20×30 Concrete Slab (6 inches): Yards, Feet & Bags
This 20 ft x 30 ft slab is perfect for a driveway section or shop floor. Decide between bags or ready-mix based on your access and waste factor.
Choose Your Calculator
Slab Calculator
Enter any custom length, width, and thickness to get cubic yards and bag counts.
Open Slab CalculatorBag Calculator
Choose 40/60/80 lb bag sizes and estimate counts for your exact volume.
Open Bag CalculatorWant other sizes? Browse all slab size pages
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 to ensure you have enough material.
Ensure your base is well-prepared and forms are secure to prevent shifting or cracking.
Apply a smooth finish and keep the slab moist during curing to enhance strength and durability.
Is 6-inch Right for a 20x30 driveway section?
For a driveway section, a 6-inch slab fits many heavy load jobs, but subgrade quality, reinforcement, and local requirements still matter.
Current spec
This page estimates a 20×30 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.
driveway section Base Work for a 6-inch 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 20×30 slab does not settle unevenly.
- Check form height and diagonal measurements before concrete arrives.
- Plan reinforcement, joints, and edge support based on the intended driveway section use.
Worked Example for 20×30 at 6 Inches
Step-by-step calculation for this exact slab:
- Convert thickness to feet: 6 ÷ 12 = 0.5000 ft
- Compute area: 20 × 30 = 600 sq ft
- Volume in cubic feet: 600 × 0.5000 = 300.00 ft³
- Convert to cubic yards: 300.00 ÷ 27 = 11.111 yd³
- Add 10% waste buffer: 11.111 × 1.10 = 12.222 yd³
Bag Mix or Ready-Mix for a 6-inch driveway section?
This slab needs about 11.11 yd³ of concrete, or 500 80 lb bags before waste. With a waste cushion, plan around 12.22 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 20×30
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 | 11.67 | 525 | 700 | 1050 |
| +10% typical | 12.22 | 550 | 734 | 1100 |
| +15% rough subgrade | 12.78 | 575 | 767 | 1150 |
6-inch Slab Curing Stages
For a driveway section, cure time matters more because wheel loads and concentrated weight can punish green concrete too early.
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 30 ft, thickness 6 in.
Compute Volume
Area: 600 sq ft → volume: 300.0 ft³ → 11.11 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 is the best option, bags or ready-mix?
Bags offer flexibility for smaller projects, while ready-mix is efficient for larger slabs, reducing labor and time.
How much waste should I account for?
It's wise to include a buffer for waste, as spills and over-excavation can occur during mixing and pouring.
How should I prepare the base for the slab?
The base should be compacted and level, ensuring it can support the slab's weight and prevent cracking.
What thickness should I choose for my slab?
A thickness of inches is recommended for heavy loads, but consider the specific use case to determine if adjustments are necessary.
Related Slab Sizes
Ready to Calculate Your Project?
Get instant estimates for cubic yards, cubic feet, and bag counts. No signup required.