The short answer is 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. None of those numbers come from the weight on the sack. They come from its yield — how much wet concrete the bag actually makes — which the manufacturer prints on the bag precisely because weight alone would not tell you.
The arithmetic
A cubic yard is 27 cubic feet. Divide that by the yield:
- 80 lb sack → 0.6 cu ft → 27 ÷ 0.6 = 45 bags
- 60 lb sack → 0.45 cu ft → 27 ÷ 0.45 = 60 bags
- 40 lb sack → 0.3 cu ft → 27 ÷ 0.3 = 90 bags
In metric the same logic gives roughly 80 sacks of 25 kg to the cubic metre, at about 12.5 litres of fresh concrete per sack.
Why the count matters more than the price per bag
Forty-five bags is about a ton and a half of material, and mixing it is a day of work for one person with a mixer. That is the real comparison, not the shelf price: a small pour done in sacks costs a weekend, and every batch is slightly different, which shows up as colour variation in a slab you will look at for years.
The threshold worth knowing is your supplier’s minimum. Most plants deliver from 3 to 5 cubic yards and charge a short-load fee of roughly $50 to $150 below it. So a one-yard pour is not “a truck or 45 bags” — it is “a truck plus a fee, or 45 bags”, and the fee is often less than the day.
Where the joints come in
The one thing sacks cannot buy you is continuity. Concrete that arrives in one truck goes in as one pour; concrete mixed in batches sets while you mix the next one, and a batch that meets a set edge leaves a cold joint. On a driveway or a garage floor that joint is a crack waiting for its first winter.
Work the volume out first, then decide. The concrete calculator takes the slab or footing dimensions and returns both the cubic yards and the bag count, so the comparison is in front of you rather than in your head.