Estimate the cost of extra project scope.
Scope creep costs the hours you absorb, priced at your rate. What makes it worse than it looks is the denominator: the unbilled hours still consume capacity, so the fee is spread across more hours and the effective rate falls. Twenty-two extra hours on an eighty-hour project with only 40 percent recoverable turns a 65 quoted rate into roughly 56 — a 14 percent erosion that never appears on an invoice and is the single most common reason fixed-fee work becomes unprofitable.
Absorbed cost
Unbilled = added hours x rate x (1 - billable share)
Rate erosion
Effective rate = (fee + recovered) / (quoted hours + added hours), compared against fee / quoted hours
Sometimes that is right, but small additions are how relationships are maintained. The point of quantifying it is to know when the total has crossed from goodwill into a change order that needs pricing.
With a written change process agreed before work starts. Freelancers who quote a rate for out-of-scope work up front recover far more of it than those who raise it after the hours are spent.