Franklin makes two related points about planning. Beginning a costly project without first estimating what it will require is clearly imprudent. Yet even someone who carefully calculates the expense beforehand may later discover that the estimate was inadequate. The second half gives the saying its irony: planning is essential, but plans cannot perfectly anticipate reality.

The quotation therefore combines prudence with a warning against false confidence. Large undertakings can involve overlooked work, changing circumstances and unexpected expenses, so an initial calculation should not be treated as certain. Franklin does not argue that estimating costs is pointless; rather, he suggests that sensible planning should recognise its own limitations. The broader lesson is to investigate costs before committing resources while also allowing a margin for uncertainty, because careful preparation reduces risk without eliminating it.