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.
Quick reference: bags for a typical job
Figures are at zero waste - add five per cent for a formed pour, ten when you are casting straight against dug ground.
| Job | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| Step landing, 3 × 3 ft at 4 in | 0.11 cu yd | 5 | 7 |
| Ten fence posts, 12 in dia × 30 in | 0.61 cu yd | 28 | 37 |
| Shed pad, 8 × 8 ft at 4 in | 0.79 cu yd | 36 | 48 |
| Garage slab, 16 × 10 ft at 4 in | 1.98 cu yd | 89 | 119 |
| Driveway, 30 × 10 ft at 5 in | 4.63 cu yd | 209 | 278 |
The table shows where hand mixing stops making sense. Five bags is done before lunch. Thirty-six is a full day with a mixer and someone to carry. Two hundred is no longer a question about price - it is a question about whether the slab sets as one piece.
Three mistakes that only show up after the pour
Measuring the excavation instead of the form. Take a few spade depths, average them, and you will order short: dug ground is never level and concrete fills every dip. Against soil, allow ten per cent rather than five.
Ordering by weight instead of yield. “About a ton of concrete” means nothing useful. A ton of dry mix is roughly 25 eighty-pound bags and a little over half a cubic yard of finished concrete - half of what most people picture when they hear “a ton”.
Forgetting that rebar sits on chairs. Steel displaces almost no volume, but mesh or bar has to sit clear of the bottom. A 4 in form with mesh on 1 in chairs still needs concrete for the full 4 in, not for three.
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.