Concrete Slab Cost Calculator
This concrete slab cost calculator turns the size and thickness of a slab into cubic yards, a bag count and a priced breakdown: concrete, gravel base, reinforcement, delivery fees and labour. Every price is editable, so you can match it to your local quotes. Tap the i button beside any field for an explanation.
Quick answer: cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324. Add about 10% extra, then multiply by your price per yard. A 20 × 20 ft slab that is 4 in thick needs about 5.4 cubic yards, ordered as 5.5. At the example price of $165 per yard that is about $908 of concrete, and roughly $2,630 for the finished slab once base, mesh, fees and labour are added.
Calculate your concrete slab cost
Example values are loaded so you can see a result straight away. Change any field and the numbers update.
1. Slab size and shape
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2. Prices (replace the examples with your own quotes)
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3. Bagged mix option (for the comparison)
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Where the money goes
Total cost by slab thickness
Same footprint and prices, only the thickness changes. Your slab is the orange bar.
| Cost item | Amount | Share |
|---|---|---|
| Total estimated cost | 100% |
Ready-mix truck or bagged mix?
Estimates for planning only. The prices shown are examples you can edit, not quotes. Confirm measurements on site and follow your local building code. See our Disclaimer.
What a concrete slab really costs
A slab bill has more lines than most people expect: a compacted gravel base, the concrete, reinforcement, delivery, and the labour to form, pour, finish and cure it. The calculator above prices each one separately, so you can see which one drives the total and which one you can change. Concrete is sold by the cubic yard in the United States and by the cubic metre elsewhere, which is why volume always comes first.
Every price starts as an example, not a quote. Delivered concrete, gravel and labour vary with region, season and distance from the plant, so replace them with figures from two or three local suppliers. With your own prices in, the cost per square foot becomes a quick check on any contractor quote you receive.
How to calculate concrete slab cost, step by step
To check the maths by hand, or to work out a slab cost on site with no signal, the method is short:
- Find the volume in cubic feet. Multiply length by width in feet, then by the thickness in feet (inches divided by 12). A 4 in slab is 0.333 ft thick.
- Convert to cubic yards. Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft.
- Add an allowance. Add 5% to 10% for uneven subgrade, spillage and forms that bow slightly. The calculator starts at 10%.
- Round up for the order. Many plants quote in quarter-yard or half-yard steps, so 5.43 yd³ becomes 5.5.
- Multiply by your price per yard, then add delivery or short-load fees.
- Add the rest of the slab: gravel base, reinforcement and labour.
Worked example: a 20 × 20 ft slab, 4 in thick
The footprint is 400 sq ft. At 4 in thick the slab holds 133.3 cubic feet, which is 4.94 cubic yards. Adding 10% gives 5.43 cubic yards, and rounding up for the order gives 5.5. At the example price of $165 per yard, that is $907.50 of concrete.
The rest, at the calculator’s default prices: a 4 in gravel base is about $244, welded wire mesh is $180, delivery and extra fees are $100, and labour and finishing at $3 per sq ft is $1,200. The slab totals about $2,632, or $6.58 per square foot. Here the concrete is only 34% of the total and labour is 46%, which is why a slab quoted per square foot can look expensive next to the price of the concrete alone.
Which numbers move the total most
Running the same slab through the calculator shows where your time is best spent. Each $10 change in the price per yard moves the total by $55. Each $1 per square foot of labour moves it by $400. Making the slab 5 in thick instead of 4 in adds about $250, about 9% of the total. The flat delivery fee barely matters at this size. Pin down the labour rate and the price per yard first.
How much is a yard of concrete?
A “yard” of concrete is one cubic yard, which is 27 cubic feet. That covers 81 square feet at 4 in thick, or 108 square feet at 3 in. When people ask how much concrete costs per yard, they usually mean what a ready-mix plant charges for that volume delivered by truck. The price depends on the mix strength, how far the truck travels, the season and your local market. Small orders often carry a short-load or minimum-delivery fee, which is why the calculator has its own field for delivery and extra fees. Ask two or three plants for the price per yard, the delivery charge and the minimum load, and enter those numbers.
There is also a shortcut for figuring yards of concrete in your head: length in feet times width in feet times thickness in inches, divided by 324. The 20 × 20 × 4 example gives 6,400 ÷ 324, or 4.94 cubic yards, the same answer as the long method.
A yard of concrete weighs roughly 4,000 pounds, a little over two tons. The calculator uses 150 lb per cubic foot, which is 4,050 lb per yard.
How many bags of concrete do I need?
