How many 80lb bags are in a cubic yard of concrete?

There are exactly 45 bags of 80lb concrete in one cubic yard (27 cubic feet / 0.60 yield = 45).

Does an 80lb bag of high-strength concrete yield less?

No. High-strength 5000 PSI concrete bags and standard 4000 PSI concrete bags both yield approximately 0.60 cubic feet per 80lb bag. The difference is in the cement ratio, not the physical volume.

How much water do I add to an 80lb bag of concrete?

You generally need about 3 quarts (or 6 pints) of clean water per 80lb bag. However, you should always start with slightly less water, mix thoroughly, and add small amounts until you reach an oatmeal-like consistency. Too much water drastically weakens the concrete.

How Many 80lb Bags of Concrete Do You Actually Need?

The quick answer: One 80lb bag yields 0.60 cubic feet. Calculate your project volume in cubic feet (Length × Width × Depth in feet), add 10% for waste, then divide by 0.60. For example, a 10×10 slab at 4” needs 62 bags. There are 45 bags in a cubic yard.

Standing in the hardware store aisle staring at pallets of concrete is a rite of passage for every DIYer. The most common question asked in that aisle is always: “How many 80lb bags of concrete do you actually need?”

Guessing is a recipe for disaster. Buy too few, and you risk a ruined project with a cold joint. Buy too many, and you are breaking your back loading and unloading heavy bags you didn’t even need—not to mention wasting money.

The Magic Number: 80lb Bag Yield

To answer how many bags you need, you first have to know how much space one single bag fills. In the concrete world, this is called the “yield,” and it is measured in cubic feet.

One 80lb bag of standard concrete mix yields exactly 0.60 cubic feet.

This is your magic number. No matter what you are building, if you can figure out the total cubic feet of your project, you just divide that number by 0.60 to get your bag count.

How to Calculate Bags for a Slab or Patio

Slabs, sidewalks, and patios are the most common DIY concrete projects. Because they are rectangular prisms, the math is straightforward.

The Formula: Length (ft) x Width (ft) x Depth (ft) = Total Cubic Feet

Important note: You must convert your thickness from inches to feet by dividing by 12 (e.g., 4 inches / 12 = 0.333 feet).

Example: A 12x12 Patio at 4 inches thick

Find Square Footage: 12 x 12 = 144 sq ft
Multiply by Depth in feet: 144 x 0.333 = 48 cubic feet
Add 10% Waste Factor: 48 x 1.10 = 52.8 cubic feet
Divide by Bag Yield: 52.8 / 0.60 = 88 bags.

You would need 88 bags of 80lb concrete for this patio.

How to Calculate Bags for Fence Posts

Fence post holes are cylinders, so the math is a little different.

The Formula: Pi (3.14) x Radius squared x Depth (ft) = Cubic Feet

Example: A 10-inch diameter hole, 3 feet deep

Find Radius in feet: 10 inches / 2 = 5 inch radius. 5 / 12 = 0.416 feet radius.
Square the Radius: 0.416 x 0.416 = 0.173
Multiply by Pi and Depth: 3.14 x 0.173 x 3 = 1.63 cubic feet per hole.
Add 15% Waste Factor (holes are rarely perfect): 1.63 x 1.15 = 1.87 cubic feet.
Divide by Bag Yield: 1.87 / 0.60 = 3.1 bags per hole.

You would need 3 to 4 bags of 80lb concrete per post hole.

Want an easier way? Use our Fence Post Calculator to get instant results per hole!

How to Calculate Bags for a Sonotube

Sonotubes (cardboard concrete forms used for deck footings) are also cylinders, but they are perfectly uniform, meaning you need a smaller waste factor than a rough dirt hole.

Quick Reference for 4-Foot Deep Sonotubes:

8-inch tube: Requires approx 2.5 bags (80lb)
10-inch tube: Requires approx 3.8 bags (80lb)
12-inch tube: Requires approx 5.5 bags (80lb)

When Should You Order a Truck Instead?

Mixing 80lb bags of concrete by hand (or even with a small portable mixer) is incredibly physically demanding. As a general rule of thumb:

Under 1 Cubic Yard (45 bags): Mix it yourself using bags.
Over 1 Cubic Yard (45+ bags): Strongly consider having ready-mix concrete delivered by a truck.

While a truck might have a minimum delivery fee, the time, labor, and physical toll saved by having the concrete dumped directly into your forms is almost always worth the extra cost for large projects.

Conclusion

Figuring out how many 80lb bags of concrete do you actually need doesn’t require a degree in engineering. By understanding that every 80lb bag yields 0.60 cubic feet, and remembering to convert all your measurements to feet, you can accurately estimate materials for any project around your home.

Don’t want to break out the notepad and calculator? We don’t blame you. Head over to our Concrete Bag Calculator and let our automated tools do the heavy lifting for you!