Concrete Slab Calculator
Calculate cubic yards, premix bags, and estimated material cost.
0% (Exact)
5% (Recommended)
10% (Heavy Waste)
Project Summary
Total Volume (Cubic Feet):
0
Total Volume (Cubic Yards):
0
Premix Bags (60lb):
0
Premix Bags (80lb):
0
Estimated Material Cost:
$0.00
*Includes the selected safety margin.
How to Calculate Concrete for Slabs, Patios, and Driveways
Planning a construction project involving concrete requires precise measurements to ensure you order enough material without wasting money on significant excess. Whether you are pouring a patio, a driveway, or a shed foundation, the formula relies on three dimensions: length, width, and thickness.
The Concrete Formula
Concrete is sold by volume, typically in Cubic Yards (often just called "yards"). The mathematical formula to determine the volume of a rectangular slab is:
Volume = Length × Width × Height (Thickness)
The challenge usually lies in unit conversion. Most people measure length and width in feet, but thickness in inches. To get an accurate calculation, all units must be consistent before multiplying.
Step-by-Step Calculation Guide
- Measure Area: Determine the length and width of your pour site in feet. Example: A patio that is 10 feet by 10 feet.
- Determine Thickness: Most residential patios and sidewalks are 4 inches thick. Driveways often require 5 to 6 inches for heavy vehicle support.
- Convert Units: Divide your thickness (inches) by 12 to convert it to feet. (e.g., 4 inches / 12 = 0.33 feet).
- Calculate Cubic Feet: Multiply Length × Width × Thickness (in feet). (e.g., 10 × 10 × 0.33 = 33 cubic feet).
- Convert to Cubic Yards: Divide your cubic feet result by 27 (since there are 27 cubic feet in one cubic yard). (e.g., 33 / 27 = 1.22 cubic yards).
Why You Need a Safety Margin
Professional contractors never order the exact amount calculated. Several factors can lead to material shortages during a pour:
- Uneven Subgrade: If the ground isn't perfectly flat, low spots will require more concrete to fill.
- Spillage: Some concrete is inevitably lost during the transfer from the truck or wheelbarrow to the form.
- Form Deflection: Wooden forms may bow slightly outward under the weight of the wet concrete, increasing the volume.
Recommendation: Always add 5% to 10% to your final calculation. Our calculator above includes a selectable "Safety Margin" to handle this automatically.
Bags vs. Ready-Mix Truck
Should you mix it yourself or call a truck? As a general rule of thumb:
- Under 1 Cubic Yard: Use premix bags (60lb or 80lb). It is labor-intensive but cost-effective for small pads.
- Over 1 Cubic Yard: It is usually better to order ready-mix delivery. Mixing over 40 bags of concrete by hand is physically exhausting and difficult to finish before the first batch starts to set.