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Concrete Bag Calculator: How Many Bags Do I Need?
Enter your slab size and thickness and pick a bag size. The calculator converts the volume to cubic feet and cubic yards, then to the number of premix bags, the number everyone actually needs at the store and always underestimates on the first trip.
How This Calculator Works
Volume is length times width times thickness, with thickness converted from inches to feet. One 80 lb bag of premix yields about 0.6 cubic feet of concrete, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. The calculator divides your volume by the bag yield and rounds up, because a slab cannot be left a bag short.
Worked Example
A 4 by 4 foot pad at 4 inches thick is 5.33 cubic feet (about 0.20 cubic yards). At 0.6 cubic feet per 80 lb bag, that is 8.9, so 9 bags. The same pad in 60 lb bags needs 12, which is why bigger bags are worth it even on small jobs.
Bag yields and a quick bag count per cubic yard
| Item | Figure | Result |
|---|---|---|
| 80 lb bag | 0.60 cu ft yield | 45 bags per cubic yard |
| 60 lb bag | 0.45 cu ft yield | 60 bags per cubic yard |
| 40 lb bag | 0.30 cu ft yield | 90 bags per cubic yard |
Frequently Asked Questions
How many 80 lb bags make a cubic yard?
45 bags. A cubic yard is 27 cubic feet and each 80 lb bag yields about 0.6 cubic feet, so 27 divided by 0.6 is 45. For anything over about a third of a yard, a ready-mix delivery usually beats bags on cost and effort.
How thick should a concrete slab be?
4 inches for patios, pads and walkways is the standard. Driveways and heavy loads want 5 to 6 inches, usually with reinforcement, which is beyond a bag job for most people.
How many bags for a fence post hole?
A typical post hole 10 inches wide and 2 feet deep is about 1.1 cubic feet, roughly 2 bags of 80 lb mix per post, including a small crown above ground level.
Should I buy extra bags?
Yes, buy 1 or 2 extra beyond the calculator. Spillage, uneven ground and a slightly deeper dig all eat volume, and unopened bags can be returned, while an unfinished slab cannot wait.