Order-of-magnitude estimate

An order-of-magnitude (or ballpark, or Class 5) estimate is prepared when the project is little more than a capacity and a location. It is built from analogy and scaling: costs of comparable past projects adjusted for size using capacity-factor relationships, parametric models, or factored methods that grow a total cost from early equipment figures. The design definition underneath it is a few percent at most, and the honest accuracy range is correspondingly wide — tens of percent in either direction, and asymmetric, since undiscovered scope is more common than undiscovered savings.

Its legitimate uses are comparison and screening: option A against option B, proceed or drop. The recurring failure is memorial: the first number ever attached to a project is the one everyone remembers, and an order-of-magnitude figure quoted in an early paper quietly becomes the yardstick against which every subsequent, better-founded estimate is judged as "growth". Disciplined organisations publish these estimates only as ranges and with their class stamped on them.