How Many 80 lb Bags of Concrete Do You Need?
Calculate 80 lb concrete bag quantities from cubic feet, understand typical yield, and avoid rounding or allowance mistakes before buying material.
An 80 lb bag count starts with volume, not weight. Measure the space in cubic feet and divide it by the yield printed on the specific bag. Then round up to a whole bag and apply any project allowance in a clear, consistent way.
The bag-count formula
bags needed = project volume in cubic feet ÷ yield per bag in cubic feet
QUIKRETE’s training material gives an approximate yield of 0.60 cu ft for an 80 lb concrete mix bag. At that yield, one cubic yard takes 45 bags because 27 ÷ 0.60 = 45.
Product yield varies, so 45 bags per yard is a useful check—not a substitute for the bag label.
Worked example: a 10 ft × 6 ft × 4 in slab
- Convert 4 inches to feet: 4 ÷ 12 = 0.333 ft.
- Calculate volume: 10 × 6 × 0.333 = about 20 cu ft.
- Divide by the stated yield: 20 ÷ 0.60 = 33.33 bags.
- Round up: 34 bags before any additional allowance.
The US concrete calculator shows the measured cubic feet, cubic yards and estimated 80 lb bag count together.
When to add the allowance
There are two valid ways to include an allowance:
- increase the measured volume first, then calculate bags; or
- calculate the exact bag count, apply the percentage, then round up.
Do one or the other, not both. Double-counting an allowance is a common reason online estimates disagree.
For example, 20 cu ft with a 10% allowance becomes 22 cu ft. At 0.60 cu ft per bag, 22 ÷ 0.60 = 36.67, so the planning quantity is 37 bags.
Bags or ready-mix?
Bagged concrete is convenient for repairs, posts, small pads and places a truck cannot reach. As volume grows, the number of bags, mixing time, water control and placement time become part of the decision. Compare the total bag count with the cubic-yard volume and talk to local suppliers before committing.
Do not assume the cheapest material total is the cheapest pour. Include delivery, equipment, labor, access, cleanup and the practical need to place the concrete within the required time.
Checks before buying
- Verify the actual bag yield and required water on the current product.
- Measure the excavation or forms at several points.
- Keep inches and feet consistent in the volume calculation.
- Round bags up, not down.
- Check that the mix is suitable for the project specification.
- Plan how the batch will be mixed, moved, placed and finished safely.
The reference yield above comes from QUIKRETE’s Mixing and Placing Concrete & Cement Mixes training material. Always use the yield on the product you are actually purchasing.