In 2026, the cost to pour a 20×20 concrete slab (400 square feet) ranges from $2,400 to $4,800 for a standard 4-inch slab ($6 to $12 per square foot installed), with a national average of $3,400. For a heavy-duty 6-inch garage or workshop slab with #4 steel rebar reinforcement and thickened perimeter edges, total costs range from $3,200 to $5,800 ($8 to $14.50 per square foot).
| Project Specification | 4-Inch Slab (Patio / Shed / Gazebo) | 6-Inch Heavy Slab (2-Car Garage / Shop) |
|---|---|---|
| Total Slab Footprint | 400 sq ft (20 ft × 20 ft) | 400 sq ft (20 ft × 20 ft) |
| Installed Cost Range | $2,400 – $4,800 ($6–$12/sq ft) | $3,200 – $5,800 ($8–$14.50/sq ft) |
| National Average Price | $3,400 | $4,500 |
| Concrete Volume Required | 4.94 cu yds (5.5 yds w/ 10% waste) | 7.41 cu yds (8.2 yds w/ 10% waste) |
| Pre-Bagged Concrete (80 lb) | 250 bags (not recommended to hand-mix) | 370 bags (ready-mix delivery required) |
| Crushed Gravel Base (4″ Deep) | 5.0 cu yds (~7.0 US tons) | 5.0 cu yds (~7.0 US tons) |
| Steel Reinforcement | 6×6 welded wire mesh or #3 rebar @ 18″ O.C. | #4 rebar (1/2″) grid @ 16″ O.C. on dobie chairs |
| Typical Planning Use | Patios and light-use pads; verify the actual load and base | Heavier-use starting point only; project-specific design may differ |
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Your Planning Estimate Breakdown
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Range breakdown
| Component | Low | Average | High |
|---|---|---|---|
| Base installation | - | - | - |
| Thickness adjustment | - | - | - |
| Finish adjustment | - | - | - |
| Calculated subtotal | - | - | - |
Itemized Cost Breakdown for a 20×20 Concrete Slab
To understand where your money goes when hiring a professional concrete finisher, here is a line-item takeoff of materials, equipment, and labor for a standard 400 sq ft pour:
| Line Item / Component | Quantity / Sizing | Cost per Unit | Total Estimated Cost |
|---|---|---|---|
| Delivered Ready-Mix Concrete (4000 PSI) | 5.5 cubic yards (incl. 10% waste) | $150 / cu yd | $825 |
| Crushed Stone Base (3/4″ Road Base) | 7.0 tons (4″ compacted depth) | $35 / ton delivered | $245 |
| #4 Steel Rebar (1/2″) & Dobie Chairs | 32 sticks (20-ft bars) + 45 chairs | $14 / stick + $0.85/chair | $485 |
| Vapor Barrier (10-mil Polyethylene) | 400 sq ft roll | $0.15 / sq ft | $60 |
| Formwork Lumber (2×4 or 2×6 & Stakes) | 80 linear feet perimeter | $1.50 / linear ft | $120 |
| Ground Prep, Excavation & Compaction | Skid steer / mini-excavator + plate compactor | $1.50 – $2.50 / sq ft | $600 – $1,000 |
| Finishing Labor & Control Joint Cutting | Pour crew (3–4 finishers for 6–8 hrs) | $3.00 – $5.00 / sq ft | $1,200 – $2,000 |
| Total Turnkey Contractor Project | 400 sq ft Installed | $8.84 / sq ft (avg) | $3,535 – $4,735 |
4-Inch vs. 6-Inch Thickness: Which Sizing Does Your 20×20 Slab Need?
When 4 Inches (0.33 ft) May Be Considered:
- Outdoor Living Patios: Patio tables, fire pits, outdoor kitchens, and seating.
- Backyard Storage Sheds: Prefabricated wood or metal sheds up to 10,000 lbs total weight.
- Hot Tub Pads: A 4-inch reinforced pad is a common manufacturer starting point for some models, but this calculator cannot verify capacity. Use the exact filled weight, footprint, manufacturer base instructions, soil, drainage, reinforcement, joints, and local frost conditions before selecting a slab design.
When a 6-Inch (0.50 ft) Option May Be Evaluated:
- 2-Car Detached Garages: Heavier vehicles can justify evaluating a thicker slab, but loads, soil, base, reinforcement, and local code control the final specification.
- Workshop & Machinery Floors: Vehicle lifts, machinery, storage racks, and stabilizer feet create concentrated loads that need equipment data and project-specific review.
