Everything goes right.
Scope creep, revisions, a difficult client.
Calls, emails, project management you do not bill for.
An estimate. PERT weighting assumes your three figures are honest. The most common error is a best case that is actually a hope and a worst case that is not really the worst.
Calculated entirely in your browser. Nothing you enter is sent anywhere.
How to use it
Give three honest numbers
Best case, most likely, and worst case hours. The worst case is the one people fudge.
Add your overhead
Calls, emails and project management you do not bill for separately.
Quote nearer the ninety percent figure
The expected figure is right on average and leaves you short half the time.
How the calculation works
The weighted estimate uses PERT: best case plus four times the likely case plus worst case, divided by six. It leans heavily on your most likely figure while still letting the extremes pull it.
The uncertainty is the range divided by six, which approximates one standard deviation. The ninety percent figure adds 1.28 of those, which is the point where you would come in under budget nine times out of ten.
For a fixed price, quote nearer the ninety percent figure. Quoting the expected figure means you lose money on half of all projects, and the ones you lose on are the ones that were already painful.
What this calculator does not do
PERT weighting assumes your three figures are honest. The most common error is a best case that is really a hope and a worst case that is not really the worst.
It does not model dependencies. If the project waits on a client decision or a third party, elapsed time will exceed the hours by a long way.
It cannot see scope that has not been defined. If your range is very wide, the answer is usually to quote a paid discovery phase rather than guess harder.
Questions
What is PERT?
A weighted average that gives the most likely figure four times the weight of the extremes. It is standard practice in project estimating.
Why is my worst case so important?
Because it sets the uncertainty, which sets the gap between the expected quote and the safe quote. A dishonest worst case produces a confident wrong answer.
Should I show the client the range?
Often yes. A range with a reason behind it reads as competence, where a single number that later moves reads as a problem.
What if the range is huge?
Then the scope is not defined. Quote a short paid discovery phase, then quote the build properly once you know what it is.
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