The prolongation cost needs building and we don't know what goes in it

The period is there, the account needs the cost of it, and the first draft is nearly always the same: the tender prelims divided by the contract weeks, multiplied by the overrun. It is quick, it is neat, and it is the version assessors discount on sight, because it prices what somebody hoped the job would cost, not what the delay actually cost.

Prolongation is the actual time-related cost of the extended period, taken from the records: the staff who stayed, the establishment that ran on, the plant that sat there, costed over the weeks it ran on. Thickening is different money, the extra supervision and establishment poured into the job during the works because of events, while the job was still running to time. The two get mixed constantly, and mixing them is how a claim double-counts, and how a good claim gets dismissed with a bad one's reputation.

The calculator holds the line between them: running-on costs built over the period, thickening costed against the events that caused it, every line pointing at a record. Built that way, the figure survives the first hostile read, which is the only read that counts.

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