This concrete slab calculator works out both ways to buy your slab — cubic yards of ready-mix from a truck, or the number of bags to mix yourself — with the cost of each. No sign-up. The working is shown under every number.
Quick example: a 10 × 10 ft slab at 4 in thick needs 1.5 cubic yards of ready-mix ($240.00) or 62 bags ($372.00) at the example prices.
| Item | Buy | Cost |
|---|---|---|
| Estimated total | $0.00 |
The two lines are alternatives — buy one or the other, not both. The total line compares the two options.
Estimate only — not a guarantee. Mistakes are possible, so always check the quantities with your supplier or a builder before you buy or build.
A 10 × 10 ft slab at 4 in thick is 1.23 cubic yards of concrete exactly. This calculator adds 10% for spillage and uneven ground (1.36 cubic yards) and rounds up to the nearest half yard, so the order quantity is 1.5 cubic yards — $240.00 at the example ready-mix price of $160 per cubic yard.
If you mix it yourself instead, the same 10 × 10 ft, 4 in slab needs 62 × 80 lb bags — each bag makes 0.6 cu ft — at $372.00 at the example bag price of $6.
The 10% covers spillage, uneven ground under the slab and settling, because concrete cannot be stretched once the truck or mixer starts. The calculator adds the 10% before rounding the ready-mix order up to the nearest 0.5 cubic yard.
At 4 in thick, a 20 × 20 ft slab takes 5.5 cubic yards of ready-mix ($880.00) or 245 bags ($1,470.00) at the example prices. At 6 in thick it takes 8.5 cubic yards ($1,360.00) or 367 bags ($2,202.00).
Both options are priced on this page so you can compare them: for the 10 × 10 ft, 4 in slab the ready-mix option is $240.00 and the bag option is $372.00 at the example prices. The two lines are alternatives — buy one or the other, not both.
| Slab size | Ready-mix | Ready-mix cost | Bags (80 lb) | Bag cost |
|---|---|---|---|---|
| 10 × 10 ft | 1.5 cu yd | $240.00 | 62 | $372.00 |
| 12 × 12 ft | 2.0 cu yd | $320.00 | 88 | $528.00 |
| 10 × 20 ft | 3.0 cu yd | $480.00 | 123 | $738.00 |
| 20 × 20 ft | 5.5 cu yd | $880.00 | 245 | $1,470.00 |
Ready-mix includes the 10% extra and is rounded up to the nearest 0.5 cubic yard. Costs use the example prices in the calculator above, and the two options are alternatives — buy one, not both.
These figures are planning estimates only — not a guarantee and not professional advice. Mistakes are possible, including through an error in this calculator. Always check the quantities, sizes, spacings and depths with your supplier, a licensed builder, and your local building code before you buy materials or start work. The owner of this site accepts no responsibility or liability for any loss, cost, damage, injury, wasted material or failed inspection that results from using these estimates. Prices are examples to start from, not live prices or quotes.