20x30 Concrete Slab Calculator (2026) - Cubic Yards, Bags, Rebar & Cost
Get an instant, accurate estimate for your 20x30 concrete slab project. This free calculator gives you exact cubic yards, bags needed, rebar quantity, gravel base, form boards, and a full cost breakdown for 2026 USA prices - whether you're pouring a garage floor, shed pad, patio, or workshop slab.
Key 20x30 Slab Facts for 2026
Concrete Volume (4")
Cubic yards needed for a standard 4-inch 20x30 slab - add 10% waste for 8.15 yd³ ordered
Installed Cost Range
Professional installation of a 600 sq ft slab at $6-$12 per square foot (2026 USA average)
Standard Thickness
4" for patios and sheds, 5-6" for garage floors and driveways per ACI standards
Full Cure Time
Concrete reaches 100% design strength at 28 days - walkable in 24-48 hours, drive on after 7 days
Who Uses This 20x30 Slab Calculator?
DIY Homeowners
Plan a backyard patio, shed foundation, or workshop floor. Know exactly how many yards to order and what it will cost before calling a supplier.
Concrete Contractors
Generate fast, accurate material bids for residential slab jobs. Share PDF reports with clients and reduce over-ordering waste on every pour.
General Contractors
Estimate concrete, rebar, gravel, and form costs for 20x30 slabs on building projects. Compare DIY vs. subcontractor pricing instantly.
Property Developers
Budget material and labor costs for multiple slab pads across a development site. Get consistent, code-referenced estimates for permit applications.
🧮 20x30 Concrete Slab Calculator
How the 20x30 Slab Calculator Works
Enter Thickness
The 20x30 dimensions are pre-filled. Select your slab thickness - 4 inches for patios, 5-6 inches for garage floors and driveways, or enter a custom depth.
Choose Specifications
Select your PSI rating, waste factor, and bag size. Open advanced options to add rebar, gravel base, form boards, and your local concrete pricing.
Click Calculate
Hit the Calculate button. Results appear instantly - no page reload. The calculator runs all math server-side using 2026 ACI and NRMCA standards.
Get Full Report
Review cubic yards, bag count, rebar, gravel, total cost, and timeline. Download a PDF report to share with your supplier or subcontractor.
Everything You Need to Know About a 20x30 Concrete Slab
A 20x30 foot slab covers 600 square feet - a common size for detached garages, large sheds, workshops, and outdoor patios. At 4 inches thick, you need 7.41 cubic yards of concrete. At 6 inches thick for a garage floor, that jumps to 11.11 cubic yards. Ordering the right amount with a proper waste factor prevents both short pours and excess material costs.
Concrete prices in 2026 average $125 to $165 per cubic yard delivered, depending on your region and PSI grade. For a full breakdown of material and labor costs, use our concrete slab cost calculator. If you need to calculate a different size, our general concrete slab calculator handles any dimensions.
How to Calculate Cubic Yards for a 20x30 Slab
The formula is straightforward: multiply length (ft) x width (ft) x thickness (ft), then divide by 27 to convert cubic feet to cubic yards. For a 20x30 slab at 4 inches (0.333 ft) thick: 20 x 30 x 0.333 / 27 = 7.41 cubic yards. Add your waste factor on top. Our concrete yardage calculator handles all the conversions automatically.
| Thickness | Cubic Yards (Base) | With 10% Waste | Common Use |
|---|---|---|---|
| 4 inches | 7.41 yd³ | 8.15 yd³ | Patio, shed floor, walkway |
| 5 inches | 9.26 yd³ | 10.19 yd³ | Light driveway, workshop |
| 6 inches | 11.11 yd³ | 12.22 yd³ | Garage floor, heavy driveway |
| 8 inches | 14.81 yd³ | 16.30 yd³ | Commercial, heavy equipment |
Rebar vs. Wire Mesh for a 20x30 Slab
Wire mesh (6x6 W1.4) works well for light-duty slabs like patios and shed floors. It costs less and is faster to install. For garage floors, driveways, or any slab that will carry vehicle loads, use #4 rebar on an 18-inch grid. You will need roughly 134 linear feet of rebar in each direction for a 20x30 slab at 18-inch spacing - about 268 total linear feet. For precise rebar quantities, see our slab calculator.
Always check local building codes before choosing reinforcement. Many jurisdictions require minimum reinforcement specs for permitted structures. Improper reinforcement is one of the top causes of slab cracking and settlement. If the slab is near a structure or will support heavy loads, consult a structural engineer. For load-bearing slabs, use our concrete load calculator to verify your design.
Gravel Base Requirements
A 4-inch compacted gravel sub-base is standard under most residential slabs per IBC guidelines. It improves drainage, reduces frost heave risk, and gives the concrete a stable, uniform bearing surface. For a 20x30 slab at 4-inch gravel depth, you need roughly 7.4 tons of crushed stone. Use a plate compactor to achieve 95% compaction before pouring. On unstable or clay-heavy soils, increase gravel to 6 inches. To dial in your exact slab thickness needs, use our concrete thickness calculator.
💡 Pro Tip: Order 10% Extra
Always order at least 10% more concrete than your base calculation. Subgrade variations, form flex, and spillage add up fast on a 600 sq ft pour. Running short means a cold joint - a structural weak point that cannot be fixed. A partial yard of extra concrete costs far less than a repair.
⚠️ Permit Required in Most Jurisdictions
A 20x30 slab typically requires a building permit, especially when attached to a structure or used as a garage floor. Check with your local building department before pouring. Unpermitted concrete work can cause problems when selling your property and may require costly removal.
