Margin drift: how small estimating errors erode job profit
Margin is not a percentage typed at the end. It erodes in waste, prelims, missed exclusions and rates that were copied forward.
6 min read · Published 2026-10-30
The margin on the summary page and the margin you make are often different. The gap is built from small misses: a waste allowance, a prelim that did not follow the programme, a subcontractor exclusion nobody flagged.
See the margin before the quote leaves
A minimum-margin warning on the way out is more useful than a post-mortem. The estimator should see overheads, contingency and markup as separate lines, not one number that was nudged.
Price variations from the same rates
If a change on site cannot be priced from the original build-up, the margin on that variation is a guess. Keep the rates that built the job.
Construction, civil and landscaping quotes lose margin in the allowances, not only in the headline percentage.