Time and materials or fixed price: how to choose without regret

The choice between time and materials and fixed price is usually presented as a preference. It is not. It is a question of who carries the overrun risk, and the answer has to match how you will actually steer the assignment.

What each model transfers

Under a fixed price, the provider commits to a scope, a price and a deadline, and carries an obligation of result. If the work takes twice as long as planned, their margin absorbs the gap, not your budget.

Under time and materials, you buy days of skill. The provider carries a best-efforts obligation. If the work takes twice as long, you pay for twice the days.

That difference has a direct consequence: fixed price is only possible if the scope is written down. A provider who gives you a firm price on a verbally described need is doing one of two things. Either they have built in a large safety margin, and you are paying for that uncertainty. Or they have not, and they will make it back through change orders by month three.

The most common mistake

The scenario we see most often: the scope is fuzzy, the IT department knows it, and picks time and materials for that reason. That is the right instinct. Then, three months later, they ask the provider to commit to a delivery date.

That is incoherent. You cannot keep the freedom to change priorities every fortnight and demand a date commitment. If you want the date, freeze the scope; if you want the flexibility, accept that the date moves.

The four questions that decide

Can the scope be frozen for six weeks? If yes, fixed price is workable. If your users discover their requirements by seeing the application, time and materials is the more honest model.

Do you have an available internal product owner? Embedded work only functions if someone on your side prioritises the backlog continuously and settles questions within forty-eight hours. Without that person, an embedded team becomes a team that waits, at your expense.

Does the budget need to be locked in front of a finance director? Fixed price gives an enforceable figure. Time and materials gives a forecast, which is sometimes unacceptable at an investment committee.

Is the need lasting or one-off? For a capacity need that will last more than a year, time and materials generally costs less: you do not pay the fixed-price risk premium, and the team accumulates knowledge of your domain.

The hybrid case, which is often the right one

In practice the best structure is frequently a combination: a fixed-price framing phase of one to three weeks, producing the specification and a firm price, then an informed choice between the two models for delivery.

That costs a few thousand euros and it solves the problem at the root. At the end of framing you know whether your scope is stable enough for a fixed price. If it is, you have the number. If it is not, you know exactly why, and time and materials becomes a deliberate choice rather than an admission of defeat.

The three clauses to write in either case

1. The defect classification grid. What counts as blocking, major or minor, defined before acceptance rather than during it.
2. The replacement clause. On embedded work, how quickly a profile who is not right gets replaced, and what happens to the days already invoiced.
3. Reversibility. Notice period, contents of the exit pack, assistance duration. Written at signature, never negotiated at the moment of departure.

A provider who refuses to write those three clauses is telling you something useful about the rest of the relationship.

In short

Fixed price protects your budget but requires a written scope and a framing phase first. Time and materials protects your flexibility but requires real internal steering. The only choice that fails systematically is the one that takes the constraints of one and the expectations of the other.