Concrete
Ready-mix or bags. For slabs over 1.5 yd3, ready-mix is usually cheaper and faster.
Free estimator · Updated 2026
Concrete Slab Calculator: Cubic Yards, Bags, Rebar & Complete Material List
Select your project type, enter dimensions, and get cubic yards, bag count, rebar or mesh quantity, and a full material list with costs. Supports L-shaped and multi-section slabs.
Concrete Slab Thickness Guide
| Application | Thickness | Reinforcement |
|---|---|---|
| Patio / walkway | 4 in | Wire mesh optional |
| Shed / outbuilding | 4 in | Wire mesh |
| Garage floor | 4-5 in | Wire mesh or rebar |
| Driveway | 4-5 in | Wire mesh or rebar |
| RV pad | 6 in | Rebar required |
| House foundation | 6 in | Rebar required |
1 yd3 covers 81 sq ft at 4 in, or 54 sq ft at 6 in.
Step 1
What are you pouring?
Patio / Walkway: 4 inches is standard for foot-traffic-only surfaces. Most jurisdictions require a permit for patios over 200 sq ft.
Step 2
Slab sections
Section 1
Area: 400 sq ft · Volume: 4.94 yd3
Step 3
Material options
Results
Patio / Walkway
400 sq ft · 5.43 yd3
Concrete: raw 4.94 yd3, order 5.43 yd3. Ready-mix $977 or 80 lb bags 245 ($1,838).
Subbase: 400 sq ft, about 7.41 tons, $320.
Vapor barrier: $60 at $0.15/sq ft.
Reinforcement: 441 sq ft, 3 rolls. Cost $220.
Forms: 80 lin ft perimeter, 92 lin ft boards, $60.
Materials Total
$1,637
Installed estimate: $3,198 - $4,326 including labor and permit allowance.
| Item | Qty | Cost |
|---|---|---|
| Concrete | 5.43 yd3 | $977 |
| Gravel | 7.41 tons | $320 |
| Vapor barrier | 1 roll | $60 |
| Reinforcement | 3 rolls | $220 |
| Form boards | 92 lin ft | $60 |
| Stakes | 16 pcs | $15 |
Thickness planning
How Thick Should Your Concrete Slab Be?
Thickness affects both structural performance and material cost. Going from 4 inches to 6 inches adds about 51% to concrete material, but far less to total installed cost because labor is mostly area-driven.
| Thickness | Vol per 100 sq ft | Ready-Mix Cost | 80 lb Bags | vs. 4 in | Use Case |
|---|---|---|---|---|---|
| 3.5 in | 1.08 yd3 | $194 | 49 bags | -12% | Minimum code; light patios |
| 4 in | 1.23 yd3 | $221 | 56 bags | Baseline | Patios, sheds, walkways |
| 5 in | 1.54 yd3 | $277 | 70 bags | +25% | Driveways, heavy garage use |
| 6 in | 1.85 yd3 | $333 | 84 bags | +51% | Foundations, RV pads |
Thickness Decision Guide
3.5 inches is code-minimum light-duty work. 4 inches handles most patios, shed bases, and walkways. 5 inches is a useful upgrade for garages, trucks, and freeze-thaw climates. 6 inches is required for RV pads, foundations, mobile home slabs, and engineered loads.
Concrete Slab Calculator - Quick Reference by Size
All values use 4 inches thickness with 10% waste. Ready-mix uses $179.89/yd3 and bags are 80 lb bags.
