Estimated Materials:
*Includes calculated safety margin.
How to Estimate Concrete for Your Project
Whether you are pouring a patio, a driveway, or a simple shed foundation, accurately estimating the amount of concrete needed is crucial to the success of your project. Ordering too little can lead to expensive "short load" fees or cold joints in your slab, while ordering too much is a waste of money and labor.
The Basic Concrete Formula
Concrete is typically sold by the Cubic Yard (when ordering from a ready-mix truck) or by the bag (for smaller DIY projects). The math to calculate the volume involves three dimensions:
- Length (measured in feet)
- Width (measured in feet)
- Thickness (measured in inches, then converted to feet)
To determine the cubic footage, use this formula:
Length (ft) × Width (ft) × [Thickness (in) ÷ 12] = Total Cubic Feet
To convert Cubic Feet to Cubic Yards, divide your result by 27.
How Many Bags of Concrete Do I Need?
If you are mixing the concrete yourself using premixed bags found at hardware stores, you need to know the yield of each bag. Standard yields are approximately:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
Our calculator above automatically does this conversion for you, rounding up to the nearest whole bag to ensure you have enough material.
Why Include a Safety Margin?
Professional contractors never order the exact amount of concrete calculated mathematically. In the real world, the ground is rarely perfectly flat, sub-bases settle, and forms can bow outward under the weight of the wet mix. We recommend adding a safety margin (or "waste factor") of:
- 5% for simple, rectangular forms on a flat base.
- 10% for irregular shapes or uneven sub-grades.
Common Slab Thicknesses
Choosing the right thickness depends on the intended use of the slab:
- 4 Inches: Standard for sidewalks, patios, and residential shed floors.
- 5-6 Inches: Recommended for driveways and garage floors that will hold vehicles.
- 6+ Inches: Heavy-duty applications or areas with heavy machinery.