Definitive Estimate
A "definitive estimate" is a very accurate prediction of how much a project (or part of a project) will cost or how long it will take. The special part is the range attached to it: −5% to +10%. This range means the real final number is expected to land somewhere between 5% less than your estimate and 10% more than your estimate. No estimate is ever perfectly exact, so professionals attach a range to show how confident they are.
You create a definitive estimate late in planning, once you understand the work in detail. Because you know so much by then, the range is narrow — the final result should be close to your number. To read it: a narrow range (like this one) means high confidence and low uncertainty. Early in a project you would instead use a "rough order of magnitude" estimate with a much wider range (often −25% to +75%), because you know very little yet.
A project manager uses a definitive estimate when they need to commit to a budget or schedule and stakeholders expect reliable numbers. It's the estimate you'd stand behind when finalizing a contract or setting the official project baseline.
Think of it like checking the weather. A week out, the forecast is vague ("maybe rain, maybe sun"). But one hour before you leave the house, you can look out the window — your prediction is now tight and reliable. A definitive estimate is that last-minute, look-out-the-window forecast for cost and time.
Imagine your team estimates that building a new website will cost $100,000, and you're at a late stage where the work is well understood, so you use the definitive range of −5% to +10%. The low end is $100,000 − 5% = $100,000 − $5,000 = $95,000. The high end is $100,000 + 10% = $100,000 + $10,000 = $110,000. So you'd tell stakeholders the project should cost between $95,000 and $110,000. This narrow range signals you're confident the true cost won't wander far from $100,000.
Every PMP formula explained free — plus worked examples and practice in PMP Math, and full timed mocks in the simulator.