What is a rough order of magnitude estimate?

A rough order of magnitude, or ROM, estimate is a preliminary cost or duration estimate made early in the project lifecycle when limited information is available. ROM estimates typically have an accuracy range of negative 25 to positive 75 percent, meaning the actual cost could be 25 percent less or 75 percent more than estimated. As the project progresses and more detail emerges, estimates are refined: a budget estimate has a range of negative 10 to positive 25 percent, and a definitive estimate has a range of negative 5 to positive 10 percent.