Bags of concrete are easier to haul than a ready mix truck for fence posts, mailbox pads, and small patio slabs. It tells DIYers and small contractors how many 60 lb or 80 lb bags to buy before digging.
From Slab Size to Bag Count
Start with volume: length x width x thickness in feet, divided by 27 for cubic yards. A 10 x 10 ft slab at 4 inches deep is 33.3 cubic feet, about 1.23 cubic yards.
An 80 lb bag yields about 0.6 cubic feet, so that slab takes roughly 56 of them, or about 74 bags in the 60 lb size.
Using the In Bags Mode
Pick Imperial units in feet, inches, and lbs, switch to In Bags, and the calculator turns dimensions into a bag count by weight class. Stay in bag mode up to about 60 bags; past that, In Bulk wins.
Add 5 to 10 percent for spills and uneven excavation, then round up to full bags. Doubling depth from 4 to 8 inches doubles the count.
For a fence line, work the posts one at a time before pressing Calculate Concrete. A 10 inch diameter hole dug 24 inches deep holds pi x 0.417 squared x 2, about 1.09 cubic feet, and three such holes need near 3.3 cubic feet. That volume maps to 8 bags of 60 lb mix at 0.45 cubic feet each, or 6 of the 80 lb class at 0.6 apiece, both landing at 3.6 cubic feet with a little spare. Round every partial bag up, since an under filled post footing wobbles once the soil is wet.
Frequently Asked Questions
How many 80 lb bags make one cubic yard?
One cubic yard is 27 cubic feet and an 80 lb bag yields about 0.6 cubic feet, so budget 45 bags per yard. Add one or two spare.
Bags needed for a 10×10 slab, 4 inches deep?
That slab is 1.23 cubic yards, needing about 56 bags of 80 lb mix or roughly 74 of 60 lb mix. Buy a couple extra either way.
How much concrete is in an 80 lb bag?
An 80 lb bag yields about 0.6 cubic feet, roughly 1.8 sq ft of slab at 4 inches thick. The 60 lb size yields about 0.45 cubic feet.
When do bags stop making sense financially?
Past about 60 bags, near one cubic yard, a ready mix truck beats bag price and mixing labor. A two car patio sits well past that point.
How long does bagged concrete take to cure?
It sets firm in 24 to 48 hours but reaches full strength at 28 days, near 4,000 psi. Keep it damp the first week.