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MaterialCalc — 20x20 Concrete Slab Cost Calculator

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

20x20 Concrete Slab Cost Calculator

Four inches for a patio, five if you will park on it.

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Result

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A 20x20 slab is 400 square feet, which puts it firmly in ready-mix territory — roughly five cubic yards at 4 inches, or about 90 bags of 80 lb mix if you were foolish enough to try. This page starts at those dimensions and lets you change them, and prices the base and mesh alongside the concrete.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

80 lb bag yield
0.6 ft³

Yield of one 80 lb bag of standard concrete mix.

Quikrete product data sheet — Published product data, checked 09/01/2026

60 lb bag yield
0.45 ft³

Yield of one 60 lb bag of standard concrete mix.

Quikrete product data sheet — Published product data, checked 09/01/2026

Ready-mix short-load threshold
1 yd³

Below roughly one cubic yard, most suppliers add a short-load fee that erases the price advantage over bags.

Regional ready-mix supplier schedules — Rough figure — widens the estimate range, checked 09/01/2026

Bags per pallet
42 bags

Above roughly one pallet of 80 lb bags, ready-mix is usually cheaper and always faster.

Home Depot / Lowe's pallet quantities — Industry standard, varies by supplier, checked 09/01/2026

Compacted gravel base under slab
4 in

Drains water away from the underside of the slab. Skipping it is the usual cause of a slab that heaves.

International Residential Code (2021) — Industry standard, varies by supplier, checked 09/01/2026

Crushed gravel density
105 lb/ft³

Compacted 3/4 in crushed stone. Varies by quarry and moisture; loose stone runs lighter.

USGS aggregate bulk density range — Industry standard, varies by supplier, checked 09/01/2026

Compaction allowance
0.2

Loose stone loses roughly a fifth of its volume once compacted, so order above the finished volume.

Standard earthwork practice — Rough figure — widens the estimate range, checked 09/01/2026

Vapor barrier roll coverage
1000 ft²

Home Depot / Lowe's published product data — Published product data, checked 09/01/2026

Welded wire mesh sheet coverage
150 ft²

One 5x10 ft sheet, less the overlap at the seams.

Home Depot / Lowe's published product data — Industry standard, varies by supplier, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

400 ft² × thickness in feet = cubic feet; divide by 27 for cubic yards. At 4 in that is 400 × 0.333 = 133 ft³ = 4.94 yd³ before waste. Gravel base is the same calculation at the base depth, converted to tons by density.

Worked examples

20x20 patio at 4 in

Length: 20 Width: 20 Thickness: 4 Waste: 10

About 5.4 cubic yards with waste. One truck, one pour, and roughly 3.5 tons of gravel underneath it.

20x20 parking pad at 6 in

Length: 20 Width: 20 Thickness: 6 Waste: 10

Just over 8 cubic yards — half again as much concrete for two extra inches. Worth it if a truck or RV parks here; wasted money if it is a patio.

20x24 slab at 4 in

Length: 20 Width: 24 Thickness: 4 Waste: 10

Adding four feet of length adds about a cubic yard. Area scales linearly, which is why thickness is the expensive decision and footprint is not.

Choosing materials

  • Five yards is one truck with room to spare, so there is no short-load fee and no reason to split the pour. Order the whole thing at once.
  • At 400 square feet you need help. A slab this size has to be screeded and floated before it sets, and two people cannot finish it alone in summer heat.
  • Control joints every 10 to 12 feet, cut within 24 hours of the pour. On a 20x20 slab that is a cut down the middle each way — the slab will crack, and joints decide where.
  • If this is a patio, slope it a quarter inch per foot away from the house. Flat patios pond, and ponded water finds its way into the foundation.
  • Check whether your jurisdiction wants a permit. Many treat a 400 ft² slab as a structure, especially if anything is going to be built on it later.

Common questions

How much concrete for a 20x20 slab?

At 4 inches thick, 4.94 cubic yards before waste, or about 5.4 yards with a 10% allowance. At 5 inches it is 6.2 yards and at 6 inches 7.4. Order to the quarter yard and round up.

How much does a 20x20 concrete slab cost?

Material — concrete, gravel base, mesh and vapor barrier — typically lands between $1,200 and $1,900 depending on local ready-mix prices. Poured and finished by a contractor, expect roughly $3,200 to $5,600. Toggle labor above to switch between the two.

Can I pour a 20x20 slab myself?

You can form and prep it yourself, but the pour itself needs three or four people and it needs to happen fast. Most owner-builders do the excavation, base and forms, then pay a finisher for the day of the pour — that split saves the most money for the least risk.

How many bags of concrete for a 20x20 slab?

About 223 bags of 80 lb mix at 4 inches. That is five pallets and several days of mixing, which is why nobody bags a slab this size. The comparison table shows the bag numbers anyway so you can see how badly it loses.

How deep should the gravel base be?

Four inches of compacted crushed stone under a 20x20 slab, which works out to roughly 3.5 tons. On clay or poor-draining soil, six inches is money well spent.

Do I need rebar in a 20x20 slab?

Wire mesh is adequate for a patio or shed slab on a good base. Move to rebar on a 12 to 18 inch grid if vehicles will park on it, if the soil is expansive, or if the slab is thicker than 5 inches.

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