For small jobs such as a shed base, a set of steps or a 4 × 8 ft pad, bagged mix is practical. The bag size tells you the weight, and the yield is printed on the bag. The table shows the usual yields, and the calculator uses the same figures, so its bag count fills your slab including the extra allowance.
| Bag size | Typical yield (cu ft) | Bags per cubic yard | Bags for 100 sq ft at 4 in (10% extra) |
|---|---|---|---|
| 40 lb bag | 0.30 | 90 | 123 |
| 50 lb bag | 0.375 | 72 | 98 |
| 60 lb bag | 0.45 | 60 | 82 |
| 80 lb bag | 0.6 | 45 | 62 |
One cubic yard is 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix. Check the yield on the brand you buy, because it can differ a little between manufacturers.
The 20 × 20 ft example needs 245 bags of 80 lb mix. That is 19,600 lb, close to 9.8 tons of bags to lift, open and mix, and it is why anything beyond a small pad is normally poured from a truck.
Concrete slab cost per square foot by thickness
Thickness is the biggest lever on the concrete line of your budget. The table shows what 100 square feet of slab needs at five common thicknesses, priced at the example $165 per yard with a 10% allowance and no rounding or fees.
| Thickness | Cubic yards | With 10% extra | 80 lb bags | Concrete at $165/yd | Concrete per sq ft |
|---|---|---|---|---|---|
| 3.5 in | 1.08 | 1.19 | 54 | $196 | $1.96 |
| 4 in | 1.23 | 1.36 | 62 | $224 | $2.24 |
| 5 in | 1.54 | 1.70 | 77 | $280 | $2.80 |
| 6 in | 1.85 | 2.04 | 92 | $336 | $3.36 |
| 8 in | 2.47 | 2.72 | 123 | $448 | $4.48 |
Going from 4 in to 6 in adds half again to the concrete. The total project cost rises by less than that, because the gravel, labour and fees barely change with thickness. The thickness chart in the calculator shows that effect for your own slab and prices.
Cost of common slab sizes
These are common footprints at 4 in thick. The order column adds 10% and rounds up to the next quarter yard, the way you would place the order. The cost column is concrete only at $165 per yard, so add base, reinforcement, fees and labour from the calculator for the full picture.
| Slab size (4 in thick) | Area (sq ft) | Cubic yards | Order (10% extra) | 80 lb bags | Concrete at $165/yd |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 | 1.23 | 1.50 | 62 | $248 |
| 12 × 12 ft | 144 | 1.78 | 2.00 | 88 | $330 |
| 12 × 20 ft | 240 | 2.96 | 3.50 | 147 | $578 |
| 20 × 20 ft | 400 | 4.94 | 5.50 | 245 | $908 |
| 24 × 24 ft | 576 | 7.11 | 8.00 | 352 | $1,320 |
| 30 × 30 ft | 900 | 11.11 | 12.25 | 550 | $2,021 |
A 12 × 20 slab, for example, holds 2.96 cubic yards, so you would order 3.5 yards, which is $578 of concrete at the example price.
If you are after a concrete slab foundation cost calculator, note that a slab-on-grade foundation often has a thickened edge, or footing, around the perimeter. This calculator treats each pour as one uniform thickness. Run it once for the main slab and once for the footings, then add the two totals.
Ready-mix or bagged concrete: which costs less?
Per cubic yard, bags are far more expensive. At the example price of $7 for an 80 lb bag, 45 bags cost $315 per yard, against $165 for ready-mix. What flips the answer on small jobs is the flat fee a plant charges to send a truck.
With the example prices, the crossover lands at about two-thirds of a cubic yard, roughly 18 cubic feet or 30 bags of 80 lb mix. Below that, bags win. Above it, the truck is cheaper and far less work. Your own prices move the crossover: a plant with no delivery fee wins at almost every size. Past one cubic yard there is also a practical limit, because concrete starts to set while you are still mixing, and a pour that stops and restarts leaves weak cold joints. The comparison bars in the results show both costs for your slab, plus the total weight of the bags.
Costs beyond the concrete itself
Besides the concrete, a slab budget usually includes the items below. Here is how the calculator treats each one:
- Gravel base. Most slabs sit on compacted gravel or crushed stone. It is priced by volume, with its own depth and price.
- Reinforcement. Welded wire mesh, rebar or fibre helps control cracking. Enter a cost per square foot, or zero for none.
- Forms and labour. Setting forms, placing, screeding, finishing and curing are covered by the labour rate. Enter zero if you pour it yourself.
- Delivery and special fees. Short-load charges, pump trucks, weekend deliveries and similar extras go in the delivery and fees field.
- Vapour barrier and insulation. Slabs that will carry finished floors sometimes get a plastic barrier or foam board underneath. Add that to the fees field.
- Permits. Garage slabs, foundations and some patios can need a permit or inspection. Ask your local building department.
- Removing an old slab. Concrete slab removal is priced separately, usually per square foot or per job. It is not in the calculation, so add the quote to the fees field for a complete total.
How thick should a concrete slab be?
