Cubic Feet per 80 lb Bag of Concrete

An 80 lb bag of concrete mix yields approximately 0.60 cubic feet of wet concrete (about 0.0222 cubic yards). This is the standard figure used by most major manufacturers, and it is the basis for estimating how many bags you need for a project.

Formula

The yield of an 80 lb bag is calculated from the bag’s volume and the amount of water added. The standard formula is:

Yield (cubic feet) = Bag weight (lb) / Density of wet concrete (lb/ft³)

Where:

• Bag weight = 80 lb
• Density of wet concrete ≈ 133 lb/ft³ (typical for standard concrete mix)

So: 80 lb ÷ 133 lb/ft³ ≈ 0.60 ft³. This is the volume of concrete after mixing with water, not the dry volume of the bag.

Worked Example

Using the calculator’s default of 10 bags:

1. Each bag yields 0.60 ft³ of wet concrete.
2. Total volume = 10 × 0.60 = 6.0 ft³.
3. Convert to cubic yards: 6.0 ÷ 27 = 0.222 yd³.
4. Convert to cubic meters: 6.0 × 0.0283168 = 0.170 m³.

The calculator shows 6.00 ft³, 0.22 yd³, and 0.170 m³, matching these steps.

Why the Yield Is Not the Bag’s Dry Volume

A common misconception is that an 80 lb bag of dry mix will produce 80 lb of concrete. In reality, the dry mix contains cement, sand, and aggregate, and when water is added, the chemical reaction causes the volume to shrink slightly. The dry powder has a lower density (around 90–110 lb/ft³) than the wet concrete (around 133 lb/ft³). This density difference is why the wet yield is less than the dry volume of the bag.

Manufacturers list the wet yield on the bag, typically 0.60 ft³ for an 80 lb bag. This is the number you should use for estimating, not the dry volume.

How Many Bags Do You Need?

To calculate the number of bags for a project:

1. Measure the length, width, and depth of the area in feet.
2. Multiply length × width × depth to get cubic feet.
3. Divide by 0.60 (the yield per 80 lb bag).
4. Round up to the nearest whole bag.

Example: A slab 10 ft × 4 ft × 0.5 ft (6 inches) = 20 ft³. 20 ÷ 0.60 = 33.33 bags, so you’d need 34 bags.

For a cylindrical column, use the formula π × radius² × height (in feet) to get cubic feet, then divide by 0.60.

Common Mistakes and How to Avoid Them

• Mixing units: Always convert inches to feet (divide by 12) before calculating volume. A 6-inch slab is 0.5 ft, not 6 ft.
• Forgetting waste: Concrete can spill, and uneven ground can require more. Add 5–10% extra to your bag count.
• Using dry volume: Some people mistakenly use the bag’s dry volume (about 0.6 ft³) but that is actually the wet yield – so they’re correct, but they might not realize the density difference. The real mistake is using the bag’s weight to estimate volume (e.g., 80 lb = 1 ft³), which is wrong.
• Rounding too early: If you round down, you’ll run short. Always round up.

Comparing Bag Sizes

Common bag sizes and their approximate yields:

• 40 lb bag: ~0.30 ft³ (0.011 yd³)
• 60 lb bag: ~0.45 ft³ (0.017 yd³)
• 80 lb bag: ~0.60 ft³ (0.022 yd³)
• 90 lb bag: ~0.68 ft³ (0.025 yd³)

These yields are approximate; always check the bag for the exact figure, as mixes can vary slightly. For example, high-strength mixes might yield slightly less due to different aggregate proportions.

Cost and Efficiency Considerations

While an 80 lb bag is the most economical per cubic foot for small projects, larger projects (over 1 cubic yard) are often cheaper per cubic yard when ordered as ready-mix concrete. Ready-mix eliminates the labor of mixing and the risk of inconsistent batches. However, for small repairs, footings, or setting posts, bags are practical and avoid the cost of a delivery truck.

When comparing prices, calculate the cost per cubic foot: divide the bag price by 0.60. This lets you compare different bag sizes fairly.

Safety and Mixing Tips

• Wear protective gear: Concrete dust is caustic. Use gloves, safety glasses, and a dust mask when mixing.
• Follow water instructions: Too much water weakens the concrete; too little makes it hard to work. Use the amount recommended on the bag (typically about 4–5 quarts for an 80 lb bag).
• Mix thoroughly: Use a mixer for batches larger than one bag, or a wheelbarrow with a hoe for small jobs.
• Clean up promptly: Concrete hardens quickly; clean tools before it sets.

Frequently Asked Questions

How many 80 lb bags are in a cubic yard?

One cubic yard equals 27 cubic feet. Since one 80 lb bag yields 0.60 ft³, you need 27 ÷ 0.60 = 45 bags. So 45 bags of 80 lb concrete make approximately one cubic yard.

Does the yield change if I add more water?

Adding more water increases the volume slightly, but it also weakens the concrete. The yield on the bag assumes the recommended water amount. Adding extra water can increase the yield by a few percent, but it’s not advisable because it compromises strength.

Why does my bag say 0.6, but I measured less?

Measured volume can be less if the concrete is not fully compacted or if you lose some during mixing or pouring. Also, if you measure the dry mix before adding water, it will be more than 0.60 ft³. The 0.60 figure is for wet, compacted concrete.

Can I use an 80 lb bag for a 4-inch slab?

Yes. A 4-inch slab is 1/3 foot thick. One 80 lb bag covers roughly 0.60 ft³ ÷ 0.333 ft = 1.8 square feet. For a 10×10 slab (100 ft²), you’d need 100 ÷ 1.8 ≈ 56 bags.

How long does an 80 lb bag take to set?

Initial set typically occurs in 30–45 minutes, and final set in a few hours. However, curing continues for weeks. Wait at least 24–48 hours before placing heavy loads, and keep the concrete moist for several days for best strength.

Is it cheaper to buy bags or ready-mix?

For small projects (under 1 cubic yard), bags are usually cheaper because ready-mix has a minimum delivery fee. For larger projects, ready-mix is often more cost-effective and saves labor. Calculate both options based on your local prices.

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in bags | Number of 80 lb bags | bags | 10 | 0
out vol_ft3 | Total wet concrete | cubic feet | bags * 0.60 | 2
out vol_yd3 | Total wet concrete | cubic yards | bags * 0.60 / 27 | 2
out vol_m3 | Total wet concrete | cubic meters | bags * 0.60 * 0.0283168 | 3