20×20 Concrete Slab 4 Inches Thick Cost in Construction
When planning a small construction project like a garage floor, shed base, patio, or foundation pad, a 20×20 concrete slab that’s 4 inches thick is a common size and thickness. It’s large enough to accommodate most standard needs while being relatively affordable.
In this article, we’ll break down the cost of a 20×20 concrete slab (4″ thick), including material, labor, permits, finishing options, and factors that can influence the final price. Whether you’re a homeowner or contractor, this detailed cost guide will help you budget effectively for your concrete project.
Average Cost for a 20×20 Concrete Slab (4-Inch Thick)
The average cost to pour a 20×20 slab (400 square feet) at 4 inches thick ranges from $2,800 to $6,000, depending on several factors like location, material costs, site prep, and finishing options.
Basic Cost Breakdown:
| Item | Cost Range |
|---|---|
| Concrete (materials only) | $1,200 – $1,600 |
| Labor | $1,200 – $2,400 |
| Forms, gravel, rebar, mesh | $300 – $600 |
| Finishing (basic trowel) | $100 – $400 |
| Total Cost Estimate | $2,800 – $6,000 |
| Average Cost per Sq. Ft. | $7 – $15 |
Concrete Volume Calculation
Before calculating cost, it’s helpful to determine the volume of concrete needed for a 20×20 slab at 4 inches thick.
- Dimensions: 20 ft x 20 ft = 400 sq. ft.
- Thickness: 4 inches = 1/3 ft
- Volume: 400 sq. ft. × 1/3 ft = 133.3 cubic feet
- Convert to cubic yards: 133.3 ÷ 27 = 4.94 cubic yards
- Recommended order: Round up to 5 cubic yards to account for waste.
Concrete Cost by Yard
Concrete typically costs between $125 and $175 per cubic yard, including delivery.
- 5 cubic yards × $125 = $625 (low end)
- 5 cubic yards × $175 = $875 (high end)
If you’re using ready-mix concrete with delivery, this will likely be part of your total labor + material package when hiring a contractor.
Additional Cost Factors
1. Site Preparation
If the site needs clearing, grading, or leveling, expect to pay:
- $1 – $2 per sq. ft. for light prep
- $400 – $800 total
2. Sub-base (Gravel or Crushed Stone)
A 4-inch layer of compacted gravel beneath the concrete improves drainage and durability.
- Cost: $0.50 – $1 per sq. ft.
- Total: $200 – $400
3. Reinforcement (Wire Mesh or Rebar)
- Wire mesh: $0.20 – $0.30 per sq. ft.
- Rebar: $0.50 – $0.75 per sq. ft.
- Total for slab: $80 – $300
4. Finishing Options
| Finish Type | Cost per Sq. Ft. | 20×20 Slab Total |
|---|---|---|
| Basic smooth trowel | $0.50 – $1.00 | $200 – $400 |
| Broom finish | $0.75 – $1.50 | $300 – $600 |
| Stamped concrete | $8 – $15 | $3,200 – $6,000 |
| Colored concrete | +$2 – $4 extra | $800 – $1,600 |
Labor Cost for a 20×20 Concrete Slab
Hiring a professional concrete contractor includes:
- Site prep
- Form setup
- Pouring and leveling
- Finishing and curing
Labor typically ranges from $3 – $6 per square foot, totaling $1,200 – $2,400 for a 400 sq. ft. slab.
Permit and Inspection Fees
Depending on your location and local regulations, a permit may be required to pour a concrete slab.
- Permit cost: $50 – $200
- Inspection fees: Sometimes included in permit or extra
Always check with your municipality or building department before starting work.
Long-Term Value
A well-installed 20×20 concrete slab provides:
- Strong base for structures or patios
- Resistance to shifting and settling
- Minimal maintenance over 25–40 years
- Higher resale value for your home
Even though the upfront cost can be significant, concrete’s durability and low maintenance make it a wise investment.
How to Save Money on Your Concrete Slab Project
- Do the site prep yourself (e.g., leveling, clearing debris).
- Choose a basic finish like broom or smooth trowel.
- Buy concrete in bulk if doing multiple slabs.
- Get multiple contractor quotes to compare prices.
- Schedule work during contractor off-season (early spring or late fall).
- Use wire mesh instead of rebar for lighter loads.
Cost Summary Table
| Category | Low Estimate | High Estimate |
|---|---|---|
| Concrete (5 cu. yd.) | $625 | $875 |
| Labor | $1,200 | $2,400 |
| Gravel base | $200 | $400 |
| Reinforcement | $80 | $300 |
| Finishing | $200 | $600 |
| Permit | $50 | $200 |
| Total | $2,800 | $6,000 |
FAQs About 20×20 Concrete Slab Costs
Q1: How much concrete do I need for a 20×20 slab that’s 4 inches thick?
You’ll need approximately 5 cubic yards of concrete. The exact calculation is 400 sq. ft. × 1/3 ft = 133.3 cubic feet ÷ 27 = 4.94 cubic yards.
Q2: How thick should a concrete slab be for a shed or patio?
A 4-inch thickness is standard and sufficient for light structures like patios, sheds, or walkways. For heavier loads (vehicles or hot tubs), consider 5–6 inches with reinforcement.
Q3: Can I pour the slab myself to save money?
Yes, DIY is possible if you have experience and access to the right tools. However, errors in grading, finishing, or curing can lead to cracks and poor performance.
Q4: Does the slab need rebar or reinforcement?
Yes, especially to prevent cracking over time. Use wire mesh for light use or rebar for heavy loads like driveways or equipment pads.
Q5: How long does it take to install a 20×20 concrete slab?
The installation usually takes 1–2 days, including site prep, pouring, and finishing. Add 3–7 days for curing before using it fully.
Q6: What’s the difference in cost between stamped and regular concrete?
Stamped concrete adds about $8–$15 per sq. ft., significantly increasing the total cost. A stamped 20×20 slab can cost $6,000 – $12,000.
Q7: Will I need a permit for a 20×20 slab?
In most cities or towns, yes. Permits ensure the slab meets building codes, especially if it supports a structure or affects drainage.
Q8: How long does a concrete slab last?
A properly installed and maintained slab can last 25 to 40 years or more. Durability depends on climate, usage, and maintenance.
Whether you’re planning a new patio, shed base, or a small workshop foundation, understanding the full cost of a 20×20 concrete slab ensures your project stays on budget and meets expectations. With smart planning and material choices, you can get the most value out of your investment.
