First, it's called "rough order of magnitude estimate" (there's the word rough at the beginning). In any case, a rough order of magnitude estimate (ROM Estimate) is a ballpark time/cost estimate that is used at the beginning of the project by the project manager to satisfy the following stakeholders' questions:
- How much will it cost?
- How much time will it take?
A ROM Estimate has a range of -50% to +50%. For example, if a ROM Estimate states that the project will cost $100,000, then it might cost anything between $50,000 to $150,000.