Concrete Bag Calculator
For smaller concrete projects, bagged concrete is often more practical than ordering a ready-mix truck. Our concrete bag calculator determines exactly how many bags you need based on your project dimensions and preferred bag size. Whether you're setting fence posts, pouring a small pad, or filling footings, get accurate bag counts for 40lb, 60lb, or 80lb bags. We account for waste factor to ensure you don't run short mid-project. Nothing is worse than stopping to make another hardware store run.
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How many bags of concrete do I need?
To work out how many bags of concrete you need, find the volume of your pour and divide it by what one bag yields. Multiply length by width by depth in feet to get cubic feet (a 4 inch depth is 4 ÷ 12, or 0.333 feet), add a 10 percent waste factor for spillage and uneven ground, then divide by the bag yield: 0.60 cubic feet for an 80 pound bag, 0.45 for a 60 pound bag. The calculator above does this for you and rounds up to whole bags, so use it as your bags of concrete calculator for any post hole, footing, or small slab.
Concrete bag yields by size
Bag yield is the volume of mixed concrete one bag fills. The numbers below are how this concrete calculator counts bags by weight. Yields are approximate and vary slightly by brand and mix, and the bags per cubic yard are for a full 27 cubic feet before any waste factor.
| Bag size | Yield (cu ft per bag) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
So a full cubic yard takes about 45 80 pound bags or 60 60 pound bags. Knowing how many 80 pound bags of concrete or how many 60 pound bags of concrete a job needs is the fastest way to compare bag sizes before you buy.
How many bags of concrete for a slab
The table shows how many bags of concrete you need for common slab sizes at 4 inches thick. The cubic feet column and both bag counts include the calculator's standard 10 percent waste factor, so they match what the tool above returns for the same dimensions.
| Dimensions | Concrete needed (cu ft) | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 10 x 10 | 36.7 | 82 | 62 |
| 12 x 12 | 52.8 | 118 | 89 |
| 16 x 20 | 117.3 | 261 | 196 |
| 20 x 20 | 146.7 | 326 | 245 |
| 24 x 24 | 211.2 | 470 | 353 |
Once a slab passes about 1 cubic yard, which is roughly 45 80 pound bags, mixing bags by hand stops making sense. A 16 by 20 or 24 by 24 slab needs hundreds of bags, so ready-mix delivery is cheaper and far less work at that size.
Bags of concrete per fence post
For fence posts, figure the hole, not the fence line. A 12 inch wide, 24 inch deep post hole holds about 2 cubic feet, which is roughly 4 bags of 80 pound concrete or 5 bags of 60 pound concrete per hole with a 10 percent waste factor. To find how many bags of concrete per fence post your project needs, enter one hole's width, length, and depth in the calculator above, then multiply the bag count by your number of posts. Many homeowners round up to a full bag per post for the last hole.
When ready-mix beats bags
Bagged concrete is the right call for post holes, footings, and small pads, but the labor adds up fast once a pour passes about 1 cubic yard, which is roughly 45 bags of 80 pound concrete. At that point a ready-mix truck is usually cheaper and far less work. To compare the two options for your project, our concrete slab cost calculator prices a poured slab including ready-mix delivery, finishing labor, wire mesh, and forms, so you can weigh the full cost against the bag count above.
How It's Calculated
1. Volume (cu ft) = Length × Width × (Thickness ÷ 12) 2. Total Volume = Volume × (1 + Waste Factor) 3. Bags Needed = Total Volume ÷ Yield per Bag Bag yields: • 40lb bag = 0.30 cu ft • 50lb bag = 0.375 cu ft • 60lb bag = 0.45 cu ft • 80lb bag = 0.60 cu ft
Worked Examples
Each 12"×12"×24" post hole = 2 cu ft. For 10 posts = 20 cu ft + 10% waste = 22 cu ft. At 0.60 cu ft per 80lb bag, you need 37 bags (about 4 per hole).
A 10×3 foot walkway at 4" thick = 10 cu ft + 10% waste = 11 cu ft. You'll need 19 bags of 80lb concrete. This is the upper limit of practical bag use, so consider ready-mix for larger projects.
A 2×2 foot base at 18" deep = 6 cu ft + 10% waste = 6.6 cu ft. Using 60lb bags (0.45 cu ft each), you need 15 bags.
Assumptions & Waste Factor
- Bag yields are based on manufacturer specifications
- 80lb bag yields approximately 0.60 cubic feet
- 60lb bag yields approximately 0.45 cubic feet
- 40lb bag yields approximately 0.30 cubic feet
- Concrete mixed according to package directions
- Standard 4,000 PSI concrete mix
Standard 10% waste factor accounts for spillage, mixing losses, and uneven surfaces. For precise forms with good containment, you may reduce to 5%. For rough ground or post holes, consider 15%.
Frequently Asked Questions
How many bags of concrete do I need?
It depends on your pour volume and bag size. Multiply length by width by depth in feet for cubic feet, add a 10 percent waste factor, then divide by the bag yield: 0.60 cubic feet for an 80 pound bag or 0.45 for a 60 pound bag. A 10 by 10 foot slab at 4 inches needs about 62 80 pound bags or 82 60 pound bags. Enter your own dimensions in the calculator above for an exact count.
How many 80 lb bags of concrete are in a cubic yard?
About 45. One cubic yard is 27 cubic feet and an 80 pound bag yields about 0.60 cubic feet, so 27 divided by 0.60 is 45 bags. A 60 pound bag yields 0.45 cubic feet, which is 60 bags per cubic yard, and a 40 pound bag yields 0.30 cubic feet, which is 90 bags. At a full cubic yard, ready-mix delivery usually costs less than mixing 45 bags by hand.
How many bags of concrete for a 10x10 slab?
A 10 by 10 foot slab at 4 inches thick is about 33 cubic feet of concrete, or 36.7 cubic feet once you add a 10 percent waste factor. That is about 62 bags of 80 pound concrete or 82 bags of 60 pound concrete. A pour this size is near the practical limit for bags, so many homeowners switch to ready-mix here.
How much does a bag of concrete cover?
An 80lb bag yields about 0.60 cubic feet, covering roughly 1.8 square feet at 4 inches thick. A 60lb bag yields 0.45 cu ft (1.35 sq ft at 4"), and a 40lb bag yields 0.30 cu ft (0.9 sq ft at 4").
Which bag size should I use?
Use 80lb bags for the best value if you can handle the weight. Use 60lb or 40lb bags if you need easier handling. All sizes produce the same strength concrete when mixed properly.
How long does bagged concrete take to set?
Initial set occurs in 20-40 minutes. You can remove forms after 24-48 hours. Full cure takes 28 days. Fast-setting concrete sets in 10-20 minutes but still needs 28 days for full strength.
Can I mix concrete in a wheelbarrow?
Yes, a standard wheelbarrow holds 1-2 bags at a time. Use a hoe or shovel to mix thoroughly. For more than 10-15 bags, renting a mixer saves significant time and effort.
When should I use ready-mix instead?
Consider ready-mix for projects requiring more than 1 cubic yard (45+ bags of 80lb). The labor of mixing that many bags usually exceeds the cost savings. Ready-mix is also better for time-critical or large-area pours.
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Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements may vary based on site conditions, installation methods, and other factors. Always consult with a qualified professional before making purchasing decisions.