20x20 Concrete Slab Cost: 400 Sq Ft, 5.43 yd3, Quotes
Answer first: a 20x20 slab at 4 in is 400 sq ft, 5.43 yd3 with waste, 245 80-lb bags, and a $1,045.95 ready-mix material screen.
Quick answer
400 sq ft / 5.43 yd3
$1,045.95 material screen; $3,200-$7,200 installed screen
A 20x20 concrete slab cost estimate should start with the same four numbers every time: 400 sq ft, slab thickness, cubic yards, and installed scope. At 4 inches thick, a 20 ft by 20 ft slab needs about 4.94 yd3 before waste or 5.43 yd3 with 10% waste. That is about 245 common 80-lb bags, so most 20x20 slabs should be priced as ready-mix-first projects before anyone buys pallets.
Use the Concrete Slab Calculator for exact volume, the Concrete Cost Calculator for local material pricing, the Ready-Mix vs Bags Calculator for the buying fork, and the Concrete Quote Reviewer when a contractor bid says "20x20 slab" but does not spell out base, forms, reinforcement, finish, delivery, cleanup, warranty, or permit responsibility.
Many slab cost pages answer price per square foot. This page keeps that answer, then separates the ready-mix material check from the installed contractor quote so you can compare bids without confusing concrete cost with the whole job.
Quick answer
For a 20x20 slab at 4 inches:
| 20x20 slab item | Answer |
|---|---|
| Finished area | 400 sq ft |
| Concrete before waste | 4.94 yd3 |
| Concrete with 10% waste | 5.43 yd3 |
| Common 80-lb bags | 245 bags |
| Common 60-lb bags | 326 bags |
| 80-lb planning pallets | about 5.8 pallets |
| Ready-mix material screen | $1,045.95 at $165/yd3 + $150 delivery |
| Installed quote screen | $3,200-$7,200 at $8-$18/sq ft |
The material screen is not the installed price. It is the concrete purchase line before excavation, gravel, compaction, forms, vapor barrier, rebar or mesh, finish, control joints, curing, cleanup, access, permits, overhead, and profit.
20x20 cost formula
slab square feet = 20 x 20 = 400 sq ft
cubic yards = 20 x 20 x thickness in / 12 / 27
order quantity = cubic yards x (1 + waste percentage)
ready-mix material = order quantity x price per yd3 + delivery or short-load fee
material $/sq ft = ready-mix material / 400
installed $/sq ft = contractor quote / 400
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 material screen
$1,045.95 / 400 = $2.61 per sq ft material-only
If a contractor quote is $5,600:
$5,600 / 400 sq ft = $14.00 per sq ft installed
Those two numbers should not match. The installed number includes the work that turns a concrete order into a finished slab.
Thickness, bags, and material screen
| 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 |
These planning checks use $165 per yd3 and a $150 delivery or short-load line. Change the yard price, minimum billable yards, tax, fuel, wait time, pump, buggy, or wheelbarrow access before using the number as a budget.
For the bag-count-only version of this slab, use How Many Bags for a 20x20 Slab?. For a garage-specific slab, use the Concrete Garage Slab Calculator because garage floors often add vapor barrier, apron, thickened edges, door transition, and inspection scope.
Installed cost per square foot
Contractors often quote a 20x20 slab as a total price or a price per square foot. Use square-foot pricing only after you make the scope equal.
| Quote screen | Formula | What it means |
|---|---|---|
| Material-only $/sq ft | ready-mix material total / 400 | Concrete, delivery, and small-load fees only. |
| Installed $/sq ft | full contractor quote / 400 | Prep, forms, placement, finish, cleanup, and business costs. |
| Base-prep allowance | excavation + gravel + compaction / 400 | Whether the ground under the slab is priced clearly. |
| Access allowance | pump, buggy, chute, or wheelbarrow line / 400 | Whether concrete can reach the forms before it stiffens. |
Example installed screens:
| Contractor quote | Installed $/sq ft | What to check next |
|---|---|---|
| $3,200 | $8.00 | Very basic scope; confirm base, forms, reinforcement, finish, and cleanup. |
| $4,800 | $12.00 | Normal comparison point; check access, joints, and exclusions. |
| $7,200 | $18.00 | May include heavier prep, finish, removal, access, or local labor. |
Use the Concrete Slab Cost per Square Foot Guide when the bid is written by square foot instead of cubic yards.
Bags vs ready-mix for a 20x20 slab
A 20x20 slab is normally a truck-delivery question, not a bag checkout question. The bag count is still useful because it shows the scale.
| Buying path | 20x20 reality check | When it may still fit |
|---|---|---|
| 80-lb bags | 245 bags, about 5.8 planning pallets, about 19,600 lb dry mix. | Split pours, no truck access, or a crew that can batch fast enough. |
| Ready-mix | 5.43 yd3 with waste at 4 in; fewer batches and better finish timing. | Most continuous 20x20 slabs, patios, garage pads, and parking pads. |
| Mini-mix or volumetric | Useful when a standard plant has a high minimum or awkward access. | Smaller local trucks, uncertain quantity, or tight sites. |
| Contractor quote | Adds prep, forms, finish, warranty, and accountability. | Vehicle loads, garage slabs, shop pads, permits, drainage, or poor base. |
If the truck cannot reach the forms, check Concrete Truck Chute Reach and Concrete Wheelbarrow Distance before you approve the quote. Access can move a cheap material order into an expensive placement job.
