A rough estimate (also called a Rough Order of Magnitude or ROM) is an early-stage price you generate based on limited project information — typically with an accuracy range of -25% to +75% per PMI standards. It’s used for budgeting, feasibility checks, and go/no-go decisions before the design is complete.
Project owners ask for rough estimates constantly during the pre-construction phase. Contractors ask for them too, usually when they’re trying to gauge a change order before pricing it out properly. GCs typically want a definitive estimate or a full proposal when they’re comparing subs — not a ROM — so don’t confuse the two. The trick is knowing how to give a ROM without locking yourself into a number you’ll regret.
What Is a Rough Estimate?
A rough estimate or rough order of magnitude (ROM) is a price you generate based on a general scope of work provided by a project owner. You create a list of assumptions to calculate how much it will cost to complete the project. The rough estimate is best suited for budgeting purposes, with the detailed estimate following once the project documents are complete.
It’s not only project owners who ask for these — contractors will ask you for one too, usually when they’re trying to budget a change order. (GCs comparing subs will typically want a full proposal, not a ROM.) If you ever get asked, don’t feel obligated to give a number immediately. Even a “rough” estimate takes some time to think through.
One scenario to watch for: if your detailed estimate later comes in substantially higher than the rough number you gave, you could be in trouble — not because the customer holds you to the verbal quote, but because they may not have the budget set aside to complete the job. The project can stall while they figure out funding, or you end up squeezed on margin to make it happen. Always say “let me get back to you” and put it in writing once you’ve had time to think it through.
A ROM should still take some time to put together. You need to understand the full scope at least at a high level, and you need to figure out a rough cost for each element that goes into the price. Don’t confuse “rough” with “fast” — the rough refers to the precision of the inputs, not the effort.
How Accurate Is a Rough Estimate?
A rough estimate is deliberately broad. The most widely cited accuracy range comes from the Project Management Institute (PMI), which puts ROM accuracy at -25% to +75%. AACE International, which classifies cost estimates for engineering projects, defines a “Class 5” ROM as accurate within -50% to +100% depending on how much project scope is defined.
In practice: if you give a rough estimate of $100,000, the actual project cost might land anywhere between $75,000 and $175,000 (per PMI), or as wide as $50,000 to $200,000 (per AACE).
The textbook hierarchy lists multiple tiers between a rough estimate and a final price — ROM, preliminary, budget, definitive — but on real projects, those middle tiers blur together. In my experience, you go from a ROM at the early planning stage straight to a detailed estimate once you have plans and specs — or once you’ve had enough time to dig into the scope and work out everything the job needs. The detailed estimate is what you submit when bidding the job, and it should be within roughly -5% to +10% of the actual cost.
When you provide a rough estimate, always communicate the range to the customer. Don’t let them treat your ROM as a fixed price.
Benefits of a Rough Estimate
Rough estimates serve a few key purposes on a project:
1. Feasibility checks. A ROM lets the customer decide whether the project is even worth pursuing. If the rough number comes back well above their budget, they might put the project on hold while they come up with funds, scale down the scope, or find other ways to get it underway. There are different approaches — but they need real numbers to choose between them.
2. Time savings for the contractor. I send rough estimates via email rather than spending the time to build out a full proposal. State the work covered, state the price, send. Make sure the scope is agreed upon by both parties before you do this — and put it in writing. You’ll want documentation to fall back on if there’s any dispute later about what your number was supposed to cover.
3. Change order budgeting. GCs often ask for a rough number when they’re trying to gauge how a change order will land with the owner. The GC can take that number back to the owner and prepare them for what the cost might look like. From there, it can kick off negotiations to land on the final price — but you’re already under contract with that GC, so they’re not shopping the work around. Bringing in another sub for a small scope change isn’t worth their time.
One thing to remember: you don’t have to perform every change that comes out of a directive or a revised drawing set. If the GC keeps pushing your number down to where you’d be losing money, you can tell them take it or leave it. Don’t eat margin just to keep the GC or owner happy — unless you’re deliberately building the relationship and it’s worth it on that job.
How to Create a Rough Estimate
The first and most important step is agreeing on the scope of work with the customer. You and the customer must be on the same page before you calculate anything. Get the scope in writing — even an email confirming what you discussed is enough.
Once the scope is clear, gather any details that affect price. The more information you have, the more accurate the estimate. A preliminary construction drawing set is ideal, but some owners don’t have the budget for a design team and you’ll be making more assumptions.
One important note: the scope write-up isn’t usually heavily detailed. As the contractor, you still need to consider all the logistical pieces that won’t be spelled out. For HVAC, that means trucking equipment, storage, freight, rigging, and any other small items that don’t make it into a scope statement but are real costs you’ll incur. This is why I keep coming back to visualizing how you would install the job. Walking through the install in your head helps you spot the components and costs that aren’t on paper.
