Bags or ready-mix?
The comparison everyone makes is between two prices. The comparison that actually decides it is between 50 bags and a truck — and the number that matters most is not on either invoice.
What a cubic yard is, in bags
A cubic yard is 27 cubic feet. An 80 lb bag of concrete mix yields 0.6 cubic feet, so a yard is 45 bags of concrete — and 50 bags of order, once the 10% waste allowance every calculator on this site applies is in. The bags calculator will do this for any volume, and in any of the sizes below.
| Bag | Bags per yard | Weight to carry |
|---|---|---|
| 80 lb bag | 50 | 4,000 lb |
| 60 lb bag | 66 | 3,960 lb |
| 50 lb bag | 80 | 4,000 lb |
| 40 lb bag | 99 | 3,960 lb |
| 90 lb bag | 44 | 3,960 lb |
The right-hand column is the one to read twice. A cubic yard in 80 lb bags is two tons of dry material that you lift twice: once into the car and once into the mixer. In 40 lb bags it is the same two tons in 99 lifts.
What the crossover actually looks like
Ready-mix is not sold by the yard alone. It is sold by the yard, plus a delivery charge, plus a short-load fee on anything under the plant's minimum — typically the difference between what you ordered and what they will get out of bed for. That fee is fixed, which means it is crushing on a small pour and invisible on a large one, and it is why the answer flips so sharply rather than sliding.
The arithmetic is straightforward once you have your own two prices:
Put your own numbers in the bags calculator and the slab calculator and you will have both sides of that line in about a minute. We deliberately do not print a crossover volume here, because it depends on two prices that differ by region, by month and by whether the plant likes the sound of your job.
Four real pours
| Pour | Volume | 80 lb bags | Dry weight |
|---|---|---|---|
| A 3 × 3 ft pad, 4 in deep | 0.12 yd³ | 6 | 480 lb |
| A 4 × 8 ft shed base, 4 in deep | 0.43 yd³ | 20 | 1,600 lb |
| A 10 × 10 ft patio, 4 in deep | 1.36 yd³ | 62 | 4,960 lb |
| A 12 × 20 ft driveway, 5 in deep | 4.07 yd³ | 184 | 14,720 lb |
The thing that is not on either invoice
A truck arrives with everything mixed to one specification and leaves in twenty minutes. Bags arrive dry, and every one of them has to be mixed by you at the same water ratio, in an order that keeps the pour continuous — because concrete that goes off before the next batch arrives leaves a cold joint, and a cold joint is a crack that has already decided where it is going.
That is the real limit on bags, and it is a limit of hands rather than money. One person with one mixer can keep a small pad continuous. Nobody keeps a driveway continuous alone, whatever the arithmetic says.
When bags win anyway
- Anything a truck cannot reach. A back garden with a side gate is a barrow job either way, and bags do not sit in a hopper going off while you barrow.
- Post holes and footings. The post hole calculator counts them by the hole; nobody orders a truck for twelve posts.
- Anything you want to stop and start. Bags keep. A truck does not.
Work out your own
The slab calculator gives the volume and both purchase routes for a rectangle or an L-shape; the bags calculator goes the other way, from bags you already have to the volume they will pour. For footings and piers, the deck footing calculator counts tubes rather than yards. All three take your own prices and none of them stores one.