18×20 Concrete Slab (6 inches): Yards, Feet & Bags
This 18 ft x 20 ft slab is perfect for a patio or gazebo base. Decide between bagged concrete or ready-mix based on your access and volume needs.
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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)
Always round to 0.25 yd³ when ordering and check yield on the label for accurate quantities.
Ensure your base is well-prepared and forms are secure, especially for thickened edges to prevent shifting.
Apply a smooth finish and keep the surface damp to allow for proper curing without cracking.
Thickness Planning for a 18x20 Concrete Slab
A 6-inch slab gives this patio extra stiffness and tolerance if soil conditions are imperfect or loads may grow over time.
Current spec
This page estimates a 18×20 slab poured at 6 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.
Subgrade and Form Prep for 18×20
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 6 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 6 ÷ 12 = 0.5000 ft
- Compute area: 18 × 20 = 360 sq ft
- Volume in cubic feet: 360 × 0.5000 = 180.00 ft³
- Convert to cubic yards: 180.00 ÷ 27 = 6.667 yd³
- Add 10% waste buffer: 6.667 × 1.10 = 7.333 yd³
Order Planning for a 18x20 Concrete Slab
This slab needs about 6.67 yd³ of concrete, or 300 80 lb bags before waste. With a waste cushion, plan around 7.33 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 | 7.00 | 315 | 420 | 630 |
| +10% typical | 7.33 | 331 | 441 | 661 |
| +15% rough subgrade | 7.67 | 345 | 460 | 690 |
What to Expect After Pouring This patio
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 6 in.
Compute Volume
Area: 360 sq ft → volume: 180.0 ft³ → 6.67 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 should I consider when choosing between bags and ready-mix concrete?
Consider your project's scale, access to the site, and whether you have the means to mix on-site or prefer delivered ready-mix.
How do I determine the right thickness for my slab?
The thickness should be based on the intended use; heavier loads may require greater thickness for stability.
What is the best way to prepare the base for my concrete slab?
Excavate to a suitable depth, ensuring it's level and compacted to prevent settling after pouring.
How can I minimize waste when ordering concrete?
Estimate your needs carefully and add a buffer for spills or over-excavation, rounding your order up as necessary.
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