Scope normalizer
| Quote line | Ask before approving |
|---|---|
| Dimensions | Is the finished slab exactly 20 ft x 20 ft, and are thickened edges separate? |
| Thickness | 4 in, 5 in, 6 in, or engineered detail for the actual load. |
| Base prep | Excavation depth, gravel depth, compaction, soft spots, and drainage. |
| Concrete | Yardage, waste factor, mix, delivery, short-load, fuel, tax, and wait time. |
| Reinforcement | Wire mesh, rebar, fiber, dowels, chairs, or no reinforcement. |
| Forms and joints | Forming, edge work, control joints, saw cuts, and stripping. |
| Finish | Broom, smooth trowel, sealer, decorative finish, or surface prep. |
| Access | Direct chute, pump, buggy, wheelbarrow distance, driveway protection, and washout. |
| Cleanup and warranty | Spoils, leftover concrete, form removal, curing protection, crack policy. |
If the slab replaces old concrete, add the Concrete Slab Removal Cost Guide. If it needs extra base work, add the Gravel Base Calculator for Concrete and Concrete Subgrade Compaction Cost Guide. For steel, compare the Concrete Rebar Calculator and Rebar vs Wire Mesh Cost.
20x20 slab use cases
The same 400 sq ft slab can mean different work depending on what it supports.
| Use case | Default starting point | Extra checks |
|---|---|---|
| Patio | 4 in slab with broom finish. | Drainage, access, finish, saw cuts, and furniture load. |
| Garage floor | 4-6 in slab or local detail. | Vapor barrier, apron, door transition, thickened edges, and inspection. |
| Parking pad | 5-6 in may be discussed. | Vehicle load, base depth, reinforcement, edge support, and drainage. |
| Shed or shop pad | 4-6 in depending on use. | Anchor bolts, equipment load, frost, base, and access. |
| Hot tub pad | Load and installer approval matter. | Filled weight, service clearance, drainage, and electrical clearance. |
Related guides: 24x24 Garage Slab Cost, Boat Parking Pad Concrete Cost, RV Parking Pad Concrete Cost, and Hot Tub Concrete Pad Cost.
Red flags
| Red flag | Why it matters |
|---|---|
| Quote only says "20x20 slab" | It may hide thickness, base, reinforcement, finish, or cleanup assumptions. |
| No concrete quantity | You cannot compare the bid with the 5.43 yd3 material screen. |
| No base-prep line | A cheap slab over weak base can crack or settle early. |
| No access plan | Pump, buggy, wheelbarrow, wait time, or driveway damage can appear later. |
| Bags treated as the default | 245 80-lb bags is a logistics warning, not a normal shopping list. |
| No joint or curing plan | Crack-control and surface quality expectations are unclear. |
Sources and checks
This page uses planning math and quote-screening ranges. It is not a live local bid or structural design.
| Check | Source | How it is used |
|---|---|---|
| Slab volume and bag-yield cross-check | QUIKRETE calculator and Sakrete concrete calculators | Cross-checks common 40/60/80-lb bag yield and slab volume assumptions. |
| Installed flatwork cost context | Homewyse concrete slab estimator and ConcreteNetwork driveway cost guide | Keeps installed square-foot screens separate from material-only concrete math. |
| Ready-mix practice context | NRMCA CIP guidance | Supports checking delivery, placement, yield, curing, and jobsite coordination before ordering. |
| Local quote override | Written supplier or contractor quote | Local yard price, minimums, labor, access, permits, and finish decide the final cost. |
Concrete Estimator Hub is independent and is not affiliated with these manufacturers, suppliers, contractor directories, or industry groups.
FAQ
How much concrete do I need for a 20x20 slab at 4 inches?
Plan about 4.94 yd3 before waste or 5.43 yd3 with 10% waste. That is about 245 common 80-lb bags.
How much does a 20x20 concrete slab cost?
As a material-only screen, 5.43 yd3 at $165 per yd3 plus a $150 delivery line is about $1,045.95. As an installed screen, 400 sq ft at $8-$18 per sq ft is about $3,200-$7,200 before local scope changes the quote.
How many 80-lb bags for a 20x20 slab?
At 4 inches thick with 10% waste, plan about 245 common 80-lb bags. That is about 5.8 planning pallets and about 19,600 lb of dry mix before water.
Is a 20x20 slab too big for bags?
Usually yes for a continuous pour. Bags can work when truck access is impossible or the slab is split into sections, but ready-mix is normally the first quote to get.
Is 4 inches thick enough for a 20x20 slab?
Four inches is a common planning starting point for light patios and pads. Vehicle loads, garages, hot tubs, shop equipment, poor soil, frost, permits, and local code may require 5 inches, 6 inches, reinforcement, or engineered details.
Why does one extra inch change the cost 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. At $165 per yd3, that extra inch adds about $224.40 before delivery, labor, base, finish, or reinforcement changes.
Next step
Run the exact dimensions in the Concrete Slab Calculator, compare the material path in the Ready-Mix vs Bags Calculator, then normalize contractor bids in the Concrete Quote Reviewer. Small contractors can turn the final scope into a client-ready packet with the Concrete Proposal Kit.
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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.