20x20 Concrete Slab Cost Guide 2026
Estimate 20x20 concrete slab cost by thickness, yards, 80 lb bags, ready-mix price, delivery, base prep, reinforcement, finish, and quote scope.
A 20x20 concrete slab is 400 square feet. At garage, shop, patio, or parking pad size, the estimate should move beyond bag count and into ready-mix, delivery, base prep, reinforcement, finish timing, and contractor quote scope.
Use the Concrete Slab Calculator for exact volume, the Concrete Cost Calculator for local material pricing, and the Concrete Proposal Kit when a small contractor needs to turn the estimate into a client-ready proposal.
Competitor structures such as ConcreteCalculator.pro's concrete slab cost calculator and ConcreteCalculatorMax's concrete slab cost calculator show that cost pages are a core commercial intent. The gap to cover is the quote review: what is included besides cubic yards.
Quick answer
At 4 inches thick, a 20 ft x 20 ft slab needs about 5.43 yd3 with 10% waste, or about 245 common 80 lb bags. At $165 per yd3 plus a $150 delivery or short-load fee, the ready-mix material check is about $1,045.95.
| Thickness | With 10% waste | 80 lb bags | Ready-mix material check |
|---|---|---|---|
| 4 in | 5.43 yd3 | 245 | $1,045.95 |
| 5 in | 6.79 yd3 | 306 | $1,270.35 |
| 6 in | 8.15 yd3 | 367 | $1,494.75 |
The material line is not the installed price. A real bid may include excavation, gravel, compaction, forms, vapor barrier, rebar or mesh, finish, control joints, curing, cleanup, permits, overhead, and profit.
Cost formula
cubic yards = 20 x 20 x thickness in / 12 / 27
order quantity = cubic yards x 1.10
ready-mix material = order quantity x price per yd3 + delivery fee
At 4 inches:
20 x 20 x 4 / 12 / 27 = 4.94 yd3 before waste
4.94 x 1.10 = 5.43 yd3 with waste
5.43 x $165 + $150 = $1,045.95
For broader yardage examples, see How Many Yards of Concrete Do I Need?.
Quote checklist
| Quote line | Ask before approving |
|---|---|
| Thickness | Is the quote based on 4, 5, or 6 inches? |
| Base prep | Is excavation, gravel depth, and compaction included? |
| Reinforcement | Wire mesh, rebar, fiber, chairs, or none? |
| Delivery | Price per yard, minimum load, short-load fee, and chute reach. |
| Finish | Broom, trowel, stamped, sealer, edge, and control joints. |
| Access | Can the truck reach, or is pump, buggy, or wheelbarrow labor needed? |
| Cleanup | Spoils, washout, form removal, and excess concrete. |
If the slab is for a garage, also read the Concrete Garage Slab Cost Calculator Guide. If it is a parking pad, compare RV Parking Pad Concrete Cost.
Bags or ready-mix?
A 20x20 slab is normally a ready-mix planning problem. Hundreds of bags create batching, fatigue, consistency, and finish-timing risk.
| Option | Reality check |
|---|---|
| Bags | 245 80 lb bags at 4 inches is rarely efficient. |
| Ready-mix | Better for continuous placement and finish timing. |
| Contractor | Useful when base prep, forms, finish, and warranty matter. |
Run the Ready-Mix vs Bags Calculator before you treat the bag count as a shopping list.
FAQ
How much concrete for a 20x20 slab at 4 inches?
Plan about 5.43 yd3 with 10% waste.
How many 80 lb bags for a 20x20 slab?
At 4 inches thick with 10% waste, plan about 245 common 80 lb bags.
Why does one extra inch change the price so much?
A 20x20 slab has 400 square feet of area. Increasing thickness from 4 inches to 5 inches adds about 1.36 yd3 after waste.
Next step
Use the Concrete Quote Reviewer to compare a material-only estimate with contractor bids that include prep, finish, and warranty.
Quote planning next step
Turn this guide into a concrete buying check
Run the matching calculator, then compare ready-mix, bagged concrete, delivery fees, access needs, and quote gaps before you buy materials or approve a contractor number.