16×20 Concrete Slab (4 inches): Yards, Feet & Bags
A 16×20 slab at 4 inches is a common size for patio or gazebo base. 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 check yield on the label to ensure you have enough material.
Ensure your base is well-prepped and forms are secure to prevent shifting, especially at the edges.
Finish the surface as desired and keep it moist during curing for best results.
Is 4-inch Right for a 16x20 patio?
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 16×20 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.
Before You Pour: 16x20 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 16×20 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 16×20 at 4 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 4 ÷ 12 = 0.3333 ft
- Compute area: 16 × 20 = 320 sq ft
- Volume in cubic feet: 320 × 0.3333 = 106.67 ft³
- Convert to cubic yards: 106.67 ÷ 27 = 3.951 yd³
- Add 10% waste buffer: 3.951 × 1.10 = 4.346 yd³
How to Order Concrete for 16×20
This slab needs about 3.95 yd³ of concrete, or 178 80 lb bags before waste. With a waste cushion, plan around 4.35 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 16×20
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 | 4.15 | 187 | 249 | 374 |
| +10% typical | 4.35 | 196 | 261 | 392 |
| +15% rough subgrade | 4.54 | 205 | 273 | 409 |
Cure Timeline for a 16x20 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 16 ft, width 20 ft, thickness 4 in.
Compute Volume
Area: 320 sq ft → volume: 106.7 ft³ → 3.95 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?
Ready-mix is convenient for larger projects, while bagged concrete allows for flexibility in smaller areas.
How should I prepare the base for my slab?
Excavate to a stable substrate, ensuring it's level and compacted to support the slab's weight.
How do I account for waste when ordering concrete?
Always order a bit extra to cover potential spillage and variations in the base preparation.
What thickness should I choose for my slab?
A thickness of four inches is generally suitable for light to moderate loads, but consider your specific use case.
Related Slab Sizes
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