Progress billing: percent complete, carry-forward and retention

Updated: 2026-08-13 · ISKRIT

Every billing period asks the same four questions of every line: what was done before, what was done this period, what that leaves, and how much of it is being held. Getting them consistent across a two-year job is the whole discipline.

The line arithmetic

For each item: completed to date equals previous plus this period, plus any materials delivered and stored on site. Percent complete is completed over scheduled. Balance to finish is scheduled minus completed. Retention is a percentage of completed, held back and carried forward.

The carry-forward is what makes the record cumulative rather than a stack of independent invoices. Each period starts from the last one, so a correction in period seven has to be made as a correction, not by quietly restating period six.

Why retention is per line

Applying the retention rate to the application total instead of to each line gives the same answer only while the rate is uniform and nothing is released. As soon as one section reaches practical completion and its retention is released while the rest continues, the total-based calculation cannot express the state of the job. Per-line retention also survives a rate that differs by work type, which is common where plant and materials are treated separately from labour.

Stored materials

The third row is where the error lives. Material billed as stored in one period and again as completed in the next, without the stored figure coming down, is an overclaim that survives until someone reconciles the whole job.

The release nobody bills

Retention held at five or ten percent across a long contract is real money, and it is released in two stages: part at practical completion and the balance after the defects period. The second stage falls due months after the site team has moved on, and it is routinely forgotten. A running total of held-to-date, visible on every application, is what makes the final claim obvious rather than something to remember.

The number that reconciles everything

Contract sum, less completed to date, equals balance to finish. If that identity fails, either a variation was billed outside the schedule or a correction was made by restatement. Checking it every period localises the error to one month instead of leaving it to be found at closeout.

The cumulative work record tool carries each line forward across periods with its retention, and reports held-to-date so the release is billed rather than remembered.