Rough Order of Magnitude (ROM)
A Rough Order of Magnitude, or ROM, is a very early, rough guess at how much a project will cost (or how long it will take) before you know many details. It is not meant to be precise — it is meant to give sponsors and decision-makers a ballpark figure so they can decide whether an idea is even worth pursuing. Because so little is known this early, the estimate is expressed as a wide range rather than a single exact number.
The typical ROM range is from −25% to +75% of your initial estimate. That means the real cost could come in as much as 25% below your guess, or as much as 75% above it. To use it, you take your rough estimate and calculate those two boundaries: the low end and the high end.
How do you read it? The wide, lopsided range is a signal that says "don't treat this number as a promise." A project manager uses a ROM during the very beginning of a project — the initiation phase — when someone asks "roughly, what are we talking about?" As the project moves forward and more is known, this rough range is later replaced by tighter, more accurate estimates.
Think of it like guessing how much a home renovation will cost after just walking through the front door for two minutes. You can say "probably somewhere between forty and eighty thousand dollars," but you'd never sign a fixed contract on that glance — you need to inspect the wiring, plumbing, and walls first.
Suppose a sponsor asks what a new mobile app might cost, and your rough estimate is $100,000. Using the ROM range: the low end is $100,000 minus 25% ($25,000), which is $75,000. The high end is $100,000 plus 75% ($75,000), which is $175,000. So you would tell the sponsor, "It'll likely land somewhere between $75,000 and $175,000." That wide range honestly reflects how little you know at this stage.
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