Concrete Slab Calculator
0% (Exact)
5% (Standard)
10% (Irregular Base)
15% (High Safety)
Total Volume Required:
0.00 Cubic Yards
Volume in Cubic Feet:
0.00 ft³
80lb Bags Needed (Premix):
0
60lb Bags Needed (Premix):
0
Estimated Truck Cost:
$0.00
Estimated Bag Cost (80lb):
$0.00
How to Calculate Concrete for Your Project
Whether you are pouring a patio, a driveway, or a foundation for a shed, accurately calculating the amount of concrete needed is crucial for project success. Ordering too little leads to "cold joints" and structural weaknesses, while ordering too much wastes money and creates disposal issues.
The Concrete Formula
Concrete is measured by volume. To find the volume of a rectangular slab, use the following formula:
Volume = Length × Width × Thickness
However, since length and width are usually measured in feet and thickness in inches, you must convert the thickness to feet first. To do this, divide the thickness inches by 12.
- Step 1: Calculate area (Length × Width).
- Step 2: Convert thickness to feet (Inches ÷ 12).
- Step 3: Multiply Area by Thickness (in feet) to get Cubic Feet.
- Step 4: Divide Cubic Feet by 27 to get Cubic Yards (the standard ordering unit for ready-mix trucks).
Accounting for Waste
Professional contractors never order the exact mathematical amount. Variables such as spillage, uneven subgrade depth, and form bowing can increase the required volume. We recommend adding a safety margin:
- 5%: For perfectly formed projects on a flat, compacted base.
- 10%: For standard projects where the ground may be slightly uneven.
- 15%+: For complex shapes or very irregular subgrades.
Premix Bags vs. Ready-Mix Truck
When should you mix it yourself versus calling a truck? generally, if your project requires more than 1 to 1.5 cubic yards of concrete, it is often more economical and labor-efficient to order a ready-mix truck.
Yield Reference:
- One 80lb bag of premix yields approximately 0.60 cubic feet.
- One 60lb bag of premix yields approximately 0.45 cubic feet.
- It takes roughly 45 bags (80lb) to equal one cubic yard.
Use the calculator above to compare the estimated bag count against the yardage to decide the best route for your pour.