Real 20x30 Slab Project Examples
🏠 Example 1: Backyard Patio Slab
Dimensions: 20 ft x 30 ft x 4 inches thick
Reinforcement: 6x6 W1.4 wire mesh
Concrete: 3000 PSI, 10% waste factor
Base: 4-inch compacted gravel
Total Cost (DIY Materials): $1,600-$2,100
This is the most common 20x30 application. At 8.15 cubic yards ordered, you need 2 ready-mix trucks. Professional installation adds $1,800-$3,000 in labor. Wire mesh runs about $180 for the full 600 sq ft at standard overlap. Use our concrete pad calculator for patio-specific estimates.
🚗 Example 2: Detached Garage Floor
Dimensions: 20 ft x 30 ft x 6 inches thick
Reinforcement: #4 rebar at 18-inch grid spacing
Concrete: 3500 PSI, 10% waste factor
Base: 6-inch compacted gravel
Total Cost (Installed): $4,500-$6,500
A 6-inch garage slab requires 12.22 cubic yards with waste - plan for 2 trucks. Rebar at 18-inch spacing needs about 400 linear feet total. Gravel base adds roughly 11 tons of crushed stone. Most contractors charge $7-$10/sq ft for garage slabs due to the extra thickness and reinforcement. See full garage estimates with our garage floor calculator.
🔧 Example 3: Workshop / Metal Building Slab
Dimensions: 20 ft x 30 ft x 5 inches thick
Reinforcement: #4 rebar at 12-inch grid spacing
Concrete: 4000 PSI, 10% waste factor
Base: 4-inch compacted gravel
Total Cost (Installed): $3,800-$5,800
Workshop slabs need higher PSI and closer rebar spacing for heavy tool and equipment loads. At 5 inches thick, you need 10.19 cubic yards with waste. #4 rebar at 12-inch spacing requires approximately 600 linear feet. 4000 PSI concrete adds $15-$20/yd³ over standard 3000 PSI. For detailed cost breakdowns, use our concrete price per yard calculator.
Frequently Asked Questions
A 20x30 foot slab at 4 inches thick requires 7.41 cubic yards of concrete before waste. With a standard 10% waste factor, order 8.15 cubic yards. At 6 inches thick, you need 11.11 cubic yards base (12.22 with waste). Use the calculator above to enter your exact thickness and get a precise material estimate instantly.
For a 20x30 slab at 4 inches thick (7.41 cubic yards), you need approximately 302 bags of 80 lb concrete, 403 bags of 60 lb, or 604 bags of 40 lb - before waste. However, for any slab over 1 cubic yard, ready-mix delivery is far more cost-effective. Mixing 300+ bags by hand or with a rented mixer is extremely labor-intensive and rarely saves money on a project this size.
A professionally installed 20x30 concrete slab (600 sq ft) costs $3,600 to $7,200 in 2026, or $6 to $12 per square foot. Materials alone run $1,400 to $2,400 depending on thickness and PSI. Labor accounts for $1,800 to $4,800. DIY installation can cut that labor cost but requires forming, pouring, screeding, and finishing skills. Regional prices vary - use our slab cost calculator for a full breakdown.
Standard residential slabs are 4 inches thick for patios, walkways, and shed floors. Driveways require 5 to 6 inches. Garage floors should be 5 to 6 inches minimum. Workshop floors or slabs supporting heavy equipment need 6 inches or more. Always check your local building codes - some jurisdictions require minimum 5-inch slabs for permitted structures. Our thickness calculator can help you determine the right depth for your load requirements.
Yes, most 20x30 slabs benefit from reinforcement. For patios and light-duty slabs, 6x6 W1.4 wire mesh is sufficient. For driveways, garage floors, or slabs subject to vehicle loads, use #4 rebar on an 18-inch grid. Heavily loaded workshop floors should use #4 rebar at 12 inches on center in both directions. Rebar prevents cracking from settling, thermal expansion, and load stress across the 600 sq ft span.
3000 PSI is the standard for most residential 20x30 slabs including patios, shed floors, and light driveways. Use 3500 to 4000 PSI for garage floors and driveways that will see regular vehicle traffic. 4000+ PSI is recommended for commercial use or slabs supporting heavy equipment. Higher PSI costs $10 to $20 more per cubic yard but extends slab life significantly, especially in freeze-thaw climates.
It takes approximately 45 bags of 80 lb concrete to fill 1 cubic yard. Each 80 lb bag yields about 0.60 cubic feet of mixed concrete, and 1 cubic yard equals 27 cubic feet (27 / 0.60 = 45 bags). For a 20x30 slab at 4 inches thick requiring 7.41 cubic yards, that is roughly 334 bags of 80 lb mix with the 10% waste factor included. Ready-mix delivery is strongly recommended for volumes over 1 yard.
A 20x30 concrete slab reaches 50% of its design strength in 3 to 7 days, 75% in 14 days, and 100% in 28 days. You can walk on it after 24 to 48 hours and park vehicles after 7 days. Keep the slab moist during the first 7 days using wet burlap, plastic sheeting, or a spray-on curing compound. Avoid pouring in temperatures below 40°F or above 90°F without special precautions.
Data Sources and Accuracy
- Concrete specifications: American Concrete Institute (ACI 318-19)
- Material costs: National Ready Mixed Concrete Association (NRMCA) 2026
- Labor rates: Bureau of Labor Statistics (BLS) 2026 Construction Wage Data
- Building codes: International Building Code (IBC 2024)
- Rebar standards: CRSI (Concrete Reinforcing Steel Institute)
- Gravel compaction: ASTM D698 Standard Proctor Test
📅 Last Updated:
Disclaimer: All results are estimates based on standard construction practices and 2026 national averages. Verify all quantities and costs with your local concrete supplier, contractor, and building department before ordering materials or beginning construction.
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