How many bags of concrete in a yard is a division with one input the bag supplies: a cubic yard is 27 cubic feet, every bag of mix states the volume it yields when mixed, and the count is 27 divided by that yield, rounded up. The number is large for any bag size, which is the real answer to the question behind the question: bags are for jobs measured in bags, and a job measured in yards is a truck. This page works the division for the common bag sizes using the yield printed on each, says what changes the yield, and works the line where the truck wins. The concrete calculator on this site works the yards; the concrete pad calculator works a pad both ways.
The division: 27 cubic feet over the yield on the bag
A bag of concrete mix states its yield in cubic feet when mixed with the stated water, and the yield is the number to use, not the bag's weight, because the weight includes the water still to be added and the aggregate's density varies by product. The count for a yard is 27 divided by the yield, rounded up. As a worked example, if the bag on the shelf states a yield of two thirds of a cubic foot, a yard takes 27 over two thirds, 40.5, so 41 bags; if it states half a cubic foot, 54; if it states three quarters, 36. Read the bag: the figure is on it, and the calculator on this site takes it as an input rather than assuming one.
What changes the yield: water, mixing and the bag itself
The stated yield assumes the stated water and full mixing. More water makes a wetter mix with the same solids and no more real yield, and it weakens the concrete; less water makes a stiff mix that will not fill the form. Partial mixing leaves dry pockets. Different products in the same bag size, a fast-setting mix, a high-strength mix, a mix with larger aggregate, yield differently and say so. The construction standard's concrete requirements, linked below, govern the placing side; the yield side is the bag's own statement, and two bags of the same weight from two makers can differ.
From bags to yards on a real job
A post hole is bags: a hole a foot across and three feet deep is under three cubic feet, a handful of bags. A pad is yards: the 18 by 18 pad on this site's concrete pad calculator is 6 yards, 6.6 with waste, which at the worked yield of two thirds of a cubic foot is about 270 bags, each mixed and placed before the previous one stiffens. That is the point of the count: it says whether the job is a wheelbarrow job or a truck job. The line falls well below a yard for most crews, at the volume they can mix and place in the time the mix allows, and the concrete calculator on this site shows both the yards and the bag count from the yield typed in.
The truck, the price and the bag count that remains
A ready-mix truck delivers the whole volume mixed in one batch, and its price per yard plus any short-load charge is compared to the bags' price times the count plus the labor of mixing them, which for a yard is a day. Bags remain useful beside a truck for the corner the chute could not reach, the post the truck came too early for, and the day after. NIST Handbook 44, linked below, is why the yield on the bag and the yard on the ticket are quantities that can be held to. The concrete price calculator on this site works the truck's delivered price; this page works the bags.
Questions people ask about how many bags of concrete in a yard
How many 80 pound bags of concrete are in a yard?
Twenty-seven divided by the yield printed on that bag, rounded up. If the bag states two thirds of a cubic foot the answer is 41; read the bag rather than a table, because yields differ by product.
Why not use the bag's weight?
Because the weight is the dry mix and the yield is the mixed volume, which depends on the aggregate and the water. Two bags of the same weight from two makers can yield differently, and each states its own figure.
When should I order a truck instead of bags?
When the job is measured in yards rather than bags. A pad of several yards is hundreds of bags, each mixed and placed before the last stiffens; a truck delivers it in one batch, and the short-load charge is the price of that.