 in. thick: 80 ft³ = 2.96 yd³. With 10% waste, order 3.26 yd³, or about 147 bags of 80 lb mix.
Frequently asked questions
How thick should a slab be?
4 inches is typical for a patio or walkway and a residential garage floor is usually 4 to 6 inches. Driveways are often 5 to 6 inches.
How much does a bag cover?
An 80 lb bag yields about 0.6 cubic feet, which covers roughly 1.8 sq ft at 4 in. thick. Use the calculator for your exact thickness.
Should I add extra?
Yes, 5 to 10%. Running short mid-pour is a worse problem than ordering a bit extra.
Related calculators
This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.