Concrete Slab
Calculator
How much concrete do you need for a slab? Calculate volume, bags, and cost for square or round concrete pours in seconds.
Used for driveways, patios, sidewalks, garage floors, fence posts, and sonotube footings. Supports imperial and metric units with an optional waste allowance for real-world accuracy. This concrete slab calculator calculates the exact amount of concrete needed for a slab by converting project dimensions into concrete volume, total weight, and concrete bag count.
What This Concrete Slab Calculator Does
Total Volume
Cubic yards or cubic meters
Bag Requirement
Based on your selected bag size
Total Weight
Precise concrete weight calculations
Material Cost
Estimated total project material cost
Waste Buffer
Optional buffer to prevent under-ordering
Instant Results
All calculations update as you type
Square & Rectangular
Use square or rectangular mode for flat slab pours with straight edges. Common uses include driveways, patios, sidewalks, garage floors, shed foundations, and concrete pads.
Inputs Required
Round Slabs & Posts
Use round mode for cylindrical concrete pours. Common uses include fence posts, deck footings, sonotube foundations, and concrete piers.
Inputs Required
How Concrete Slab Calculations Work
Concrete volume for square or rectangular slabs is calculated as: length × width × thickness. Thickness is internally converted to feet or meters before calculation. Concrete volume for round slabs, posts, and footings is calculated as: π × radius² × depth × quantity. Radius is calculated from the entered diameter.
Rectangular Formula
L × W × T
Round Formula
π × R² × D × Qty
Waste Allowance
To account for spillage, uneven ground, and form leakage, a waste multiplier is applied. 5-10% is recommended.
Weight Conversion
Imperial uses ~145 lbs/ft³. Metric uses ~2,400 kg/m³. Standard density values ensure weight accuracy.
Bag Count
Calculated by dividing total weight by bag weight. Results are always rounded up so you don't run short.
Cost Estimate
Calculated by multiplying the number of bags by the price per bag. Essential for budgeting.
Recommended Concrete Slab Thickness
| Application | Standard Thickness |
|---|---|
| Patios, Sidewalks, and Walkways | 4 Inches |
| Driveways and Load-Bearing Slabs | 5 - 6 Inches |
| Heavy-Duty or Commercial Applications | 8+ Inches |
Always follow local building codes and engineering requirements.
Why Use This Concrete Slab Calculator?
Prevents Errors
Helps avoid over-ordering or under-ordering concrete.
Flexible Units
Supports both imperial (feet/inches) and metric (meters/cm) units.
Versatile
Works for multiple slabs or footings using the quantity input.
Real-World Ready
Includes a waste buffer for real-world accuracy.
No Manual Math Required
No manual formulas. No spreadsheets. Just accurate estimates for DIY homeowners and contractors.
Frequently Asked Questions
How many bags of concrete do I need for a slab?
Enter the slab dimensions, thickness, and bag size in the calculator. The tool converts volume to weight and determines the total number of concrete bags required instantly.
How much concrete do I need for a driveway or patio?
Use square or rectangular mode, enter length, width, and thickness (usually 4 to 6 inches), and add a waste allowance. The calculator instantly returns total volume and bag count.
Should I add a waste allowance?
Yes. A 5 to 10 percent waste allowance is recommended to account for spillage, uneven surfaces, and form leakage. It's better to have two extra bags than to run out during the pour.
Can this calculator be used for fence posts?
Yes. Use round mode to calculate concrete for fence posts, deck footings, and sonotube foundations. Simply enter the diameter of the hole and the depth.