Bags per cubic foot and per yard
A bag of concrete mix is sold by its weight and used by its volume: what it yields once water is added. Quikrete’s data sheet gives about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. Divide the volume of the pour by the yield, and round up: a part of a bag is a bag.
That makes 45 bags of 80 lb to a cubic yard, 60 of 60 lb, or 90 of 40 lb.
Worked example. A slab 10 × 10 ft, 4 in thick: 33.33 cu ft.
- In 80 lb bags: 33.33 ÷ 0.60 = 56 bags
- In 60 lb bags: 75 bags
- In 40 lb bags: 112 bags
- With 10% extra, in 80 lb bags: 62 bags, 4,960 lb to carry
Posts, steps and small slabs
Bags are made for the pours a truck is too big for.
- Fence posts. A hole 10 in across and 30 in deep round a 4 × 4 post takes 1.15 cu ft: 2 bags of 80 lb for one hole, since a bag cannot be split between two holes that are far apart. Counted together, fourteen holes take 16.11 cu ft, 27 bags of 80 lb or 36 of 60 lb.
- Steps. Three solid steps 36 in wide, with a 7 in rise and an 11 in run, are 9.63 cu ft.
- A thin pour. Self-leveling underlayment goes on a quarter of an inch at a time, and its yield is its own. Choose “Slab by its area” and “Another product, by its yield”, and type the figure from the bag: 100 sq ft at 1/4 in is 2.08 cu ft, 5 bags at a yield of 0.5 cu ft.
Add every piece of the job before you read the count: the pieces are added up first and rounded once.
Fast-setting and other mixes
The mix changes the yield, the water and the time you have. Fast-setting mix comes in 50 and 60 lb bags and sets in minutes, which suits a post and not a slab you have to finish; high-strength mix comes in 50, 60 and 80 lb bags. The calculator has the yields of each from its maker’s data sheet. For any other product, choose “Another product, by its yield”: the yield is printed on the bag, and the page uses yours.
Water is on the bag too. For the standard mix, the same data sheet says about 6 pt for each 80 lb bag to start with, and up to 9 pt in all. More water makes concrete easier to place and weaker.
When to order ready-mix instead
The slab of the example is 62 bags with its extra, more than two tons to lift, mix and place before the first batch sets. A truck brings the same 1.5 cubic yards already mixed.
Whether it is worth it depends on your supplier’s smallest load. Under “Options”, enter that minimum and the page says on which side of it your pour falls; add the price of a bag and of a cubic yard, and it puts the two totals side by side. The concrete calculator gives the order in cubic yards, by the quarter yard.
Questions about bags of concrete
How many 80 lb bags make a yard?
45. A cubic yard is 27 cu ft, and an 80 lb bag yields about 0.60 cu ft.
How many bags for a fence post?
For a hole 10 in across and 30 in deep with a 4 × 4 post: 1.15 cu ft, so 2 bags of 80 lb for a single post, and 27 for fourteen. Your hole decides: enter its diameter and its depth.
Can I mix bags for a 10×10 slab?
It is 62 bags of 80 lb at 4 in thick with 10% extra. It can be done with a mixer and help; it is also the size at which a ready-mix price is worth asking for.
How much water goes in a bag of concrete?
For an 80 lb bag of standard mix, about 6 pt to start with and 6 to 9 pt in all, by Quikrete’s data sheet. Other sizes and mixes differ: read the bag.
Sources
- Quikrete, Concrete Mix (No. 1101), data sheet. A 40 lb bag yields approximately 0.30 cu ft, a 50 lb bag 0.375, a 60 lb bag 0.45, an 80 lb bag 0.60 and a 90 lb bag 0.675. An 80 lb bag takes about 6 pt of water to start with, and 6 to 9 pt in all. Read on .
- Quikrete, Fast-Setting Concrete Mix (No. 1004), data sheet. A 50 lb bag yields approximately 0.375 cu ft and a 60 lb bag 0.45 cu ft. Read on .
- Quikrete, Quikrete 5000 High Early Strength Concrete Mix (No. 1007), data sheet. A 50 lb bag yields approximately 0.375 cu ft, a 60 lb bag 0.45 and an 80 lb bag 0.60. Read on .
- Archtoolbox, Lumber Dimensions: Nominal vs Actual. Surfaced softwood lumber: a 2 x 10 is 1 1/2 x 9 1/4 in and a 2 x 12 is 1 1/2 x 11 1/4 in; a 4 x 4 is 3 1/2 x 3 1/2 in and a 6 x 6 is 5 1/2 x 5 1/2 in; a 1 x 4 is 3/4 x 3 1/2 in and a 1 x 6 is 3/4 x 5 1/2 in. Standard lengths run from 6 to 24 ft in steps of 2 ft. Read on .
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