From there:
1. Equipment and material costs. Identify what equipment and material the project needs. Reach out to your vendors and suppliers for pricing. The numbers they give you go into your rough estimate.
2. Labor hours. Without full plans this is the hardest piece to estimate. If you have a construction superintendent you trust, work with them on the hours. If not, use your best judgment — and stay conservative. Going conservative on labor hours protects you when it comes time to put together the detailed estimate later. Multiply hours by your labor rate to get labor costs.
3. Overhead and non-field labor. Your PM time, project engineer time, drafters, warehouse, project admin — all of that needs to be in the number. These are overhead costs, and if you don’t account for them, they eat your profit margin.
4. Logistics and operational costs. Trucking, storage, freight, rigging, equipment delivery coordination — these don’t show up in the scope but they show up in your final cost. Include them.
5. Union considerations. If you’re using union labor, you have to cover wages and fringes at minimum. Whether you raise the rate above that for your estimate is your call. The goal is to make money on the project.
Here’s a graphic that summarizes how to create a rough estimate:
Factors to Consider When Creating a Rough Estimate
A few things shape how you approach a rough estimate:
- Purpose of the estimate. Owner budgeting calls for a conservative number. A customer shopping for prices across contractors might call for something more competitive.
- Time available. More time means you can get real pricing from vendors and subs. Less time means more guessing — and the more guessing, the more conservative you should be.
- Information available. If you have preliminary drawings, your estimate gets tighter. If you’re working off a verbal description, your accuracy range will be wider.
How to Use a Rough Estimate
A rough estimate gives the project owner a real number to work with at the earliest stage. They can decide whether to push forward into design or pause until funding catches up. For some projects, the ROM determines whether the project moves at all.
The other common use is change orders. When something on the project shifts — a design revision, an unforeseen condition, a scope addition — the GC or owner needs a rough number quickly to gauge the impact. A fast, reasonable ROM keeps the project moving instead of stalling while everyone waits for a formal estimate. The ROM also gives the owner a clean go/no-go on the change itself — sometimes the added cost isn’t worth it to them and they drop the scope entirely rather than pay for it.
Limitations of Rough Estimates
Because contractors tend to stay conservative on ROMs, owners sometimes get sticker shock from the rough number — only to find the detailed estimate later comes in lower. That’s not always a problem (clients are happy about savings), but it can scare an owner away from a project that actually fits their budget. Always frame the ROM with the accuracy range so they don’t take it as final.
ROMs also take time to put together, even when they look casual. If a project is too large for your company, contains work outside your scope, or involves too much risk, it may not be worth providing a ROM at all. Use your judgment — sometimes the answer is to politely decline rather than spend hours pricing something you won’t bid on.
Frequently Asked Questions
How accurate is a ROM estimate?
A ROM is intentionally broad. The PMI puts the accuracy range at -25% to +75%. AACE International defines its “Class 5” estimate (similar to a ROM) at -50% to +100%. In practice, this means a $100,000 ROM could land anywhere from $75,000 to $175,000 — or wider — depending on which standard you reference. Always communicate the range when you give a customer a ROM, so they don’t treat it as a fixed price.
What’s the difference between a ROM and a detailed estimate?
A ROM is based on a general scope and limited information, with an accuracy range of -25% to +75%. A detailed estimate comes after you have full plans and specs, vendor quotes, and a clear scope — and it should be within about -5% to +10% of the actual project cost. The ROM is for early-stage budgeting and feasibility. The detailed estimate is what you submit when bidding a job.
When should I use a rough estimate?
Use a ROM when the customer needs a budget number early — before the design is complete, before final scope is locked in, or before they commit funding. Common scenarios: initial project feasibility, change order budgeting during construction, and quick comparisons across contractors when a formal bid isn’t worth the time. Always confirm the scope in writing before sending the number.
Is a rough estimate legally binding?
A rough estimate isn’t a fixed price, but that doesn’t mean you can ignore the number you gave. Customers may have set their budget based on your ROM, and if your detailed estimate comes back substantially higher, the project can stall while they sort out funding. Always put the ROM in writing along with the assumptions and scope it’s based on. That documentation protects you if there’s a dispute about what your number was supposed to cover.
Conclusion
A rough estimate is an early-stage cost projection used for budgeting, feasibility, and quick pricing on change orders. It’s not a fixed price, but the number you provide carries weight — customers plan around it. Always lock in the scope in writing before sending one, and communicate the accuracy range so it’s treated as what it is: a rough number, not a final commitment.
Take time to put a ROM together properly. The “rough” refers to the precision of your inputs, not the effort you put into thinking it through.
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