How this instrument works
A concrete column poured into a round form — a structural porch post, a deck footing, a fence or pergola support — is a cylinder, and its volume follows the standard cylinder-volume formula: pi times the radius squared times the height. This instrument takes the column's diameter in inches, the way round forms are almost always specified, and its height in feet, converts the diameter to a radius in feet, and returns the volume in cubic yards, the unit ready-mix concrete is ordered and priced by.
The conversion has two steps folded into one formula: diameter in inches divides by 24, not 12, because that single division both halves the diameter into a radius and converts inches to feet in the same step (diameter/2/12 = diameter/24). The resulting radius, in feet, gets squared, multiplied by pi and by the column's height, and the whole cubic-foot volume divides by 27, the number of cubic feet in a cubic yard, to land on the ordering unit a concrete supplier actually uses.
Round column forms come in standard diameters — commonly 8, 10, 12, 16, and 24 inches — sold as rigid fiber tubes that get stripped away after the pour cures. Getting the volume right before ordering matters because ready-mix concrete is typically sold in minimum-load increments and short-load surcharges apply below a truck's minimum, so an accurate cubic-yard figure for one or several columns helps avoid both an expensive short load and a costly leftover.
This instrument computes the volume of a single round column. A construction project with multiple identical columns needs the per-column figure multiplied by the column count, and any columns of different diameters or heights calculated separately and then summed.
- Enter the round form's diameter into Column diameter (in) — the size printed on the fiber tube form, such as 12 for a 12-inch tube.
- Enter the column's height into Column height (ft) — the full height of concrete the form will hold.
- Read Concrete needed (cubic yards) beneath the inputs — the volume figure to order from a ready-mix supplier.
- Multiply the result by the number of identical columns being poured, since this instrument computes the volume of one column at a time.
Worked example — a 12 in diameter, 8 ft tall column
Enter 12 into Column diameter (in) and 8 into Column height (ft). The instrument converts the radius to 12/24 = 0.5 ft, computes pi x 0.5² x 8 = pi x 0.25 x 8 ≈ 6.2832 cubic feet, then divides by 27: 6.2832 / 27 ≈ 0.2327. Concrete needed (cubic yards) reads 0.2327 yd³.
A quarter of a cubic yard is well under most ready-mix trucks' minimum load, so a single 12 in x 8 ft column like this one is a case where hand-mixing bagged concrete, or grouping several columns into one order, is typically more practical than calling for a dedicated truck delivery.
Questions
Why does the formula divide diameter by 24, not 12?
Because two conversions happen in one step: dividing by 2 turns the diameter into a radius, and dividing by 12 converts inches to feet, and 2 x 12 = 24 combines both into a single division. Diameter divided by 24 gives the radius directly in feet, ready to square and multiply by pi and the height without any separate conversion step.
What column diameters do round forms typically come in?
Common rigid fiber tube forms are sold in standard diameters including 8, 10, 12, 16, and 24 inches, though larger and smaller sizes exist for specific structural needs. Enter whatever diameter is printed on the form or specified in the structural plans — the formula works identically at any diameter.
Do I need to order exactly the calculated volume?
No — order a modest overage beyond the calculated figure to cover minor form irregularities, spillage, and the fact that ready-mix concrete is typically sold in set load increments rather than arbitrary fractions of a yard. Many contractors add 5 to 10 percent to a calculated volume as a standard buffer before placing an order.
How do I calculate concrete for several columns at once?
Run this calculation once per distinct diameter-and-height combination, then multiply each result by how many identical columns use that size, and add the totals together. Columns that share the same diameter and height can simply be multiplied by their count; columns of different sizes need separate calculations before summing.
Is this the same formula used for a concrete cylinder sample?
The underlying cylinder-volume geometry is identical, yes, but the two instruments serve different purposes. This one sizes a full-scale structural column, typically several feet tall and ordered in cubic yards from a ready-mix supplier; this site's concrete cylinder instrument sizes a small ASTM-standard compression-test sample, typically inches tall and reported in cubic feet, used to check a concrete mix's strength rather than to build anything load-bearing.
What if my column form isn't perfectly round?
This formula assumes a true circular cross-section, which standard fiber tube forms provide. A square, rectangular, or otherwise non-circular column form needs a different volume formula based on its actual cross-sectional shape — entering an average or estimated 'diameter' for a non-round form will not give an accurate volume.