How the two models fail differently, and which one fits the work in front of you.
Most firms choose a billing model out of habit, or because the client asked for one. It is worth choosing deliberately, because the two models do not just price work differently. They fail differently, and the failure mode determines what you need to watch.
You bill for hours worked at agreed rates. The client carries scope risk. If the work takes longer, they pay more.
This sounds like the safe option and mostly is, but it has a specific and expensive failure mode: allocated people who are not billing.
You win the deal, allocate four engineers starting Monday, and then Monday arrives and the client has not issued VPN credentials. Or the kickoff slips a week. Or two of the four are waiting on repository access. Those people are on your payroll and are not on the client's invoice. Nobody notices because everyone is technically assigned to a paying project.
This leak is quiet because your resource plan looks fully utilized. The gap only appears when you compare allocated hours against billed hours, which most firms do monthly, if at all.
Time and materials is the right choice when scope is genuinely unclear, when the client wants to steer as they learn, or for ongoing work with no defined end.
You agree a price for a defined outcome. You carry scope risk. If the work takes longer, you absorb it.
The failure mode is the obvious one, but it is worth being precise about where it actually happens. Fixed-price projects rarely die from one catastrophic underestimate. They die from a series of small unbilled accommodations: a quick extra screen, a revision round nobody counted, a data migration that turned out to be dirtier than the sample suggested.
Each is too small to justify a change order conversation. Together they eat the margin.
The number that matters is burn against completion. If you have spent 60 percent of the budget and delivered 40 percent of the scope, you are in trouble, and you are in trouble now, not at delivery. Firms that catch fixed-price overruns catch them because someone is watching that ratio weekly.
Fixed price is the right choice when scope is genuinely well understood, usually because you have built something close to it before, and when the client needs budget certainty more than flexibility.
The common middle path. You bill hourly, but agree a ceiling. The client gets a worst-case number, you get paid for actual work below it.
It is a reasonable compromise and it inherits both failure modes in diluted form. You still need to watch unbilled allocated time, and you still need to watch burn against the cap.
Ask one question: can you describe what done looks like precisely enough to defend it in an argument eight months from now?
If yes, fixed price is available to you, and it is usually the more profitable option because you keep the upside of your own efficiency.
If no, price it time and materials. Fixed-price work on genuinely unclear scope is not a pricing decision. It is a bet, and the house is the client.
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