8×20 Concrete Slab (6 inches): Yards, Feet & Bags
An 8 ft x 20 ft concrete slab is perfect for a patio or a gazebo base. Decide between bags or ready-mix based on your 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)
When ordering, round to 0.25 yd³ and check yield on the label to ensure you have enough material.
Ensure your base is well-prepared and forms are sturdy, especially if you're adding thickened edges.
Apply a smooth finish and keep the slab moist during curing for optimal strength.
Is 6-inch Right for a 8x20 walkway?
A 6-inch slab gives this walkway extra stiffness and tolerance if soil conditions are imperfect or loads may grow over time.
Current spec
This page estimates a 8×20 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 8×20
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 8×20 slab does not settle unevenly.
- Check form height and diagonal measurements before concrete arrives.
- Plan reinforcement, joints, and edge support based on the intended walkway use.
Worked Example for 8×20 at 6 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 6 ÷ 12 = 0.5000 ft
- Compute area: 8 × 20 = 160 sq ft
- Volume in cubic feet: 160 × 0.5000 = 80.00 ft³
- Convert to cubic yards: 80.00 ÷ 27 = 2.963 yd³
- Add 10% waste buffer: 2.963 × 1.10 = 3.259 yd³
Concrete Delivery Strategy for 8×20
This slab needs about 2.96 yd³ of concrete, or 134 80 lb bags before waste. With a waste cushion, plan around 3.26 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 8×20
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 | 3.11 | 140 | 187 | 280 |
| +10% typical | 3.26 | 147 | 196 | 294 |
| +15% rough subgrade | 3.41 | 154 | 205 | 307 |
Cure Timeline for a 8x20 Concrete Slab
For a walkway slab, light foot traffic may come sooner than vehicle loads, but finishing quality still depends on keeping the slab damp and protected early on.
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 8 ft, width 20 ft, thickness 6 in.
Compute Volume
Area: 160 sq ft → volume: 80.0 ft³ → 2.96 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 bagged concrete and ready-mix?
Consider your project's location and access; ready-mix is often easier for large pours, while bags offer flexibility for smaller jobs.
How do I estimate the right amount of concrete needed for my slab?
Measure the area accurately and always include a buffer for waste, as it's common to miscalculate or have spillage.
What is the best way to prepare the base for my concrete slab?
Clear the area of debris, ensure proper drainage, and compact the soil to create a stable foundation for your slab.
How thick should my concrete slab be for a patio or walkway?
A thickness of inches is generally sufficient for patios and walkways, providing durability and support for foot traffic.
Related Slab Sizes
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