Concrete Slab Cost Calculator
Find out what the concrete for your slab will cost, and whether bags or a ready-mix truck is cheaper. Enter the slab size, then type in the prices your local store and ready-mix plant quote you: the calculator shows both totals side by side.
How to use the concrete slab cost calculator
- Enter the slab's length, width and thickness. The example is a 20 × 20 ft slab, 5 in thick, about the size of a two-car parking pad.
- Keep a 10% waste allowance: running short in the middle of a pour costs far more than a few spare bags.
- Open Cost estimate and enter your price per 80, 60 or 40 lb bag, and the ready-mix price per cubic yard you were quoted.
- Add the plant's delivery or short-load fee, if any, to see the true ready-mix total.
- Compare the totals. Use Copy link to save the numbers and get a second quote.
How slab cost is worked out
The concrete itself is priced one of two ways:
- Bagged: number of bags × price per bag
- Ready-mix: cubic yards × price per yard, plus any delivery, short-load or waiting-time fees
Both start from the same volume: length × width × thickness ÷ 12 gives cubic feet, ÷ 27 gives cubic yards, and the waste allowance goes on top. An 80 lb bag yields about 0.60 ft³ (QUIKRETE, Sakrete), so one cubic yard takes 45 bags before waste.
The calculator doesn't pre-fill any prices. Concrete prices vary a lot by region, season and order size, and we found no current national figure we could cite, so a built-in price would only mislead you. Two phone calls give you better numbers than any average.
Worked example: a 20 × 20 ft slab, 5 in thick
This is the slab loaded above.
- Volume: 20 × 20 × 5 ÷ 12 = 166.7 ft³, which is 6.17 yd³ before waste.
- With 10% waste: 183.33 ft³, or 6.79 yd³ to order.
- In bags: 306 bags of 80 lb mix (or 408 of 60 lb), weighing over 12 tons in total.
- Cost: if an 80 lb bag costs P, the bagged total is 306 × P. If ready-mix costs R per yard plus a fee F, the truck costs 6.79 × R + F.
A handy shortcut: ready-mix is cheaper whenever its price per yard is below about 45 × the price of one 80 lb bag, less a share of any delivery fee. For a slab this size that is almost always the case, before you even count the labor of mixing 306 bags.
What else goes into the cost of a slab
The concrete is usually the biggest single line, but not the only one. Budget for:
- Base gravel: several inches of compacted crushed stone under the slab (see the gravel calculator).
- Forms and stakes: lumber or steel forms around the edge.
- Reinforcement: wire mesh, rebar or fiber mix, as your plans or local code require.
- Tools and rentals: mixer, plate compactor, bull float, edgers, groover, and a pump if the truck can't reach.
- Permits and labor: many towns need a permit for driveways and structural slabs; a finishing crew is worth it on large pours.
Write these down next to the concrete total so you compare complete quotes, not just the price per yard.
What to price for common slab sizes
| Slab | Order (yd³, +10%) | 80 lb bags | 60 lb bags | Weight of the bags | Usually the practical choice |
|---|---|---|---|---|---|
| 8 × 8 ft, 4 in | 0.87 | 40 | 53 | 3,200 lb | Bags |
| 10 × 10 ft, 4 in | 1.36 | 62 | 82 | 4,960 lb | Compare both |
| 12 × 12 ft, 4 in | 1.96 | 88 | 118 | 7,040 lb | Compare both |
| 12 × 20 ft, 4 in | 3.26 | 147 | 196 | 11,760 lb | Ready-mix |
| 16 × 20 ft, 4 in | 4.35 | 196 | 261 | 15,680 lb | Ready-mix |
| 20 × 20 ft, 5 in | 6.79 | 306 | 408 | 24,480 lb | Ready-mix |
| 24 × 24 ft, 5 in | 9.78 | 440 | 587 | 35,200 lb | Ready-mix |
| 24 × 24 ft, 6 in | 11.73 | 528 | 704 | 42,240 lb | Ready-mix |
| 30 × 40 ft, 6 in | 24.44 | 1,100 | 1,467 | 88,000 lb | Ready-mix |
To price bags: bags × your price per bag. To price ready-mix: cubic yards × the plant's price per yd³, plus any delivery or short-load fee. No prices are pre-filled because they vary by region and month; get two local quotes.
Ways to keep the cost down
- Get the base right first. A flat, compacted base keeps the slab at its design thickness. Every extra inch over a 20 × 20 ft slab is another 1.2 yd³ of concrete.
- Ask about short-load fees. Many plants add a charge below a minimum load. Combining jobs (slab plus steps or footings) into one delivery can avoid it.
- Be ready when the truck arrives. Plants often charge for waiting time beyond a set unloading window. Have the forms, helpers and tools in place before the truck comes.
- Price per pound for bags. 40, 60 and 80 lb bags yield the same concrete per pound (about 0.0075 ft³ per lb), so compare bag prices per pound, not per bag.
- Don't cut the waste allowance to save money. A second delivery or a cold joint costs more than the 10% extra.
Frequently asked questions
How much does a 20×20 concrete slab cost?
It depends on your local prices. At 5 in thick, a 20 × 20 ft slab needs about 6.79 yd³ of concrete including 10% waste, or 306 bags of 80 lb mix. Multiply the cubic yards by your ready-mix price (plus fees) or the bags by your bag price, then add base gravel, forms, reinforcement and labor.
Is it cheaper to mix your own concrete?
For small jobs under about 1 yd³, bags are usually cheaper and simpler because you avoid delivery and short-load fees. For larger slabs, ready-mix usually wins on price per yard, and it saves hours of heavy mixing. Compare using the rule of thumb: ready-mix is cheaper when its price per yard is below about 45 × the price of an 80 lb bag.
How many bags of concrete are in a yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb or 90 bags of 40 lb, based on bag yields of 0.60, 0.45 and 0.30 ft³. See the concrete bags calculator for pallet counts.
Why doesn't the calculator show prices?
Concrete prices change by region, season and order size, and we found no current, citable national average. Rather than show a number that could be wrong for you, the calculator leaves the price fields empty for you to fill in with real local quotes.
What is a short-load fee?
An extra charge some ready-mix plants add when you order less than their minimum load, because the truck makes the trip either way. Ask for the full delivered price, including any short-load, delivery and waiting-time charges, when you request a quote.
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- Concrete Bags CalculatorHow many 40, 60 or 80 lb bags make a yard, and how many per pallet.
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