20x20 Concrete Slab Cost Worksheet

A 20 by 20 foot slab covers 400 square feet and contains about 4.94 cubic yards at 4 inches before allowance. Confirm the required section, then compare quotes for base, reinforcement, concrete, delivery, placing and finish; the dollar examples are replaceable assumptions.

Key takeaways

  • A 20x20 footprint is 400 square feet and uses about 4.94 cubic yards at 4 inches before allowance.
  • Thickness and any thickened edges must come from the approved project details.
  • Compare self-performed and contractor budgets with equipment, labor, delivery and finish all visible.
  • The calculator returns volume and bag counts from entered dimensions; it does not design the slab.
  • The installed and materials-only figures are illustrative assumptions, not market data.
A finished 20x20 concrete slab being floated smooth by two workers on a residential backyard job site
Concrete Slab Calculator
Calculate cubic yards, bags and material cost from slab thickness and an entered price.
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Illustrative cost inputs for a 20x20 concrete slab

Apply current rates only to the 20x20 slab quantities, thickness, base, reinforcement and finish.

Illustrative calculation inputs only. Replace every price with a current, scope-matched local quote.
Use caseThicknessAssumed installed input
Patio, shed, light use4 in2,400 to 4,800 dollars
Residential garage floor5 in2,800 to 5,600 dollars
Heavy trucks, workshop6 in3,200 to 6,400 dollars
DIY, materials only4 inabout 1,200 to 1,600 dollars

To price your own slab with your exact thickness and local rate, enter the dimensions in the concrete slab calculator and it returns yards, bags and an estimated cost.

How much concrete a 20x20 slab needs

A 20x20 slab at 4 inches thick needs about 4.94 cubic yards of concrete, which most contractors round up and order as 5.5 yards to cover waste and an uneven subgrade. The math is simple: 400 square feet times 0.333 feet of thickness is 133 cubic feet, divided by 27 gives 4.94 cubic yards.

Cubic yards = Length(ft) x Width(ft) x (Thickness(in) ÷ 12) ÷ 27

Apply current rates only to the 20x20 slab quantities, thickness, base, reinforcement and finish.

ThicknessCubic yards neededOrder with waste80 lb bags (DIY)
4 in4.945.5about 222
5 in6.177about 278
6 in7.418about 333

Labor versus materials: where the money goes

The gap between material and total is why DIY tempts people, but a 5.5-yard pour has a 60 to 90 minute working window and needs a crew of three or four to place and finish before it sets. To weigh hiring out against pouring yourself, the breakdown in cost to pour a concrete slab walks through the labor side in detail.

What drives the price up: rebar, base and site prep

If the slab will carry vehicles or a structure, obtain the approved slab, edge and foundation details. A turned-down edge or perimeter footing cannot be selected or credited with preventing cracks from area alone.
A contractor measuring formwork with a tape measure before pouring a 20 by 20 foot concrete slab

Finishes, region and other add-ons

State the 20x20 slab quantities, thickness, base, reinforcement and finish in each quote.

How to get an accurate price for your slab

Apply current rates only to the 20x20 slab quantities, thickness, base, reinforcement and finish.

Rather than juggle the conversions by hand, the concrete slab calculator takes your dimensions and returns cubic yards, bag counts, and an estimated installed cost, so you walk into the conversation knowing what the job should run.

By Calcnaut
Formula and declared-case checks run during the build; sources are listed for readers to review. Read our method and editorial policy.
Illustrative worksheet, not a quote. These dollar figures are illustrative assumptions, not market data. Replace every rate with a current local quote for the stated scope: All monetary tables and examples on this page; replace rates with current, scope-matched local quotes.

Frequently asked questions

How much does a 20x20 concrete slab cost?
In the page's illustrative worksheet, a 20x20 area is multiplied by assumed inputs of 6 to 12 dollars per square foot, giving 2,400 to 4,800 dollars; an assumed 8 dollar input gives 3,200 dollars. Replace the input with a current, scope-matched local quote.
How many yards of concrete do I need for a 20x20 slab?
A 20x20 slab at 4 inches thick needs about 4.94 cubic yards of concrete, but you should order about 5.5 cubic yards to cover waste and an uneven subgrade. At 5 inches order 7 yards, and at 6 inches order 8 yards.
How should I compare self-performed and contractor slab budgets?
For self-performed work, price the approved base, forms, reinforcement, concrete, delivery or bags, equipment, curing and waste. Compare that total with contractor bids covering those same items plus placing, finishing and cleanup; include the value and feasibility of the required labor rather than comparing concrete alone.
How thick should a 20x20 concrete slab be?
A 20x20 footprint does not determine thickness or reinforcement. Use the section specified for the loads, supporting soil, concrete properties, joints and local requirements, then enter that thickness in the calculator to determine volume.
How much does the concrete itself cost for a 20x20 slab?
At 4 inches, the geometric volume is about 4.94 cubic yards before allowance. Multiply the order quantity by a current ready-mix quote for the required specification and include delivery, minimum-load, waiting and pumping terms. Base, forms, reinforcement, placing and finishing are separate unless quoted.

Sources and references

These references and structured source records support the method and material data used on this page.

Related

Concrete Slab Calculator→Cost to Pour a Concrete Slab→How Thick Should a Concrete Slab Be?→How Much Does a 12x12 Concrete Slab Cost?→