Concrete calculator
Enter the size of a slab, a footing or a round column to see how much concrete it needs in cubic yards, and how many bags of each size that comes to.
Bags are counted for the volume with the extra.
How it works
The calculator turns the size of the pour into cubic feet and divides by 27 to get cubic yards. A slab is its length times its width times its thickness, with the thickness in inches divided by 12. A footing is its length times its width and its depth, with those two in inches. A round column is pi times its radius squared times its height, for each column you pour.
Then it adds the extra you choose, 10 percent to start, and counts bags for the volume with that extra. The bag sizes are the ones QUIKRETE sells its Concrete Mix in: 40, 50, 60, 80 and 90 pounds. Its data sheet gives their yields as approximately 0.30, 0.375, 0.45, 0.60 and 0.675 cubic feet. Bags are always rounded up to a whole bag.
A worked example
A patio 10 feet long, 10 feet wide and 4 inches thick holds 33.3 cubic feet of concrete: 10 times 10 times one third of a foot. That is 1.23 cubic yards.
With 10 percent extra the job needs 36.7 cubic feet, or 1.36 cubic yards. An 80 pound bag yields about 0.60 cubic feet, so 36.7 divided by 0.60 is 61.1, which rounds up to 62 bags. The same job takes 82 bags of 60 pounds, or 123 bags of 40 pounds.
Before you buy
Other brands print their own yields on the bag, and the bag wins over this page. The data sheet also says this mix is designed for concrete 2 inches thick or more, so the calculator warns you about a thinner slab.
The calculator counts concrete and nothing else. It adds nothing for the gravel under a slab, the forms, the rebar or the delivery.
Sources
- QUIKRETE Concrete Mix data sheet (revised October 2022), read on 2026-10-31.
- QUIKRETE concrete calculator, read on 2026-10-31.
Questions
How much concrete do I need for a 10 by 10 slab?
A slab 10 feet by 10 feet and 4 inches thick needs 1.23 cubic yards, or 1.36 cubic yards with 10 percent extra. In 80 pound bags that is 62 bags.
How many 80 pound bags make a cubic yard?
An 80 pound bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet. It takes 45 bags to make a cubic yard.
What is the thinnest slab this mix is for?
The data sheet says QUIKRETE Concrete Mix is designed for pouring concrete 2 inches thick or more. For a thinner layer, use a product made for it.
Should I order extra concrete?
A little, yes. Quikrete’s own calculator says its yields leave out waste and uneven ground. This calculator starts at 10 percent extra, and you can set it from 0 to 50.
How do I figure a round post hole?
Choose the round column shape, then enter the diameter of the hole in inches and its depth in feet. If you are digging several holes, type how many.
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