Definitive estimate

A definitive estimate is what the estimating process has been converging toward: quantities taken off from near-complete drawings and models, pricing from firm quotations and current rates rather than allowances, indirects built from an actual execution plan and schedule, and contingency reduced to reflect how little remains undefined. In class terms it corresponds to the top of the scale — Class 2 and Class 1 — with expected accuracy in the single digits to low teens of percent.

Because of that accuracy, it carries the most weight: final investment decisions, lump-sum contract prices and control baselines are set against it. From this point, cost management stops being estimating and becomes change control — every deviation is measured against the definitive baseline.

Its cost is its cost. A definitive estimate requires most of the engineering to exist, which means most of the engineering money is spent before the most reliable number exists. Projects that demand definitive accuracy at concept stage are not asking for a better estimate; they are asking for the design to be finished early, and no estimating method substitutes for that.