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Concrete Slab Calculator
Calculate exactly how much concrete you need for slabs, patios, driveways, and foundations. Get cubic yards, bag counts, rebar estimates, and material costs instantly.
Concrete Slab Calculator
Calculate concrete volume, bags, cost, and rebar for any slab size
Concrete Slab Thickness Guide
Choosing the right slab thickness is critical for structural integrity and longevity. Under-building leads to cracking; over-building wastes material and money.
| Thickness | Application | Notes |
|---|---|---|
| 4" | Sidewalks, patios, shed floors | Minimum for foot traffic and light use. Most common for residential patios. |
| 5" | Residential driveways, garage floors | Standard for passenger vehicles. Reinforcement with wire mesh or fiber recommended. |
| 6" | Heavy vehicle driveways, commercial slabs | Required for trucks, RVs, and commercial traffic. Rebar reinforcement standard. |
| 8" | Foundations, loading docks | Structural grade for building foundations and heavy loading areas. |
| 10-12" | Industrial, heavy equipment pads | Engineered slabs for warehouses, factories, and machinery foundations. |
How to Calculate Concrete for a Slab
Calculating concrete for a slab is straightforward once you know three measurements: length, width, and thickness. All measurements must be converted to the same unit (feet) before plugging into the formula.
Step 1: Measure the Slab
Measure the length and width of your slab in feet. Measure thickness in inches, then convert to feet by dividing by 12. For example, a 4-inch slab is 4 / 12 = 0.333 feet thick.
Step 2: Calculate Volume in Cubic Feet
Multiply all three dimensions together to get the volume in cubic feet.
Step 3: Convert to Cubic Yards
Concrete is sold by the cubic yard. There are 27 cubic feet in one cubic yard (3 x 3 x 3 = 27).
Step 4: Add Waste Factor
Always add 10% extra for waste, spillage, and uneven subgrade. For irregular shapes or first-time pours, consider adding 15%.
Worked Examples
Example 1: 10 x 10 Patio Slab (4 inches thick)
1. Convert thickness: 4 inches / 12 = 0.333 ft
2. Volume = 10 ft x 10 ft x 0.333 ft = 33.3 cu ft
3. Convert: 33.3 / 27 = 1.23 cu yd
4. With 10% waste: 1.23 x 1.10 = 1.36 cu yd
You need approximately 1.36 cubic yards of concrete (about 56 x 60lb bags or 42 x 80lb bags).
Example 2: 20 x 24 Driveway Slab (6 inches thick)
1. Convert thickness: 6 inches / 12 = 0.5 ft
2. Volume = 20 ft x 24 ft x 0.5 ft = 240 cu ft
3. Convert: 240 / 27 = 8.89 cu yd
4. With 10% waste: 8.89 x 1.10 = 9.78 cu yd
You need approximately 9.78 cubic yards -- just under one ready-mix truck load (10 cu yd per truck).
Common Slab Sizes Reference
Quick reference table with pre-calculated concrete amounts for common slab sizes. All values shown before waste factor -- add 10% for your actual order.
| Slab Size | Thickness | Cubic Yards | 80lb Bags | Typical Use |
|---|---|---|---|---|
| 10 x 10 ft | 4" | 1.23 | 56 | Small patio, shed pad |
| 12 x 12 ft | 4" | 1.78 | 80 | Patio, hot tub pad |
| 20 x 20 ft | 4" | 4.94 | 222 | Two-car driveway pad |
| 24 x 24 ft | 4" | 7.11 | 320 | Large garage slab |
| 10 x 20 ft | 5" | 3.09 | 139 | Single-car driveway |
| 20 x 24 ft | 6" | 8.89 | 400 | Heavy-duty driveway |
Concrete Slab Calculator FAQ
How many bags of concrete do I need for a 10x10 slab?
For a 10x10 foot slab at the standard 4-inch thickness, you need approximately 1.23 cubic yards of concrete. That equals about 56 bags of 60lb concrete mix or 42 bags of 80lb mix (before waste factor). With the recommended 10% waste factor, order 62 bags of 60lb or 46 bags of 80lb mix.
How thick should a concrete slab be?
Slab thickness depends on the application. Sidewalks and patios need a minimum of 4 inches. Residential driveways and garage floors should be 5 inches. Driveways for heavy vehicles (trucks, RVs) and commercial slabs require 6 inches. Foundations and loading docks typically require 8 inches, while industrial and heavy equipment pads may need 10 to 12 inches.
How much does a concrete slab cost per square foot?
A basic concrete slab costs between $4 and $8 per square foot for materials and labor. Materials alone (concrete, rebar, forms) typically run $2 to $4 per square foot. Ready-mix concrete costs $125 to $150 per cubic yard delivered. Finishing options like stamping, coloring, or exposed aggregate can add $2 to $12 per square foot.
How many cubic yards of concrete do I need?
Use the formula: Length (ft) x Width (ft) x Thickness (ft) / 27 = cubic yards. Convert thickness from inches to feet first by dividing by 12. For example, a 20x20 foot slab at 4 inches thick: 20 x 20 x 0.333 / 27 = 4.94 cubic yards. Always add 10% for waste.
Do I need rebar in a concrete slab?
Rebar is recommended for any slab thicker than 4 inches and is essential for driveways, garage floors, and structural slabs. For residential slabs, #4 rebar spaced at 12 inches on center in both directions is standard. For light-duty slabs like patios, welded wire mesh (6x6 W1.4/W1.4) is an acceptable alternative. Rebar prevents cracking from soil movement and load stress.
How long does concrete take to cure?
Concrete reaches about 70% of its full strength in 7 days and 99% in 28 days. You can walk on a slab after 24 to 48 hours. Light vehicle traffic is safe after 7 days. Heavy loads and vehicles should wait the full 28-day cure. Keep the slab moist during the first 7 days for optimal strength -- cover with plastic sheeting or use a curing compound.
Need Automated Material Takeoffs?
BuildVision AI automatically calculates concrete, rebar, and formwork quantities from your blueprints -- eliminating manual measurement and calculation errors.
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