| Slab Size | Sq Ft | Cubic Yards | 80 lb Bags | Ready-Mix Cost | Bag Cost | Use Case |
|---|---|---|---|---|---|---|
| 10x10 ft | 100 | 1.36 yd3 | 62 bags | $245 + short load | $465 | Small patio, shed |
| 12x12 ft | 144 | 1.96 yd3 | 88 bags | $352 + short load | $660 | Standard shed base |
| 12x16 ft | 192 | 2.61 yd3 | 118 bags | $469 + short load | $885 | Medium shed |
| 16x16 ft | 256 | 3.48 yd3 | 157 bags | $626 + short load | $1,178 | Large shed |
| 16x20 ft | 320 | 4.35 yd3 | 196 bags | $782 | $1,470 | 1-car garage |
| 20x20 ft | 400 | 5.43 yd3 | 245 bags | $977 | $1,838 | 2-car compact |
| 20x24 ft | 480 | 6.52 yd3 | 293 bags | $1,172 | $2,198 | 2-car standard |
| 24x24 ft | 576 | 7.83 yd3 | 352 bags | $1,407 | $2,640 | 2-car full |
| 24x30 ft | 720 | 9.78 yd3 | 440 bags | $1,758 | $3,300 | 3-car garage |
| 24x36 ft | 864 | 11.74 yd3 | 528 bags | $2,110 | $3,960 | Large garage |
| 30x30 ft | 900 | 12.22 yd3 | 550 bags | $2,197 | $4,125 | Large patio/slab |
| 30x40 ft | 1,200 | 16.30 yd3 | 733 bags | $2,930 | $5,498 | Foundation |
| 40x40 ft | 1,600 | 21.73 yd3 | 977 bags | $3,906 | $7,328 | Large foundation |
| 40x50 ft | 2,000 | 27.16 yd3 | 1,222 bags | $4,882 | $9,165 | Large foundation |
| 40x60 ft | 2,400 | 32.59 yd3 | 1,467 bags | $5,859 | $11,003 | Full house slab |
| 50x60 ft | 3,000 | 40.74 yd3 | 1,833 bags | $7,323 | $13,748 | Large house slab |
Reinforcement
Rebar vs. Wire Mesh
Wire mesh is enough for patios, walkways, and shed bases. Rebar is better for driveways, garages, foundations, RV pads, and loads that need structural reinforcement.
Wire mesh: rolls = ceil(area x 1.10 / 150).
Rebar: calculate runs both ways at 18 inch spacing, then add 10% for lap splice.
| Wire Mesh | Rebar #3 | Rebar #4 | |
|---|---|---|---|
| Cost | $0.50-$0.60/sq ft | $1.00-$1.30/sq ft | $1.50-$2.00/sq ft |
| Strength | Holds cracks together | Structural reinforcement | Heavy structural |
| Best for | Patios, walkways, sheds | Driveways, garages | Foundations, RV pads |
| Spacing | 6x6 inch grid | 18 in both ways | 18 in both ways |
What Materials Does a Concrete Slab Require?
The calculator outputs the full material list, not just concrete volume.
Gravel Subbase
A 4-inch compacted crushed-stone base improves drainage and reduces cracking.
Vapor Barrier
Polyethylene sheet prevents moisture from wicking through interior slabs.
Reinforcement
Wire mesh or rebar controls cracking and load performance.
Forms and Stakes
Form boards define slab edges and hold concrete during placement.
Compare concrete slab cost including labor, check ready-mix concrete prices by state, estimate how many bags of concrete, or use the concrete calculator for all shapes.
Concrete Slab Calculator - Frequently Asked Questions
How much concrete do I need for a slab?+
Multiply length x width x (thickness / 12) to get cubic feet, then divide by 27 for cubic yards. Add 10% for waste. For a 20x20 ft slab at 4 inches: 20 x 20 x 0.333 = 133.3 ft3 / 27 = 4.94 yd3 x 1.10 = 5.43 yd3.
How many yards of concrete for a 24x24 slab?+
A 24x24 ft slab at 4 inches thick requires 7.83 cubic yards with 10% waste. At 5 inches thick, use 9.79 yd3. At 6 inches thick, use 11.74 yd3.
How thick should a concrete slab be?+
Use 4 inches for patios, walkways, and shed bases; 5 inches for garage floors with heavy vehicles; and 6 inches for foundations, RV pads, and engineered loads.
How much rebar do I need for a concrete slab?+
For a 24x24 ft slab with #4 rebar at 18-inch spacing both ways, plan on about 845 linear feet, or 43 sticks of 20-ft rebar, with 10% lap splice waste.
How many bags of concrete for a 12x12 slab?+
A 12x12 ft slab at 4 inches thick needs about 88 bags of 80 lb concrete with 10% waste, or 132 bags at 6 inches thick.
Can I calculate an L-shaped or irregular slab?+
Yes. Break the slab into rectangles, add each section separately, and the calculator sums the total area, volume, reinforcement, forms, and materials.
What is the coverage of one cubic yard of concrete?+
One cubic yard covers 81 sq ft at 4 inches thick, 65 sq ft at 5 inches thick, 54 sq ft at 6 inches thick, and 40 sq ft at 8 inches thick.
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