20×24 Concrete Slab (4 inches): Yards, Feet & Bags
A 20×24 slab at 4 inches is a common size for driveway section or shop floor. It’s usually manageable with bags, but if access is easy you may prefer ready-mix for speed and consistency.
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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)
Round to 0.25 yd³ when ordering and ensure delivery access for the truck.
Neglecting proper base preparation can lead to cracking; make sure to compact the subgrade and use forms that support thickened edges.
Apply a smooth finish and keep the surface moist during curing to achieve optimal strength.
20×24 Slab Thickness Check
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 20×24 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.
Before You Pour: 20x24 Base Preparation Steps
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 20×24 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 20×24 at 4 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 4 ÷ 12 = 0.3333 ft
- Compute area: 20 × 24 = 480 sq ft
- Volume in cubic feet: 480 × 0.3333 = 160.00 ft³
- Convert to cubic yards: 160.00 ÷ 27 = 5.926 yd³
- Add 10% waste buffer: 5.926 × 1.10 = 6.519 yd³
Pour-Day Planning for This patio
This slab needs about 5.93 yd³ of concrete, or 267 80 lb bags before waste. With a waste cushion, plan around 6.52 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 20×24
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 | 6.22 | 280 | 374 | 560 |
| +10% typical | 6.52 | 294 | 392 | 587 |
| +15% rough subgrade | 6.81 | 307 | 409 | 614 |
After the Pour: Timeline for 20×24
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 24 ft, thickness 4 in.
Compute Volume
Area: 480 sq ft → volume: 160.0 ft³ → 5.93 yd³.
Add Waste
Add a small buffer for spillage and uneven subgrade, then round up when ordering.
Planning Notes
Common uses: driveway section, shop floor, large patio.
For more context and conversions, see the Concrete Calculator hub.
Frequently Asked Questions
What should I consider when choosing between bags and ready-mix?
Consider your project size, access for delivery, and whether you can mix on-site.
How do I account for waste in my concrete project?
Always add extra to your estimate to cover potential spills or miscalculations.
What is the best way to prepare the base for my slab?
Excavate to a stable layer, compact the soil, and ensure proper drainage for the best results.
How thick should my concrete slab be for a driveway?
A thickness of at least four inches is generally recommended for durability under vehicle loads.
Related Slab Sizes
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