Time and Materials vs Fixed Price: How to Choose the Right Model for Each Engagement
Whoever understands the work best should carry its risk, and that principle settles the time and materials vs fixed price question for most engagements: fixed price suits work you have delivered many times and can scope tightly, while time and materials suits work where the client controls the unknowns. Between those two poles sit hybrid structures, such as a fixed-price discovery followed by phased pricing, which is where mature firms quote most of their work.
The rest of this article explains where each model places the risk, when each one protects your margin, and how the same project looks when priced both ways, so that you can choose a model for your next proposal and explain it to the client without sounding defensive.
Where each model puts the risk
Under time and materials, the client pays for the days you work at an agreed rate, which means that if the work takes longer than expected the client pays more, and your margin per day stays broadly where you set it. The risk you carry is commercial rather than financial, because a client who watches the invoices climb will start to question the value, and that conversation is harder to recover from than most founders expect.
Under fixed price, the client pays an agreed sum for an agreed output, which means that every overrun comes straight out of your margin, while every efficiency you find is yours to keep. The client buys certainty, and you are paid, in effect, for absorbing their uncertainty, which is a perfectly good trade as long as you have priced that uncertainty rather than hoped it away.
Rule of thumb: if you cannot write down the outputs, the exclusions and the assumptions on a single page, you do not yet know enough to fix the price.
When time and materials protects your margin
Time and materials is the safer model when the scope depends on things you cannot see or control, which in technology services usually means the state of a client's existing systems, the speed of their decision-making, or requirements that will only become clear once work begins. It suits discovery work, legacy integrations, staff augmentation and any engagement where the client wants to change direction as they learn.
The model does not protect you from everything, however, because it rewards hours rather than outcomes, and a team that is comfortable billing time can drift into slow delivery that the client eventually notices. It also caps your upside, since the most efficient team in the market earns exactly the same per day as the least efficient one, which matters if your pricing is part of a wider push to improve delivery margins.
When fixed price protects your margin
Fixed price is the stronger model when the work is repeatable, the inputs are known and your team has delivered something close to it several times, because in those conditions your estimates are grounded in history rather than optimism and the efficiency you have built becomes profit rather than a discount to the client. Productised services, well-defined builds and audits with a clear method tend to fit here.
Where fixed price goes badly wrong is almost always in the same place, which we call the confident guess: a price built from a sales conversation rather than a delivery estimate, with no contingency, loose assumptions and no named mechanism for handling change. The job that has just burned your margin was probably not unlucky so much as priced before anyone who would deliver it had looked at it properly, and the defences against that are covered in our piece on stopping scope creep without souring the relationship.
A worked example: the same project priced both ways
Consider a hypothetical integration project that your delivery lead estimates at 60 days, where you charge £900 a day and your fully loaded delivery cost is £450 a day. The figures are illustrative, but the arithmetic is what matters.
| Scenario | Time and materials | Fixed price (£64,800) |
|---|---|---|
| Delivered in 60 days (cost £27,000) | Revenue £54,000, margin 50% | Revenue £64,800, margin 58% |
| Delivered in 80 days (cost £36,000) | Revenue £72,000, margin 50% | Revenue £64,800, margin 44% |
| Delivered in 95 days (cost £42,750) | Revenue £85,500, margin 50% | Revenue £64,800, margin 34% |
The fixed price here includes a 20% contingency, so it is set at the equivalent of 72 days. If your estimate holds, fixed price earns you more, which is the reward for carrying the risk. If the project runs a third over, time and materials still protects your margin, but the client has paid £18,000 more than the figure they had in their head, and you should expect the reference call you dread. At 95 days the fixed price job is still profitable, though only just, which shows why contingency is not padding but the price of certainty.
The useful question for your proposal is therefore not which column looks better but how confident you are in the 60 days, and whether that confidence comes from delivery history or from wanting to win the deal.
Hybrid structures that mature firms use
Most established services firms do not choose one model for the whole business but match the model to each phase of the work, which lets them fix what they understand and meter what they do not. The common structures are these:
- Fixed-price discovery, then a priced build. A short, fixed-fee phase produces a scope, a plan and an estimate, after which the build is quoted on far better information.
- Capped time and materials. The client pays for time used up to an agreed ceiling, which gives them a budget they can defend while you keep protection below the cap, although the cap should come with assumptions that reopen it if they fail.
- Time and materials with a variance threshold. You give an estimate and agree that any forecast overrun beyond a set percentage triggers a conversation before the time is spent, not after.
- Fixed price per phase or milestone. Large projects are broken into fixed-price stages, so that an estimating error costs you one stage rather than the whole engagement.
- Fixed fee with managed change. A fixed price for a defined scope, paired with a pre-agreed day rate for change requests, so that the client knows in advance what additions will cost.
The pricing model is not a sales decision but a statement about who understands the risk.
How to choose for your next proposal
Before you pick a model, work through a short set of questions with the person who will actually deliver the work rather than the person who sold it:
- Have we delivered something close to this at least three times, and do we have the actual days recorded?
- Which unknowns sit with the client, such as access, data quality or decision speed, and which sit with us?
- Can we write the outputs, exclusions and assumptions on one page?
- What happens to this quarter if the job runs a third over, and could the business absorb it?
- Does the client need budget certainty for their own board, or flexibility to change direction?
If most answers point to known work and known inputs, fix the price with contingency. If the unknowns sit with the client, use time and materials or a capped variant. If the answers are mixed, which they usually are, split the work into phases and price each one on its own evidence.
Explaining it to the client: tell them plainly that you fix what you can predict and meter what depends on them, because that way neither side pays for the other's uncertainty. Most sensible buyers find that easier to accept than a round number with no reasoning behind it.
Where Vitori fits
Pricing models are usually a symptom rather than the problem, since a firm that keeps losing money on fixed-price work typically has gaps in estimating, scoping discipline and the handover between sales and delivery, and changing the contract template will not close them on its own. You can fix much of this internally, whether with Vitori or anyone else, if someone senior owns it and has the time.
Where we help is in the Delivery and Growth pillars of the Operational Scale Framework, diagnosing why estimates drift and, through the Operator model, embedding the estimating, pricing and change-control practices until they hold. It is not the right answer to every commercial problem, but where it is, the aim is the same as always: a business that runs, and scales, without the founder in every decision.