Whether you are setting fence posts, pouring a small patio pad, or filling a form for a planter, the first question is always how many bags of concrete do I need for the volume in front of you. Bagged concrete is a premixed blend of Portland cement, coarse aggregate, fine sand, and approved admixtures, and it only needs water on site. Mix manufacturers keep adding faster-setting and higher-strength formulas, and the annual concrete industry show is a good place to see what is new before you commit to a product.
This article explains what one bag makes, how to convert any measured space into a bag count, and where the common ordering mistakes hide. It includes yield tables for the standard bag sizes, worked examples for small pours, and per-cubic-yard math for bigger jobs, so the numbers on the shelf translate into the right number of trips to the register.
How Many Cubic Feet Does One Bag of Concrete Make?
Yield is the number that drives everything else. Standard bagged mix produces about 0.15 cubic feet for every 20 pounds of dry mix, so a 40 pound bag makes 0.30 cubic feet, a 60 pound bag makes 0.45 cubic feet, and an 80 pound bag makes 0.60 cubic feet. A 50 pound fast-setting bag yields about 0.375 cubic feet. In cubic yards, a 60 pound bag produces roughly 0.017 and an 80 pound bag about 0.022, which matters when you compare bagged concrete with a ready-mix truck order.
Yield also varies with the product line. Lightweight and fast-setting blends pack differently than standard mix, and decorative products behave differently again; for finished floor and wall surfaces where dimensional consistency and factory curing count, many builders choose colorful concrete tiles rather than a site-cast slab.
Bag Weight and Yield at a Glance
| Bag weight | Yield (cu ft) | Yield (cu yd) | Bags per cubic yard |
|---|---|---|---|
| 40 lb | 0.30 | 0.011 | 90 |
| 50 lb fast-setting | 0.375 | 0.014 | 72 |
| 60 lb | 0.45 | 0.017 | 60 |
| 80 lb | 0.60 | 0.022 | 45 |
Check the Label, Not the Weight
The printed yield on the bag is the authoritative number. Density and admixtures differ between brands, so the same 60 pound weight can finish 0.42 or 0.48 cubic feet depending on the product. Use the bag’s own number in your division and you will never be more than a bag or two off.
How to Calculate Bags for Any Cubic Footage
The calculation has three measurements and one division. Measure the space in feet, multiply length by width by thickness to get cubic feet, divide by 27 to get cubic yards, then divide the required volume by the yield of the bag you intend to buy. Most contractors cross-check their arithmetic with concrete estimate samples and estimating worksheets before placing the order, because a missed conversion is the classic cause of a half-finished pour.
- Measure the space in feet: length, width, and depth or thickness.
- Multiply the three dimensions together to get cubic feet.
- Divide by 27 to convert the volume to cubic yards.
- Divide the total by the yield of the bag you plan to buy.
- Multiply the bag count by 1.10 to cover wastage and dry volume, then round up.
The Formula in Practice: 3 Cubic Feet
A 3 cubic foot pour with 60 pound bags works out to 3 divided by 0.45, which is 6.67 bags, so you buy 7. Add the 10 percent buffer and the volume becomes 3.3 cubic feet, which needs 7.33 bags, so the order climbs to 8. With 80 pound bags, 3 divided by 0.60 is exactly 5 bags, and the buffered count rounds up to 6.
Measuring Round Post Holes
Not every pour is a rectangle. For a round post hole, use the cylinder formula: pi times the radius squared times the depth. A 10 inch diameter hole has a 5 inch radius, or 0.42 feet, and a 30 inch depth is 2.5 feet, so the volume is 3.14 x 0.42 x 0.42 x 2.5, which comes to 1.39 cubic feet. That takes about 3.1 bags of 60 pound mix, or 4 bags once you add the 10 percent buffer and round up.
Working with Cubic Yards Instead
If your supplier quotes cubic yards, remember that one cubic yard is 27 cubic feet and that a single 60 pound bag covers only about 0.017 of it. A full cubic yard needs about 60 bags of 60 pound mix, or 45 bags of 80 pound mix, before the buffer is added.
