Yield, not weight, decides the count
An 80 lb bag does not make 80 lb worth of a fixed volume — it makes the volume printed on its label, commonly around 0.60 cu ft for standard mixes. Fast-setting and high-strength products differ, and bag weight alone never establishes yield.
At typical yields: a 40 lb bag ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft. Always confirm against your product's label.
Bags per cubic yard
One cubic yard is 27 cu ft. At typical yields that is about 90 × 40 lb bags, 60 × 60 lb bags, or 45 × 80 lb bags.
That is why bags suit small jobs: setting a few posts or a small pad is convenient, but mixing 45+ bags for a full yard is slow, and ready-mix usually wins on price and effort.
Worked example
A 4 × 4 ft pad at 4 in thick is 5.33 cu ft. With 10% waste: 5.87 cu ft. At 0.60 cu ft per 80 lb bag: 5.87 ÷ 0.60 = 9.8 → buy 10 bags.
The bag calculator does this for any shape and lets you edit the yield to match your exact product.
Frequently asked questions
How many 80 lb bags of concrete do I need for a fence post?
A 10 in diameter, 36 in deep hole with a 4×4 post needs about 1.4 cu ft — 3 whole 80 lb bags at an example yield of 0.60 ft³ per bag, before waste. The post hole calculator subtracts the post for you.
Do I need extra bags for waste?
Yes. Add 5–10% before dividing by the yield, then round up. Running out mid-pour leaves a weak cold joint.