Rough Order of Magnitude (ROM) Estimate
A rough order of magnitude estimate is a first number produced when almost nothing is defined, used to decide whether a project is worth pursuing at all. It is quoted as a range, because the accuracy it carries is wide by design.
Used at the earliest stage of industrial, process and infrastructure projects, before any package is issued for bid.
How a ROM Estimate Is Built
With little defined, a ROM leans on the cost of things already built rather than on a count of what will be built.
Write Down What Is Actually Known
Capacity, location, rough footprint, the process and the schedule. A ROM is only as honest as the assumptions behind it, so the assumption list is written first and travels with the number.
Price It From Something Already Built
The number comes from a factored method rather than a takeoff: cost per unit of capacity, cost per square foot, or a parametric model built from past projects, adjusted for location and for the years since.
Quote It as a Range, With Its Class
The output is a band, not a figure. Naming the estimate class alongside it tells the reader how much to trust it, and what has to be defined before a tighter number is possible.
Where a ROM Sits Among the Estimate Classes
Cost estimates are classified by how much of the project is defined. A rough order of magnitude is the loosest class, built on almost no definition, and each class after it narrows as the design fills in.
- A ROM is a Class 5 estimate, built on 0% to 2% project definition.
- Its accuracy range runs roughly -30% to +50%, so the high side is wider than the low side.
- AACE guidance is that the range should come from risk analysis on the actual project, not be read off a table.
| Class | Project Definition | Typical Accuracy Range |
|---|---|---|
| Class 5, rough order of magnitude | 0% to 2% | -30% to +50% |
| Class 4 | 1% to 15% | -20% to +30% |
| Class 3 | 10% to 40% | -15% to +20% |
| Class 2 | 30% to 70% | -10% to +15% |
| Class 1 | 70% to 100% | -5% to +10% |
A ROM Is a Decision Number, Not a Bid
A rough order of magnitude answers whether something is worth pursuing, and nothing more. The risk is that it gets quoted once in a meeting, loses its range and its assumptions on the way, and reappears months later as a budget nobody can hit. The fix is to name the class every time the number is used, and to replace it as soon as there is enough definition to count instead of factor.
| Basis | Cost per unit of capacity |
| Project definition | Under 2% |
| Point estimate | $42,000,000 |
| Range, -30% to +50% | $29.4M to $63.0M |
| Estimate class | Class 5 |
| Assumptions logged | 14 |
Once drawings exist, the factored number can be replaced with a counted one. The Takeoff AI reads the drawings, counts and measures the scope, and returns a material and labor takeoff with a priced bill of materials, with every number traced to the sheet it came from.
- A counted scope replacing a factored one, traced to the sheet
- Material and labor takeoff with a priced bill of materials
The Range Is the Number
At Class 5 the spread between the low and high case can be most of the project value again, which means the same ROM can support two opposite decisions. Dropping the range and quoting the midpoint turns a screening tool into a commitment nobody agreed to. Carrying the class and the assumption list alongside the figure is what keeps it honest.
“Quote a rough order of magnitude without its range and you have turned a screening number into a promise.”
Source: Alberta Infrastructure, cost estimate classification
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Rough Order of Magnitude (ROM) Estimate
A first cost figure produced when almost nothing about a project is defined, used to screen whether it is worth pursuing. It is quoted as a range rather than a single number.