Bag Counts for Small Pours: 2 to 4 Cubic Feet
Small volumes are where bag math pays for itself, because the difference between 9 and 10 bags is a second trip to the store. The table below assumes a 10 percent buffer and standard mix yields of 0.30, 0.45, and 0.60 cubic feet for 40, 60, and 80 pound bags.
| Volume needed | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 2 cu ft | 8 | 5 | 4 |
| 2.5 cu ft | 10 | 7 | 5 |
| 3 cu ft | 11 | 8 | 6 |
| 3.5 cu ft | 13 | 9 | 7 |
| 4 cu ft | 15 | 10 | 8 |
Small pours still need proper compaction. The guide on consolidating concrete in congested reinforced members applies to a fence post hole too, because rodding or vibrating pushes out the entrapped air that otherwise leaves soft spots at the surface.
Common Jobs in This Range
- Setting fence, mailbox, and gate posts at 1 to 2 cubic feet per hole.
- Small equipment pads for air conditioners and generators.
- Filling CMU block cores on retaining and foundation walls.
- Anchor blocks for awnings, decks, and outdoor structures.
Mixing Small Batches by Hand
For batches up to about 2 cubic feet, a wheelbarrow and a hoe are enough. Add water in stages, mix until the color is uniform, and clean the tools before the mix sets. A mortar pan is easier on the back than a wheelbarrow for the smallest pours.
Ten Cubic Feet and Larger Pours
A 10 cubic foot pour is about 0.37 cubic yards, roughly a slab 6 feet by 5 feet at 4 inches thick. With 60 pound bags you divide 10 by 0.45 and get 22.2 bags, so 23 are needed; with the 10 percent buffer the volume is 11 cubic feet, which needs 24.4 bags and rounds up to 25. With 80 pound bags the base count is 16.7, or 17 bags, and the buffered count is 18.3, or 19 bags.
At this size the surface preparation starts to matter as much as the math. If the pour is a topping or repair over an existing slab, read the bonding requirements in the guidance on pouring new concrete over an old concrete surface first, because a clean, roughened base is what keeps the new layer attached.
What Ten Cubic Feet Looks Like
A typical contractor wheelbarrow carries about 3 cubic feet struck level, so 10 cubic feet means roughly three loads, or about 25 mixed 60 pound bags carried by hand. Plan the path from the mixer to the form before you start, and stage the bags close to the work to cut carrying time.
Splitting the Work Across Days
If you cannot place the full pour in one session, stop at a clean edge and form a construction joint. Concrete placed the next day against a cured face will not bond as well as a monolithic pour, so keep the joint in a low-stress location and roughen the face before continuing.
Bags per Cubic Yard, Bulk Buying, and Mix Selection
For planning, convert everything to cubic yards. One cubic yard needs 90 bags of 40 pound mix, 72 bags of 50 pound fast-setting mix, 60 bags of 60 pound mix, or 45 bags of 80 pound mix. With the 10 percent buffer those counts become 99, 80, 66, and 50 bags respectively, which is the number you actually order.
Super Sacks and Bulk Delivery
Once the order passes about 45 bags of 80 pound mix, bulk options start to make sense. Super sacks holding 3,000 pounds of dry mix need a crane, loader, or hoist to move, and ready-mix trucks deliver wet concrete directly to the form. Both cut the per-cubic-foot price, but both require more crew and more careful scheduling than bags.
Buying Advice That Saves Money
- Buy all bags from the same production batch so color and set time match.
- Store bags on pallets off the ground and keep them covered and dry.
- Check the manufacture date; cement loses strength as bags age.
- Keep the receipt and return unopened bags if the store allows it.
After the pour, the mix still has to prove itself. Curing, strength, and surface quality are verified, not assumed, and post-concrete inspection and testing catches problems while they are still fixable. For spans and loads beyond what a small crew can place, weigh field-bagged concrete against factory products like prestressed concrete, which delivers predictable strength without on-site batching risk.