Concrete Calculator for Slabs and Pads
Cubic yards of concrete for a slab, patio, pad or footing, and how many 40, 60 or 80 lb bags of concrete mix it takes.
How this is calculated
Volume is length times width times thickness. With feet and inches, the thickness is divided by 12 to get feet, and cubic feet are divided by 27 to get cubic yards, the unit ready-mix concrete is ordered in. With meters and centimeters, the thickness is divided by 100.
Bag counts use the yield the manufacturer prints for its standard concrete mix. QUIKRETE Concrete Mix (No. 1101) data sheet: a 40 lb bag yields about 0.30 cubic feet, 50 lb 0.375, 60 lb 0.45 and 80 lb 0.60. The number of bags is the volume divided by the yield, rounded up.
The extra percentage covers spillage, an uneven base and forms that bow a little; 5 to 10 percent is a common allowance. Set it to 0 for the exact volume.
Limits
A rectangular slab of even thickness. For round columns or footings, work out each shape separately. Yields are for one standard mix; other mixes (fast-setting, high-strength) print their own yields on the bag. This is not structural advice: thickness and reinforcement for a driveway or a load-bearing slab depend on local codes.