Four inches is the usual thickness for walkways, patios, shed bases and other light-duty slabs. Driveways and garage floors are commonly five or six inches, and workshops or anything carrying heavy equipment may need more, along with a proper design. The International Residential Code sets 3.5 inches as the minimum thickness for concrete floors on the ground (section R506.1), and the calculator flags anything thinner.
Thickness is only part of what makes a slab last. Soft spots, poor drainage and frost are common reasons slabs crack, and extra thickness does not fix them, so the subgrade deserves as much attention as the pour. This page is not engineering advice. For structural slabs, heavy loads or difficult soil, have a professional specify the slab.
Ways to lower the cost of a concrete slab
- Compare delivered prices, not yard prices. Ask each plant for the price per yard, the delivery charge and the minimum load. A low yard price with a high short-load fee can cost more on a small slab.
- Do the prep yourself. In the 20 × 20 example, labour is 46% of the total, the largest line. Excavation, forms, gravel and mesh are manageable for a patient DIYer, while the pour and finish are skilled work where mistakes are expensive.
- Combine pours. A walkway and a pad poured on the same day can reach a plant’s minimum load instead of paying two short-load fees.
- Spend on thickness where it counts. Going from 4 in to 5 in adds only about $250 to the example slab, which is modest insurance for a driveway or garage floor.
Common mistakes that blow up a slab budget
- Measuring thickness only at the edge. Low spots in the subgrade swallow extra concrete. Check the depth at the corners and the middle, and keep the allowance for the rest.
- Mixing units. Thickness in inches against length in feet is the classic error. If a result looks off, check it with the 324 shortcut above.
- Ordering the exact volume. A pour that comes up short is a bigger problem than leftover concrete.
- Leaving the base out of the budget. In the example the gravel base is $244, close to a tenth of the total.
Concrete slab cost calculator FAQ
How do I calculate concrete slab cost?
Multiply length, width and thickness (all in feet) to get cubic feet, divide by 27 for cubic yards, add 5% to 10% extra, round up to the next quarter yard, and multiply by your price per yard. Then add delivery fees, the gravel base, reinforcement and labour. The calculator above does every step and shows each line separately.
How much is a yard of concrete?
A cubic yard of ready-mix is priced by your local plant, and the price changes with mix strength, delivery distance and the market, so call two or three suppliers. The calculator starts with an example of $165 per yard that you should replace with your own quote.
How much concrete do I need for a 12×20 slab?
A 12 × 20 ft slab that is 4 inches thick holds 2.96 cubic yards. With a 10% allowance that is 3.26 cubic yards, which you would order as 3.50 yards, or about 147 bags of 80 lb mix.
How many bags of concrete are in a yard?
One cubic yard equals 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix, based on the usual yields of 0.60, 0.45 and 0.30 cubic feet per bag.
How much does a yard of concrete weigh?
About 4,000 pounds, a little over two tons. This calculator uses 150 lb per cubic foot, which works out to 4,050 lb per cubic yard.
How thick should a concrete slab be?
Four inches is common for patios, walkways and shed bases, and five to six inches for driveways and garage floors. The International Residential Code sets 3.5 inches as the minimum for concrete floors on the ground, and heavy loads need a proper design.
Is it cheaper to mix concrete myself or order a truck?
For very small pours, bags can be cheaper because trucks add delivery or short-load fees. Past roughly two-thirds of a cubic yard, with the example prices used here, ready-mix costs less per yard and is far less work. The calculator compares both for your slab.
What does a concrete slab cost per square foot?
In this calculator’s default example, a 4-inch slab with gravel base, mesh, delivery fees and labour comes to about $6.58 per square foot, and the concrete alone to about $2.27. Local prices change both numbers, so enter your own quotes.
Does the calculator include removing an old slab?
No. Demolition and haul-away are priced separately. If you need them, add the quote to the delivery and extra fees field so the total reflects the whole project.
Sources and further reading
The volume formulas are standard geometry, and one cubic yard is 27 cubic feet. Normal-weight concrete is taken as 150 lb per cubic foot, a common planning figure. For code and product details, go to the source:
- International Residential Code 2021, Chapter 5: Floors (section R506 covers concrete floors on ground, including minimum thickness)
- QUIKRETE Concrete Mix (bag sizes, with the data sheet for yield)
- National Ready Mixed Concrete Association (industry guidance on ready-mixed concrete)
- American Cement Association (cement and concrete research and education)
Codes are adopted locally and change over time, so confirm the rules with your local building department.
Plan the rest of the project
If your slab is not a simple rectangle, work out its area with the square footage calculator and use the known-area option above. To price a floor on top of the slab, try the flooring calculator, and use the concrete yard calculator or concrete bag calculator for volumes and bag counts. The concrete calculators hub lists more tools like this one.