- Freeze-Thaw Areas: Frost depth, drainage, soil, and local requirements may call for frost-protected or thickened edges; a universal 12-inch edge is not assumed here.
Complete Material Takeoff for a 20×20 Slab
If you are planning to purchase materials yourself or verify contractor supply bids, use these planning takeoff quantities and confirm product yields and the final design locally:
- Concrete Volume:
20 × 20 × (4 ÷ 12) ÷ 27 = 4.94 cubic yards. Always add a 10% spillage and grade-variation buffer, bringing your order to 5.5 cubic yards (or convert exact specs using our concrete unit converter). - Total Concrete Weight: At the page’s 4,050-lb-per-yard planning density, 5.5 cubic yards weighs about 22,275 pounds (11.14 US tons). See the concrete weight guide; supplier batch weights vary.
- Steel Rebar Takeoff: A 16-inch-on-center planning grid contains 16 bars in each direction, or 640 linear feet before laps and waste. Use the rebar calculator, then have the final bar size, spacing, cover, and laps specified for the project.
- Gravel Sub-base: 400 square feet at 4-inch depth requires 133 cubic feet (5.0 cubic yards) of crushed gravel, weighing approximately 14,000 lbs (7.0 tons).
DIY vs. Turnkey Contractor: Can You Pour a 20×20 Slab Yourself?
| Project Scope | DIY Execution | Professional Contractor |
|---|---|---|
| Total Project Cost | $1,600 – $2,100 (Materials + tool rentals) | $3,200 – $4,800 (Turnkey labor + guarantee) |
| Time Commitment | 2 full weekends (forming, gravel prep, pour) | 1 to 2 days total |
| Physical Labor Required | Heavy (screeding 22,000 lbs of wet concrete) | None (handled by 3–4 experienced crew) |
| Pour Setting Window | 60–90 minutes before concrete hardens | Finished smoothly with power trowel |
| Risk of Costly Mistakes | High (flash setting, uneven puddles, surface scaling) | Ask each bidder for written warranty terms and exclusions |
Recommendation: While you can save $1,500 to $2,000 by doing it yourself, pouring 5.5 cubic yards of concrete is beyond the scope of a single DIYer. A ready-mix truck unloads in 30 minutes, giving you less than an hour to screed, float, and edge 400 square feet before the slab hardens. If you choose DIY, assemble a crew of at least 3 to 4 physically capable helpers and rent a professional bull float and power screed.
Frequently Asked Questions: 20×20 Concrete Slab Cost
How many yards of concrete do I need for a 20×20 slab?
For a 4-inch thick 20×20 slab, you need 4.94 cubic yards (order 5.5 yards with 10% waste). For a 6-inch thick slab, you need 7.41 cubic yards (order 8.2 yards with waste).
How many 80-pound bags of concrete for a 20×20 slab?
A 4-inch 20×20 slab requires approximately 225 to 250 bags of 80 lb concrete (or 300 to 330 bags of 60 lb concrete). Hand-mixing 250 bags is strongly discouraged; ordering a ready-mix truck is cheaper, faster, and ensures uniform mix strength.
Does a 20×20 concrete slab need a building permit?
Permit and inspection rules vary by jurisdiction and project use. An open patio, hot-tub pad, detached garage, workshop, electrical work, grading, or work near setbacks can each trigger different requirements. Check the local building department before excavation and do not use a national size threshold or fee as a permit determination.
How long after pouring a 20×20 slab can I build on it?
Loading time depends on the mix design, temperature, curing, slab design, and the planned load. Ask the ready-mix supplier and project professional for a written loading schedule; do not rely on one universal 7-day rule for walls, lifts, or vehicles.
Public References
- Hot Spring Spas pre-delivery instructions — manufacturer-specific foundation, site, and electrical guidance by model and year.
- International Code Council residential-code hearing document — examples of slab provisions and authority given to the local building official.
Estimate transparency
How this estimate was prepared
- Prepared by
- GoHomeCost Cost Data Team
- Last reviewed
- August 24, 2026
- Update cycle
- Public price inputs checked monthly
Calculation method: Project quantity is multiplied by editable low, typical, and high unit rates. Selected location adjustments, waste, fixed charges, and minimum-job rules are then applied and shown in the result.
Public references used on this page:
This is a planning estimate, not a quote or structural specification. Check product yields, local code, site conditions, and written contractor bids before purchasing materials or building.
Read the full methodology Report a price or calculation issue