Cost to Pour a 400 Sq Ft Concrete Slab: $1,600 to $4,800
Four hundred square feet is the size where ready-mix economics turn favourable, the short-load fee threshold goes away, and the project moves from "weekend DIY" to "two-day contractor job." The 2026 cost range is $1,600 to $4,800 installed, with a 4-inch wire-mesh slab typically landing at $2,400 to $3,000. Common configurations include 20x20 (square patio or tight 2-car garage), 16x25 (large patio or RV pad), and 10x40 (driveway extension).
Why 20x20 Is the Default 400 Sq Ft Layout
The 20x20 footprint dominates the 400 sq ft market because it sits at the intersection of three considerations. First, the proportions feel natural for outdoor rooms because 20 ft is long enough for a dining table plus chair pullback in any orientation, and the square shape simplifies furniture arrangement. Second, the perimeter (80 ft) and the screed travel distance (20 ft maximum) are manageable for a crew of 3 in a single half-day pour. Third, the 5 cubic yard concrete order (after the 10 percent overage allowance) sits exactly at the short-load fee threshold at most ready-mix suppliers.
The 20x20 also works as a tight 2-car garage, although it is on the small end. Two standard sedans fit with roughly 6 inches of clearance on each side and almost no walking room between vehicles. A 22x22 (484 sq ft) is the more typical residential 2-car garage size, and a 24x24 (576 sq ft) is the comfortable 2-car size. If you're building a garage on a 400 sq ft footprint, expect the use to be more like covered parking with limited workshop or storage flexibility.
Other 400 sq ft configurations include 16x25 (250 sq ft long-rectangle suiting an RV pad or large patio), 14x29 (400 sq ft suiting a driveway extension or workshop apron), and 10x40 (long strip for a sidewalk, walkway, or narrow-lot driveway). Each requires different concrete-pour logistics, with the long strips requiring multiple screed pulls and more crew coordination. The square 20x20 remains the easiest to pour and the most cost-efficient for the contractor.
Where the Short-Load Fee Disappears at 400 Sq Ft
Ready-mix suppliers charge a short-load fee on orders below 5 cubic yards, typically $50 to $150 per truck delivery. The fee compensates for the operational cost of dispatching a partially-loaded truck (the truck still drives the same distance, the driver still bills the same hours, the wash-out cycle still happens regardless of how much concrete was delivered). A 200 sq ft 4-inch pour at 2.47 cubic yards pays the short-load fee. A 400 sq ft 4-inch pour at 4.94 cubic yards sits at the boundary and pays it about half the time, depending on supplier policy and how much overage the contractor orders. A 400 sq ft 6-inch pour at 7.41 cubic yards never pays it.
The economics meaningfully shift at 5 cubic yards. The contractor can order exactly the volume needed (5.5 cubic yards for a 4-inch pour, allowing 10 percent overage) and pay only the base per-cubic-yard rate. Concrete cost on a 5.5 cubic yard order at the national midpoint of $165 per cubic yard is $908, plus a $100 delivery fee, for $1,008 total. The same per-cubic-yard rate applied to a 2.5 cubic yard order would be $413 in concrete plus a $100 short-load fee and $100 delivery, for $613 total, or $245 per cubic yard effective rate. The 400 sq ft project gets concrete at $183 per cubic yard effective rate, a 25 percent discount versus the 200 sq ft project.
This is why per-sq-ft installed cost drops noticeably between 200 and 400 sq ft. The same contractor's labour rate and overhead apply to both projects, but the materials line is more efficient on the larger pour. By the time you reach 1,000 sq ft, the per-sq-ft cost is at the bottom of the residential range because both labour and materials are amortising fully. The 1,000 sq ft cost page covers the next breakpoint.
Wire Mesh vs Rebar at 400 Sq Ft
The wire mesh vs rebar choice is governed by the slab's intended use, not its size. A 400 sq ft patio with no vehicle access can use wire mesh (6x6 #10 welded wire) at $0.35 per sq ft installed, totaling $140 for the reinforcement line. A 400 sq ft driveway, garage floor, or RV pad requires rebar (#4 bar on 18-inch on-center grid) at $1.50 to $2.50 per sq ft installed, totaling $600 to $1,000 for the reinforcement line. The rebar premium of $460 to $860 over wire mesh is the largest single line-item difference between a patio and a vehicle-rated slab at this size.
