Estimate what the concrete for your slab costs. Enter size and thickness, adjust the bag price and the ready-mix price to your local quotes, and the calculator compares both routes, including the short-load fee most plants add to small deliveries.
The two price fields start at $6.45 per 80-lb bag and $165 per cubic yard of ready-mix, with a $140 short-load fee when a delivery is under 4 cu yd. Local plants differ a lot, so overwrite both fields with the quotes you have. The estimate covers the concrete only: forms, rebar, gravel base, pump, finishing and labor are not priced.
Example: a 16×16 ft slab at 4 in needs 3.48 cu yd with a 10% allowance. As bags that is 157 bags and $1,013. As ready-mix it is $714: 3.48 cu yd at $165 plus the $140 short-load fee. The truck is $299 cheaper.
At these defaults ready-mix becomes the cheaper route from about 1.1 cu yd, which is around 50 bags, because a bag works out at $290.25 per cubic yard against $165 for the truck. The short-load fee disappears at 4 cu yd, so the 6 in version of the same 16×16 ft slab (5.21 cu yd) costs $860 as ready-mix, only $147 more than the 4 in slab.
| Slab size | Concrete | 80-lb bags | Ready-mix truck | Cost per sq ft (cheaper option) |
|---|---|---|---|---|
| 10×10 ft | 1.36 cu yd | 62 bags · $400 | $364 | $3.64 (ready-mix) |
| 12×12 ft | 1.96 cu yd | 89 bags · $574 | $463 | $3.21 (ready-mix) |
| 16×16 ft | 3.48 cu yd | 157 bags · $1,013 | $714 | $2.79 (ready-mix) |
| 20×20 ft | 5.43 cu yd | 245 bags · $1,580 | $896 | $2.24 (ready-mix) |
| 24×24 ft | 7.82 cu yd | 353 bags · $2,277 | $1,291 | $2.24 (ready-mix) |
| 30×30 ft | 12.22 cu yd | 550 bags · $3,548 | $2,017 | $2.24 (ready-mix) |
For the concrete alone, the default prices give $2.24 to $3.64 per sq ft for a 4 in slab, depending on size; small slabs cost more per square foot because of the $140 short-load fee. Forms, rebar, gravel base, finishing and labor come on top, so use these numbers as a materials budget and compare them with contractor quotes.
With the default prices ready-mix wins from about 1.1 cu yd (around 50 bags) upward. Below that, bags are about even or cheaper, and they avoid minimum-order rules. Change the bag price and truck price to your local quotes, since the break-even moves with both.
Many ready-mix plants charge a flat fee for small deliveries because a truck has to be sent for a part load. The calculator adds $140 whenever the total is under 4 cu yd. Ask your plant for its minimum and fee and adjust the volume or price to match.
A 20×20 ft slab at 4 in needs 5.43 cu yd. At the default prices ready-mix costs $896 and 245 bags would cost $1,580. No short-load fee applies above 4 cu yd.
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