The formula
Effective hourly rate is what a project paid per hour of your actual time, not the rate printed on the invoice. Take what you were paid, subtract anything you spent to deliver it, and divide by the hours it really took.
A worked example
A €4,000 fixed fee that took 100 hours paid €40 an hour, not the €80 you had in mind when you wrote the proposal. That is the entire calculation. It does not adjust for how good the work was, how happy the client is, or how long ago you agreed the price. It only counts what actually cleared your hours.
What this number is actually for
Fixed fees hide the rate. A day rate makes the math obvious every time you invoice. A flat project fee makes it invisible until someone divides it out, which is exactly why studios that quote fixed fees are the ones least likely to know their real margin.
Scope creep is a rate problem before it is anything else. Two extra revision rounds do not change the fee. They do change the hours, and the hours are the number on the bottom of the fraction.
It is the fair way to compare clients. Revenue per client tells you who pays the most. Effective rate tells you who is worth the hours, which is a different ranking more often than people expect.
It only means what your time log means. Hours you never tracked do not count against the fee, so an undertracked project flatters its own rate. Log close to what actually happened, or the number describes your logging habits instead of your business.