Measure the drilled hole
Use actual hole diameter and depth rather than the nominal auger size.
Separate holes that change diameter or depth instead of averaging them.
Subtract post displacement carefully
A post reduces concrete volume only through the portion actually inside the concrete.
Keep the assumption visible so it can be replaced when project or product data is available.
Convert volume to bags
Divide required cubic feet by the manufacturer-published yield for the selected mix.
Round up to whole bags only after all matching holes and the allowance are included.
Keep structural requirements separate
A quantity estimate does not establish required footing diameter, embedment, frost depth, uplift resistance, or post spacing.
Use project documents and applicable requirements for those decisions.
Confirm the selected mix
Fast-setting and conventional concrete products can have different package sizes, yields, and placement instructions.
Check the current product data before purchase and placement.