Please enter valid positive numbers for length, width, and thickness.
Project Totals
Total Volume (Cubic Yards):
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Total Volume (Cubic Feet):
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Pre-Mixed Bag Estimates
80lb Bags Needed:
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60lb Bags Needed:
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How to Calculate Concrete for Your Slab
Whether you are pouring a patio, a driveway, or a shed foundation, calculating the correct amount of concrete is critical. Ordering too little results in expensive delays and cold joints, while ordering too much is a waste of money.
The Concrete Formula
To determine the volume of concrete needed for a rectangular slab, you use the basic volume formula:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Since concrete is typically sold by the Cubic Yard, you must divide the total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
Understanding Waste Factors
Professional contractors always include a margin of error, often called a "waste factor." This accounts for:
- Spillage during the pour.
- Uneven subgrade (dips in the ground make the slab thicker in spots).
- Compression of the subbase.
A standard safety margin is 5% to 10%. Our calculator above allows you to input your own margin to ensure you don't run short.
Bags vs. Ready-Mix Truck
When should you buy bags, and when should you call a truck?
- Under 1 Cubic Yard: It is usually more cost-effective and manageable to mix 60lb or 80lb bags yourself.
- 1 to 2 Cubic Yards: This is the "gray area." It is a lot of physical labor to mix by hand, but might be below the minimum order for some delivery trucks (or incur a "short load" fee).
- Over 2 Cubic Yards: Ordering a ready-mix truck is almost always the better option for consistency and labor saving.
Standard Thickness Guide
- 4 Inches: Standard for sidewalks, patios, and residential driveways (passenger cars).
- 5-6 Inches: Recommended for heavy-duty driveways (RVs, trucks) or shed bases.