It's a common assumption that the client billing the most is also the most valuable one to keep. Revenue and profitability aren't the same measurement, though, and a client generating the highest invoice totals can simultaneously be operating at the thinnest — or even negative — effective margin, once the actual time and cost that went into serving them is counted.
Why effective hourly rate is the number that reveals this
A fixed-fee or retainer client can look identical to another client on the invoice while requiring dramatically more hours behind the scenes. Calculating effective hourly rate — total fees divided by actual hours spent — per client, rather than looking at revenue alone, is what surfaces the gap between a client that pays well and a client that pays well per hour actually worked.
Where scope creep quietly does the damage
The most common way a strong-margin client relationship turns thin over time isn't a single bad decision — it's gradual scope creep: a few extra small requests here, an "can you also just" there, none of which individually seemed worth a difficult scope conversation, but which collectively add hours that were never re-priced into the fee.
Why this is worth checking quarterly, not just at contract renewal
Waiting until a contract renewal to evaluate whether a client relationship is still profitable means months or years of eroding margin before the number is actually reviewed. Checking effective hourly rate regularly catches the drift while it's still a small, easy conversation to have — rather than after it's become a large one.