18×20 Concrete Slab (4 inches): Yards, Feet & Bags
An 18 ft x 20 ft concrete slab is perfect for a patio or gazebo base. Decide between using bags or ready-mix based on access and waste factor.
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Quick Answer
Tip: Always confirm the yield printed on your bag label. Bag yield guide
Large pour: consider ready-mix delivery for speed and consistent results. Bag counts this high can take significant mixing time.
Project Notes (Unique)
Check yield on the label and round to 0.25 yd³ for accurate ordering.
Ensure your base is well-prepped and forms are set correctly to avoid issues with thickened edges.
Finish the surface as desired and allow it to cure properly before heavy use.
Choosing the Right Slab Thickness for 18×20
For a patio, a 4-inch slab fits many medium load jobs, but subgrade quality, reinforcement, and local requirements still matter.
Current spec
This page estimates a 18×20 slab poured at 4 inches.
Load check
Best fit depends on whether this is mainly pedestrian use, storage, or heavier wheel traffic.
Upgrade trigger
If you expect heavier loads later, compare this design against the alternate thickness before ordering.
Base Prep Checklist for a 18x20 Slab
Most slab problems start below the concrete. Keep the base level, compacted, and consistent before you pour.
- Strip loose topsoil and organic material until you reach firm subgrade.
- Compact the base in lifts so the 18×20 slab does not settle unevenly.
- Check form height and diagonal measurements before concrete arrives.
- Plan reinforcement, joints, and edge support based on the intended patio use.
Worked Example for 18×20 at 4 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 4 ÷ 12 = 0.3333 ft
- Compute area: 18 × 20 = 360 sq ft
- Volume in cubic feet: 360 × 0.3333 = 120.00 ft³
- Convert to cubic yards: 120.00 ÷ 27 = 4.444 yd³
- Add 10% waste buffer: 4.444 × 1.10 = 4.889 yd³
Bag Mix or Ready-Mix for a 4-inch patio?
This slab needs about 4.44 yd³ of concrete, or 200 80 lb bags before waste. With a waste cushion, plan around 4.89 yd³.
For this volume, ready-mix is usually the safer plan. Round up, confirm delivery minimums, and make sure your crew can place and finish the slab without delays.
Adjust dimensions in the slab calculator or compare bag counts here.
Bag Counts by Waste Buffer for 18×20
Pick a buffer based on site conditions: smooth forms and good subgrade can run leaner; uneven excavation or hard-to-reach pours need more cushion.
| Buffer | Yards (yd³) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| +5% tight pour | 4.67 | 210 | 280 | 420 |
| +10% typical | 4.89 | 220 | 294 | 440 |
| +15% rough subgrade | 5.11 | 230 | 307 | 460 |
Cure Timeline for a 18x20 Concrete Slab
Protect the slab after finishing, keep the surface from drying too fast, and avoid heavy service until the concrete has had time to gain strength.
First 24 hours
Protect edges, prevent washout, and avoid unnecessary traffic while the slab sets.
Days 2-7
Maintain curing conditions and monitor joints, surface drying, and weather exposure.
Up to 28 days
Concrete keeps gaining strength, so wait for the right service window before heavy use.
How to Estimate This Slab in 3 Steps
Measure
Length 18 ft, width 20 ft, thickness 4 in.
Compute Volume
Area: 360 sq ft → volume: 120.0 ft³ → 4.44 yd³.
Add Waste
Add a small buffer for spillage and uneven subgrade, then round up when ordering.
Planning Notes
Common uses: patio, gazebo base, walkway section.
For more context and conversions, see the Concrete Calculator hub.
Frequently Asked Questions
What’s better for my project, bags or ready-mix concrete?
Bags offer flexibility for small projects, while ready-mix is efficient for larger areas, saving time and labor.
How much extra concrete should I plan for?
Always consider a waste factor to cover spills, over-excavation, or adjustments during pouring.
What should I consider for base preparation?
A solid, level base is critical; ensure it’s compacted and free of debris to support the slab properly.
How thick should my concrete slab be?
A thickness of inches is generally suitable for patios and walkways, but consider additional thickness for heavier structures.
Related Slab Sizes
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