Estimating & Scheduling
AACE estimate classes explained: Class 5 to Class 1
November 1, 2026 · 6 min read
Every project goes through the same awkward moment: someone asks for "a number", someone gives one, and six months later that number is treated as a promise. AACE International's estimate classification system exists to stop that. It labels every estimate with how much is actually known, so nobody mistakes a napkin budget for a bid.
What the classes measure
The classes run from Class 5, the roughest, to Class 1, the most detailed. The main thing that decides the class is not how much effort the estimator spent. It is how far the project definition has progressed: drawings, specs, equipment lists, site information and execution plan. More definition means a tighter range and a higher class.
The figures below come from AACE Recommended Practice 18R-97, written for the process industries. Practice 56R-08 covers building and general construction with the same five-class structure.
| Class | Project definition | Typical use | Method | Typical accuracy (low / high) |
|---|---|---|---|---|
| Class 5 | 0% to 2% | Concept screening | Capacity factored, parametric models, judgment or analogy | -20% to -50% / +30% to +100% |
| Class 4 | 1% to 15% | Study or feasibility | Equipment factored or parametric models | -15% to -30% / +20% to +50% |
| Class 3 | 10% to 40% | Budget authorization or control | Semi-detailed unit costs with assembly-level line items | -10% to -20% / +10% to +30% |
| Class 2 | 30% to 75% | Control, or bid and tender | Detailed unit costs with forced detailed takeoff | -5% to -15% / +5% to +20% |
| Class 1 | 65% to 100% | Check estimate, or bid and tender | Detailed unit costs with detailed takeoff | -3% to -10% / +3% to +15% |
The accuracy ranges are wide on purpose. They depend on how complex the work is, how good the reference data is, and how much contingency is carried. A Class 5 number for a first-of-its-kind unit can land well outside the typical range. Treat the table as a guide, not a guarantee.
Which class do you need?
- Is this even worth pursuing? A Class 5 or Class 4 estimate. Fast, cheap, and good enough to kill a bad idea early.
- Asking the board or owner for money? Class 3 is the usual minimum for budget authorization. It needs real engineering behind it.
- Submitting a lump-sum bid? Class 2 or Class 1, built from a detailed takeoff, real labour hours and current pricing.
- Checking someone else's number? A check estimate, often Class 1 or 2 on the scope that matters most.
The mistakes we see most
- No class on the estimate at all. A number with no label gets remembered as the most accurate version of itself.
- Single-point numbers. Every estimate should carry a range. Presenting a Class 4 as one figure invites trouble.
- Contingency used as a slush fund. Contingency covers known unknowns within the defined scope. It is not for scope changes.
- Skipping the class progression. Jumping from a concept number straight to a bid means no one tested the assumptions in between.
Getting help
Our Estimating & Scheduling team builds estimates at every class, from early budgets to bid-ready numbers, and runs independent check estimates on numbers you already have. Every estimate we deliver is labelled with its class and range. Book a free consult, or try us first with a free comparison takeoff.