How Many 80lb Bags Per Cubic Foot? Concrete Math

One 80-pound bag of concrete mix yields about 0.6 cubic feet of wet concrete, so you need roughly 1.67 bags per cubic foot. For a typical project, that means 1 cubic yard (27 cubic feet) requires about 45 bags of 80-lb mix. This article explains the math, the variables that change it, and how to avoid running short or wasting material.

Formula

The standard formula is simple: Volume of concrete (cu ft) = Number of bags × 0.6 (for 80-lb bags). Conversely, Bags needed = Volume (cu ft) ÷ 0.6.

Variables:

• Bags – the number of 80-lb bags you plan to use (unit: bags)
• Volume – the volume you need to fill (unit: cubic feet)
• Yield per bag – 0.6 cubic feet per 80-lb bag (this is the industry standard for most concrete mixes)

The 0.6 figure comes from the mix design: 80 lb of dry concrete powder plus the required water expands to about 0.6 cubic feet when fully mixed. This is not the same as the bag’s dry volume, which is roughly 0.6–0.66 cubic feet depending on compaction.

Worked Example

Using the calculator defaults: you have 10 bags of 80-lb mix and want to know how much concrete that gives you.

1. Start with 10 bags.
2. Multiply by the yield per bag: 10 × 0.6 = 6.0 cubic feet.
3. So 10 bags produce exactly 6 cubic feet of wet concrete.

The calculator shows the same: total volume = 6.0 cu ft, and bags per cubic foot = 1.67 (since 1 ÷ 0.6 = 1.666…).

Why the Yield Varies Slightly

Not every 80-lb bag gives exactly 0.6 cubic feet. Here’s what changes it:

• Mix type – High-strength mixes (e.g., 5000 psi) may yield slightly less because they contain more cement and less aggregate. Some mixes yield 0.6, others 0.58 or 0.62.
• Water added – Adding more water increases the volume a bit, but weakens the concrete and can cause shrinkage cracks. Always follow the bag’s water instructions.
• Compaction – When you vibrate or tamp the concrete, it settles and reduces the apparent volume. The 0.6 figure assumes normal placement without excessive compaction.
• Waste – Spills, uneven subgrades, and over-excavation can eat 5–10% of your material. Most contractors add 10% extra.

For precise work, check the specific bag’s label – most list the yield in cubic feet. If it says “yield 0.6 cu ft,” use that.

How to Calculate for Your Project

Follow these steps to avoid under- or over-ordering:

1. Measure the length, width, and depth of the area in feet. For a slab, that’s length × width × thickness.
2. Convert all measurements to feet (e.g., 4 inches = 0.333 ft).
3. Multiply to get cubic feet.
4. Divide by 0.6 to get the number of 80-lb bags.
5. Round up to the nearest whole bag, then add 10% for waste.

Example: a 4×4 ft patio slab, 4 inches thick.

• Volume = 4 × 4 × 0.333 = 5.328 cu ft.
• Bags = 5.328 ÷ 0.6 = 8.88 → 9 bags.
• With 10% waste: 9 × 1.1 = 9.9 → 10 bags.

Always round up – you can’t buy a third of a bag, and leftover dry mix can be stored for future small repairs.

Common Mistakes and Misconceptions

One big mistake is confusing dry volume with wet volume. An 80-lb bag of dry powder might look like it takes up more than 0.6 cubic feet in the bag, but once mixed with water it shrinks to the 0.6 figure. Another error: using the bag’s weight (80 lb) to estimate volume – but concrete density is about 150 lb per cubic foot, so 80 lb would be 0.53 cu ft if it were solid. The 0.6 yield is higher because the mix includes air and water that expands it.

Also, don’t forget that a “cubic foot” is different from a “square foot.” A 1-inch-thick layer over 12 square feet is 1 cubic foot, but a 4-inch slab over the same area is 4 cubic feet.

Finally, the calculator’s 0.6 is an average – some brands yield 0.6, others 0.59 or 0.62. For critical structural work, consult the bag’s label or the manufacturer’s data sheet.

Comparing Bag Sizes

Concrete comes in 40, 60, and 80-lb bags. Here’s how they compare:

• 40-lb bag – yields about 0.3 cu ft (half of an 80-lb bag).
• 60-lb bag – yields about 0.45 cu ft.
• 80-lb bag – yields about 0.6 cu ft.

Why use 80-lb? Fewer bags to carry, less waste, and often cheaper per cubic foot. But 80-lb bags are heavy – around 80 lb – so if you’re mixing by hand, a 60-lb bag might be easier to handle. For large projects, ready-mix concrete delivered by truck is usually more economical than bags, but for small jobs like fence posts or small slabs, bags are practical.

Safety and Handling Tips

Concrete mix is caustic – the cement powder can burn skin and eyes. Always wear gloves, long sleeves, and safety glasses. Mix in a well-ventilated area to avoid inhaling dust. When mixing, add water gradually and use a hoe or mixer; never mix with bare hands. Wet concrete can cause severe burns if it sits against skin – wash immediately if it contacts you. Also, lift bags properly to avoid back injury; use a wheelbarrow or cart for moving multiple bags.

Frequently Asked Questions

How many 80lb bags do I need for 1 cubic yard?
One cubic yard is 27 cubic feet. Divide 27 by 0.6 = 45 bags. So you need 45 bags of 80-lb mix for a full cubic yard, plus extra for waste.

Can I use 80lb bags for a 4-inch slab?
Yes. For a 4-inch slab, calculate the area in square feet, multiply by 0.333 (4 inches in feet) to get cubic feet, then divide by 0.6. For example, a 10×10 ft slab (100 sq ft) needs 100 × 0.333 = 33.3 cu ft, which is 55.5 bags – so order 56 or 60 bags.

Why does the bag say “yield 0.6 cu ft” but my hole looks bigger?
The 0.6 is the volume of wet concrete after mixing, not the volume of the dry powder. When you pour it into a hole, it settles and compacts, so the final volume is slightly less than the dry powder’s volume. Also, if your hole is larger than you think, you might need more.

What if I use too much water?
Adding extra water increases the volume slightly but weakens the concrete and increases shrinkage. Always add water gradually and use the amount recommended on the bag. Too much water can also cause segregation, where the aggregate settles to the bottom.

Is it cheaper to buy bags or order ready-mix?
For small projects (under 1 cubic yard), bags are usually cheaper and more practical. For larger projects, ready-mix is often less expensive per cubic foot and saves labor. A typical 80-lb bag costs around $5–$7, so 45 bags for a yard would be $225–$315, while ready-mix might be $150–$200 per yard, but with a minimum delivery fee.

Can I store leftover dry mix?
Yes, if you keep it dry and sealed. Opened bags can absorb moisture and harden, so roll the top down and store in a dry place. Use within a few months. Once mixed, concrete must be used within about 30–45 minutes, so only mix what you can place quickly.

Related

• Bundles in a Square of Shingles Explained
• Confidence Interval for Hazard Ratio: Explained
• Tom Watts Chef: Who He Is and His Cooking

in bags | Number of 80-lb bags | bags | 10 | 0
in vol | Volume to fill | cu ft | 6 | 0
out yield | Total wet concrete | cu ft | bags * 0.6 | 1
out per_cuft | Bags per cubic foot | bags/cu ft | 1 / 0.6 | 2
out total_vol | Volume from bags | cu ft | bags * 0.6 | 1