1. How to calculate concrete for a column or post?
Calculating the concrete needed for a column (like a sonotube or deck post) requires finding the volume of a cylinder or prism. Because columns are typically tall and narrow, it is standard practice to measure the Diameter or Width in inches (or cm) and the Height/Depth in feet (or meters). Our calculator handles this exact unit conversion automatically.
2. Formulas used for column volumes
Depending on the shape of your column, we use the following standard geometric formulas to find the exact cubic volume:
- Circular (Sonotube): Volume = π × Radius² × Height
- Square / Rectangular: Volume = Length × Width × Height
- Hexagonal: Volume = (3√3 / 2) × Side² × Height
- Octagonal: Volume = 2(1 + √2) × Side² × Height
3. How many premixed bags do I need?
Once the total volume is calculated, it is divided by the yield of a standard premixed concrete bag. Common yields are:
- 50 lb Bag: Yields ~0.375 Cubic Feet
- 60 lb Bag: Yields ~0.45 Cubic Feet
- 80 lb Bag: Yields ~0.60 Cubic Feet
- 94 lb Bag (Portland): Yields exactly 1.0 Cubic Foot
- 20 kg Bag (Metric): Yields ~0.0094 Cubic Meters
- 40 kg Bag (Metric): Yields ~0.0188 Cubic Meters
4. Why add a Waste Margin?
For columns and posts, especially those poured into holes in the ground, it is crucial to include a waste margin. Soil absorption, slightly uneven digging (over-excavation), and spillage mean you will almost always use more concrete than the perfect mathematical cylinder suggests. We recommend a 10% to 15% margin for post holes.
5. Estimating Rebar for Columns
If you toggle the reinforcement estimator, the calculator approximates the linear footage of rebar needed. It assumes standard vertical bars (4 for square/circle, 6 for hex, 8 for octagon) running the height of the column, plus horizontal ties (stirrups) spaced every 12 inches wrapped around the perimeter.