Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Convert a pour size directly into 40, 60, or 80 lb bag counts using each bag's published concrete yield.
Total volume: 5.87 cu ft. Bag yields per manufacturer packaging (0.30/0.45/0.60 cu ft).
For small pours -- a fence post, a mailbox base, a small patio -- buying pre-mixed bags is usually easier than ordering a ready-mix truck. This calculator takes your pour's dimensions and tells you exactly how many bags you'd need at each of the three common bag sizes, so you can compare cost and trips to the store before you shop.
Enter your pour's length and width in feet and thickness in inches -- the same inputs as the general Concrete Calculator -- and the calculator shows bag counts for 40 lb, 60 lb, and 80 lb bags side by side.
Bags = Volume (cu ft) ÷ Bag Yield (cu ft)
| Bag Size | Yield (mixed concrete) |
|---|---|
| 40 lb | 0.30 cu ft |
| 60 lb | 0.45 cu ft |
| 80 lb | 0.60 cu ft |
These yield figures are the ones printed directly on major concrete-mix packaging (Quikrete and Sakrete both publish these same numbers) -- not an approximation. Since an 80 lb bag yields exactly twice the mixed concrete of a 40 lb bag, you'll always need exactly half as many 80 lb bags as 40 lb bags for the same pour.
This is the right tool for a small pad, a mailbox or sign post base, or setting a single fence post in concrete (for post-hole-specific math including hole diameter, see the Fence Post Calculator). For anything beyond roughly a cubic yard, mixing that many bags by hand becomes impractical and the Concrete Slab Calculator's ready-mix order sizing is usually the better option.
Bag yield figures assume the bag is mixed exactly per the manufacturer's water ratio instructions -- under- or over-watering changes the actual yield and, more importantly, the concrete's final strength. Always follow the mixing instructions printed on your specific bag.
The calculator shows 40, 60, and 80 lb counts together because the choice is a trade-off, not a formula. For the same pour, the heavier the bag, the fewer bags you carry, but the harder each bag is to lift and mix. A useful habit is to compare the number of bags against how many you can realistically mix and place before the batch starts to set. If the 80 lb count is in the dozens, hand mixing in a wheelbarrow may not be practical, and a rented mixer or a ready-mix order is worth pricing.
Suppose you are setting a fence post in a hypothetical hole that needs about 2.0 cubic feet of concrete. Dividing by 0.60 gives 3.33, so you need four 80 lb bags, or seven 40 lb bags, since 2.0 divided by 0.30 is 6.67. In this small case the 40 lb option lets you buy closer to the exact quantity, while the 80 lb option leaves a partly unused fourth bag. Compare the leftover, not just the count.
Bagged mix reacts with moisture, so keep bags dry and off the ground until use. A bag that is hard or lumpy inside may have partially hydrated and should not be used for structural work. Wear gloves, eye protection, and a dust mask when opening and mixing, because cement is caustic to skin and eyes. Follow the instructions on your bag for water quantity, mixing time, and curing.
A cubic yard is 27 cubic feet, and one 80 lb bag yields 0.60 cu ft, so 27 ÷ 0.60 = 45 bags make one cubic yard.
For small pours, bags are usually cheaper and more practical since ready-mix trucks often have delivery minimums and short-load fees. For anything approaching a full cubic yard or more, ready-mix is typically more cost-effective per unit of concrete.
80 lb bags are the most material-efficient per trip (fewest bags for the same volume) but are heavier to carry and mix; 40 lb bags are easier to handle but mean more bags and more mixing for the same job.
The 0.30/0.45/0.60 cu ft figures are standard across the major US brands (Quikrete, Sakrete) for their respective 40/60/80 lb concrete mix bags -- always double-check the yield printed on your specific bag before relying on it for a large order.
The page applies a waste allowance to the volume before dividing, as the worked example shows. Even so, uneven subgrade can eat into that margin, so buy one spare bag for small jobs.
Yes, as long as they are the same type of mix. You can combine 80 lb bags with a few 40 lb bags to land near your exact volume without a large surplus.
It estimates quantity only. Footings, structural slabs, and anything load-bearing may need a specific strength and engineering, so consult the plans or a professional.