Some contractors offer "rebar mesh" or "fibrous wire mesh" as a hybrid option at $0.75 to $1.25 per sq ft. This is fundamentally still wire mesh with thicker gauge or added synthetic fibers, not equivalent to true rebar grid. Wire mesh with added fibers improves shrinkage-crack resistance but does not match rebar for tensile strength under point loads. Vehicles exert concentrated loads at the tire contact patches (roughly 4 psi over 50 sq inches per tire for a typical sedan, or 100 lb per square inch over a single tire-patch); rebar handles this, wire mesh under shrinkage stress alone does not.
For 400 sq ft applications that mix uses (a patio that occasionally hosts a car for a party, an RV pad that mostly stays empty), the conservative call is rebar. The upgrade cost is meaningful but the failure cost of an underspec slab is much higher: cracked vehicle-rated slabs typically require full removal and replacement at $6 to $18 per sq ft, or $2,400 to $7,200 for a 400 sq ft slab, versus the $460 to $860 upgrade premium at install time. See rebar vs wire mesh cost detail for the side-by-side breakdown.
Fiber-reinforced concrete (synthetic polypropylene fibers added at the batch plant) is a third option that adds $5 to $15 per cubic yard to the concrete bill, or $25 to $75 on a 5 cubic yard order. Fibers replace wire mesh in most patio applications and complement rebar in heavy-use applications. The fiber-reinforced cost page has the detailed treatment.
Excavation and Base for a 400 Sq Ft Slab
Site prep for a 400 sq ft slab moves 6 to 9 cubic yards of soil (4 to 6 inches of excavation depth across 400 sq ft), depending on existing grade and final slab elevation. At a typical $30 to $50 per cubic yard for soil disposal at a local fill site, excavation runs $180 to $450 for the disposal alone, plus $200 to $400 for the contractor's labour and mini-excavator rental. Total excavation runs $380 to $850, or $1 to $2 per sq ft of the final project budget.
Gravel base placement at 4 inches depth across 400 sq ft requires 4.94 cubic yards of crushed stone (same volume math as the concrete). Bulk gravel delivery in 2026 runs $35 to $60 per cubic yard delivered, so the gravel bill is $175 to $300. Plate compaction takes 1 hour for a crew of 1 with a rented compactor at $75 per day, adding $100 to $150 in labour and rental. Total base prep runs $275 to $450, or roughly $0.75 to $1.25 per sq ft.
Vapor barrier (recommended for any slab in a building envelope and required for foundations) adds $0.10 to $0.25 per sq ft, or $40 to $100 for a 400 sq ft slab. The barrier is a 6 mil to 15 mil polyethylene sheet, overlapped at seams and turned up at the perimeter. It is invisible after the pour but materially extends slab life by preventing moisture migration from the sub-grade upward into the concrete. The detailed site prep cost page covers each line item.
How a 400 Sq Ft Slab Compares to Smaller and Larger Projects
At $2,400 to $3,000 mid-range, a 400 sq ft 4-inch slab costs 2 to 2.5 times a 200 sq ft slab ($1,200 to $1,400) and roughly half a 1,000 sq ft slab ($5,500 to $6,500). The per-sq-ft cost curve is steep at the low end (a 100 sq ft slab costs $5 to $8 per sq ft because of mobilisation, a 200 sq ft slab costs $4 to $7), flattens through the 400 to 800 sq ft range (around $6 to $7 per sq ft), and continues falling slowly into the multi-thousand sq ft range (around $5 to $6 per sq ft).
The shape of this curve reflects three transitions. The first is the mobilisation transition (around 200 sq ft) where fixed costs are no longer dominant. The second is the short-load transition (around 400 to 500 sq ft) where concrete delivery efficiency improves. The third is the crew-efficiency transition (around 1,000 sq ft) where crews can operate on assembly-line logic rather than project-specific tactics.
At 400 sq ft, you have crossed the first two transitions but not the third. The unit economics are reasonable, the project fits in two contractor-days, and the price is predictable. This is why 400 sq ft is the most common residential slab size and why most contractors' standard quote structures are built around projects of this scale. Use the size calculator to plug in your exact dimensions and see how moving above or below 400 sq ft affects the per-sq-ft economics.