20×20 Concrete Slab (6 inches): Yards, Feet & Bags
A 20 ft x 20 ft concrete slab serves well as a sturdy patio or gazebo base. Decide between bagged concrete or ready-mix based on your delivery access and project scale.
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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)
When ordering, round to 0.25 yd³ and confirm delivery access for the truck.
Ensure your base is properly leveled and forms are secure to prevent shifting or uneven surfaces.
Finish the surface as desired and allow it to cure naturally, keeping it moist for optimal strength.
Is 6-inch Right for a 20x20 patio?
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 20×20 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.
Base Prep Checklist for a 20x20 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 20×20 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 20×20 at 6 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 6 ÷ 12 = 0.5000 ft
- Compute area: 20 × 20 = 400 sq ft
- Volume in cubic feet: 400 × 0.5000 = 200.00 ft³
- Convert to cubic yards: 200.00 ÷ 27 = 7.407 yd³
- Add 10% waste buffer: 7.407 × 1.10 = 8.148 yd³
Bag Mix or Ready-Mix for a 6-inch patio?
This slab needs about 7.41 yd³ of concrete, or 334 80 lb bags before waste. With a waste cushion, plan around 8.15 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 20×20
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 | 7.78 | 350 | 467 | 700 |
| +10% typical | 8.15 | 367 | 489 | 734 |
| +15% rough subgrade | 8.52 | 384 | 512 | 767 |
6-inch Slab Curing Stages
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 20 ft, width 20 ft, thickness 6 in.
Compute Volume
Area: 400 sq ft → volume: 200.0 ft³ → 7.41 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 the best option for mixing concrete, bags or ready-mix?
Choose bags for smaller projects with limited access, while ready-mix is suitable for larger areas needing efficient delivery.
How should I prepare the base for my slab?
Clear the area of debris, level the ground, and compact it to create a stable foundation.
What thickness should I choose for my slab?
A thickness of inches is generally recommended for patios and walkways to support weight and resist cracking.
How do I account for waste when ordering concrete?
Always plan for extra material to cover spills, over-excavation, or miscalculations; this helps ensure you have enough for your project.
Related Slab Sizes
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