How Much Concrete for a 20×20 Slab?
Quick Answer
For a 20×20 foot slab at 4-inch thickness, you need 4.94 cubic yards of concrete. At this volume, ready-mix delivery is strongly recommended over bags.
- 4 inches thick: 4.94 cubic yards (224 bags of 80 lb — use ready-mix instead)
- 5 inches thick: 6.17 cubic yards
- 6 inches thick: 7.41 cubic yards
- Ready-mix delivery: $600–$800 for 4-inch, $900–$1,200 for 6-inch
Formula
Multiply 20 ft × 20 ft × thickness in inches, divide by 12 to convert to feet, then divide by 27 for cubic yards.
Step-by-Step Calculation
- 1
Convert thickness to feet
4 inches ÷ 12 = 0.333 feet
- 2
Calculate cubic feet
20 × 20 × 0.333 = 133.3 cubic feet
- 3
Convert to cubic yards
133.3 ÷ 27 = 4.94 cubic yards
- 4
Add 10% waste
4.94 × 1.10 = 5.43 → order 5.5 cubic yards
20×20 Slab — Concrete by Thickness
| Thickness | Cubic Yards | Ready-Mix Cost* | With 10% Waste | Total Est. |
|---|---|---|---|---|
| 4 inches | 4.94 | $590–$790 | 5.5 CY | $660–$880 |
| 5 inches | 6.17 | $740–$990 | 6.8 CY | $815–$1,090 |
| 6 inches | 7.41 | $890–$1,185 | 8.2 CY | $985–$1,310 |
| 8 inches | 9.88 | $1,185–$1,580 | 10.9 CY | $1,310–$1,740 |
*Ready-mix concrete at $120–$160 per cubic yard delivered. Does not include labor or finishing.
Pro Tips
- At 4.94 cubic yards, bagged concrete is out of the question — that is 222 bags weighing nearly 9 tons. Ready-mix delivery is the only practical option, and you should have 3–4 helpers on site plus a plan for where to start and finish the pour.
- Divide the 20×20 slab into two 10×20 pours using a temporary form board if you are a smaller crew. This gives you manageable sections and natural control joints. Set the divider board, pour one side, remove it after 24 hours, and pour the other.
- At 400 square feet, invest in a power trowel rental ($100–$150/day). Hand-troweling this area is exhausting and inconsistent — a power trowel gives a professional hard-steel finish in a fraction of the time.
- Have a concrete pump truck on standby ($200–$400) if the mixer truck cannot back up to within 10 feet of your forms. Wheelbarrowing 5 yards of concrete is brutal and you will not place it fast enough to avoid cold joints.
Key Cost Factors
- Ready-mix delivery for 5.4 yards (including 10% waste) runs $650–$860 at $120–$160 per cubic yard. Most plants waive the short-load fee at 5+ yards, so you get a better per-yard rate than smaller orders. This is the sweet spot where ready-mix cost efficiency kicks in.
- A 20×20 slab pushes DIY vs contractor economics closer together. A contractor charges $8–$12/sq ft ($3,200–$4,800) while DIY materials run $800–$1,100 including rebar, forms, gravel base, and concrete. But you need tools ($200–$300 in rentals) and at least 3 helpers — factor in pizza and beer.
- Sub-base preparation is critical at this size and often underestimated. You need 4 inches of compacted gravel ($150–$250 delivered), proper grading for drainage, and a plate compactor rental ($60–$80/day). Skipping the sub-base is the #1 cause of slab failure.
Common Mistakes to Avoid
- Trying to bag-mix it. A 20×20 slab needs about 4.9 cubic yards — that is over 220 bags. Hand-mixing this volume is impractical and guarantees cold joints. This size demands ready-mix truck delivery.
- Ordering a single short load without planning. At nearly 5 yards you fill most of a truck, but ordering exactly 4.9 with no buffer risks running short. Order 5.25–5.5 yards to cover waste and uneven grade.
- Underbuilding the base. Over 400 square feet, poor compaction anywhere becomes a crack. Spend the time on 4–6 inches of compacted gravel and verify it is level across the whole area.
- No plan for placing 5 yards fast. Concrete starts setting in 60–90 minutes. Without enough people and a clear path for the wheelbarrows or chute, part of the pour stiffens before it is finished.
Related Project Considerations
- Reinforcement is non-negotiable. A 20×20 slab needs #4 rebar on a 16–18 inch grid, not just wire mesh, to control cracking and carry loads across such a large area.
- Plan control joints in a grid. Divide the slab into panels no larger than about 10×10 with saw-cut or tooled joints, keeping panel aspect ratios close to square to steer cracks into the joints.
- Truck access matters. Confirm the mixer can get within chute range (about 18 feet) of the pour. If not, budget for a concrete pump, which adds cost but prevents a stalled, wheelbarrow-only pour.
Frequently Asked Questions
How many trucks of concrete for a 20×20 slab?
A standard concrete truck holds 8–10 cubic yards. A 4-inch thick 20×20 slab (4.94 CY + 10% waste ≈ 5.5 CY) fits in one truck. A 6-inch thick slab at about 8.2 CY also fits in one load, though you may be near the truck maximum. Confirm load capacity with the plant when ordering, and have the site prepped so the truck can pour immediately on arrival to avoid extra waiting charges.
How much does a 20×20 concrete pad cost installed?
Professionally installed, a 20×20 slab (400 sq ft) costs $2,400–$4,800 for 4-inch thick or $3,600–$7,200 for 6-inch thick, including excavation, forming, gravel base, concrete, reinforcement, and finishing. Add 10–20% if site access is difficult, the old surface needs demolition, or you want a decorative finish like exposed aggregate or stamped patterns.
Do I need a permit for a 20×20 concrete slab?
Most jurisdictions require permits for slabs over 200 sq ft, and a 20×20 slab at 400 sq ft will almost certainly need one. Permit fees are typically $100–$500 and the process usually requires a site plan and an inspection of the base and reinforcement before pouring. Check with your local building department before starting — pouring without a permit can result in fines or a required tear-out at resale.