Rough Order of Magnitude

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Rough order of magnitude (ROM) refers to an initial estimate of the cost of a project or parts of a project.

  • It has an expected accuracy of -25% to +75% according to the PMBOK (other sources suggest -50% to +50%). In other words, the actual costs of a project are typically expected to be between 75% and 175% (or 50% to 150%) of the ROM estimate.
  • While the calculation is straightforward, the main challenge is rather how to determine the estimate during the initiation phase of a project.