For round post holes, estimate concrete volume as a cylinder: π × radius² × depth, multiplied by the number of holes. Convert diameter and depth to the same unit first. Then divide by the labeled yield of the mix you plan to use to estimate bag count.
Measure the holes you plan to fill
Measure the inside diameter and depth of each hole. If hole sizes differ, calculate each group separately and add the volumes. The hole’s measured dimensions do not by themselves determine how deep or wide a post footing should be; soil, loads, frost conditions and local requirements may affect that design.
To work in feet, convert inches by dividing by 12. A 12-inch diameter hole has a 0.5-foot radius. A 30-inch depth is 2.5 feet. For four identical holes, the geometric volume is π × 0.5² × 2.5 × 4, or about 7.85 cubic feet (about 0.29 cubic yards).
Turn volume into a bag estimate
Bag weight is not the same as finished concrete volume. Read the selected mix’s stated yield and divide the total volume by that number. Illustration only: if a hypothetical bag label states a yield of 0.5 cubic feet, 7.85 ÷ 0.5 = 15.7, so the base estimate rounds up to 16 bags before any allowance.
Yield differs among mixes and package sizes. The example yield above is only to show the arithmetic; replace it with the figure on your product label. Any extra allowance should reflect your measured holes, over-excavation and site conditions, not a universal percentage.
Check the result before buying
The Concrete Calculator includes a cylindrical post-hole option and lets you enter hole count, diameter, depth and bag yield. For structural details, mix choice, water, reinforcement, placement and curing, follow the product instructions and project requirements. A volume estimate is not a footing design.
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