Pouring Concrete Curbs
Whether you are pouring a small decorative landscape border for a garden bed or a heavy-duty structural curb for a commercial parking lot, estimating the exact volume of concrete you need is critical.
Unlike flat slabs, curbs are essentially long, continuous concrete beams. Because they are often poured over uneven terrain or dug into trenches, it is easy to accidentally pour them thicker or wider than intended, causing you to run out of concrete before reaching the end of the line.
Understanding Curb Dimensions
To calculate the volume of a curb, you are simply calculating the volume of a three-dimensional rectangle.
- Length: The total linear run of the curb along the ground.
- Width (Depth): How thick the curb is from front to back.
- Height: How tall the curb is from the very bottom of the excavated trench to the top finish level.
Crucial Warning: Do not just measure the visible height of an existing curb! A curb that sticks up 6 inches above the asphalt is usually buried another 6 to 12 inches below the asphalt for structural stability. You must calculate the entire height of the concrete block, both above and below grade.
How to Calculate Curb Volume
Because concrete in the United States is ordered and sold by the Cubic Yard, we must take measurements in feet and inches and convert them to yards.
The Formula
- Keep the Length in feet.
- Convert the Width (Depth) from inches to feet by dividing by 12.
- Convert the Height from inches to feet by dividing by 12.
- Multiply Length × Width × Height to find the total Cubic Feet.
- Divide the Cubic Feet by 27 (since there are 27 cubic feet in one cubic yard).
- Add a 5% to 10% waste factor to account for trench irregularities and spillage.
Total Cubic Yards = (Length × (Width ÷ 12) × (Height ÷ 12)) ÷ 27
Example Calculation
You are pouring a straight parking lot curb that is 100 feet long. The curb will be 6 inches wide and 12 inches tall (6 inches below ground, 6 inches above ground).
- Convert Width to feet:
6 ÷ 12 = 0.5 ft - Convert Height to feet:
12 ÷ 12 = 1.0 ft - Calculate Cubic Feet:
100 ft × 0.5 ft × 1.0 ft = 50 cubic feet - Convert to Cubic Yards:
50 ÷ 27 = 1.85 cubic yards - Add 5% Waste:
1.85 × 1.05 = 1.94 cubic yards
You should order 2 cubic yards of concrete from the ready-mix plant.
Extruded Curbs vs. Formed Curbs
There are two primary ways to pour a curb:
1. Formed Curbs: This is the traditional method. Carpenters build wooden forms (usually using flexible hardboard or stacked 2x6s) pinned to the ground with steel stakes. The concrete is poured into the wood forms, vibrated, troweled smooth, and the forms are stripped the next day. This is slow but incredibly strong.
2. Extruded (Slip-Form) Curbs: For long commercial runs or landscape borders, a specialized curbing machine is used. A very dry, stiff mix of concrete is shoveled into a hopper on the machine. A large internal auger compresses the concrete and extrudes it out the back in the perfect shape of a curb, leaving a standing, unsupported concrete curb right on the ground. This method is incredibly fast but requires specialized equipment.