Fixed price, or pay by the day?
Which contract shape actually protects you depends entirely on how settled the scope is.
Fixed price works when the scope can be defined — you carry no overrun risk and the supplier carries the estimate risk. Time and materials works when priorities move, because re-scoping every change costs more than it protects. Neither is inherently safer.
Updated August 2026
Short answer
Fixed price for a well-understood build with a clear finish. Time and materials for ongoing product work where priorities change. Beware fixed price on a vague scope — it protects nobody and produces defensive behaviour on both sides.
Fixed price feels safer. On a vague scope it is the opposite.
The instinct is understandable: a fixed price means a known number, and a known number is easier to approve. When the scope is genuinely definable that instinct is correct — the supplier carries the estimate risk, you carry none, and everyone knows what done looks like.
The problem appears when the scope is not really settled and a fixed price is agreed anyway. The supplier prices in uncertainty, so you pay a premium for risk that may not materialise. Then, because their margin depends on the scope line, every change becomes a negotiation about whether it was included. Both sides get defensive and the relationship turns adversarial over things neither party anticipated.
Time and materials removes that friction and moves the risk to you. It suits work where you are still learning what to build — which describes most product development honestly assessed. The protection is not a fixed number; it is a monthly cap and seeing working software every two weeks so you can stop or redirect early.
The failure mode of time and materials is discovering three months in that progress did not match spend. That is a visibility problem, and it is solved by cadence rather than by contract shape.
Side by side.
| Fixed price | Time and materials | |
|---|---|---|
| Who carries estimate risk | The supplier | You |
| Changing your mind | Re-scope and re-quote | Just re-prioritise |
| Budget certainty | High | Requires a cap to be certain |
| Needs upfront definition | Substantial | Little |
| Supplier incentive | Finish efficiently | Be useful for longer |
| When scope is vague | Padded price, defensive scoping | Works well |
| Best for | A defined build with a clear end | Ongoing product work |
Choose fixed price when
- You can describe what done looks like
- The scope will not change much
- You need budget certainty for approval
- There is a clear finish line
Choose time and materials when
- Priorities move week to week
- You are still learning what to build
- The work is ongoing rather than a project
- Re-scoping every change would slow you more than it protects
What people get wrong.
Fixing the price on an unclear scope
You pay a risk premium and get defensive scoping. Nobody wins, and the relationship sours over changes both sides consider reasonable.
Treating time and materials as open-ended
It should have a monthly cap and a fortnightly review. Without those, the risk you took on has no boundary.
Choosing the shape before defining the work
The contract should follow how settled the scope is, not the other way round. Deciding fixed price first forces a premature specification.
Assuming fixed price means fixed date
They are different commitments. A fixed price with no agreed date protects your budget and nothing else.
Four questions that settle it.
This is genuinely about how much you already know.
Can you write down what done looks like?
If you can describe it specifically enough that two people would agree whether it was finished, fixed price works.
How often did the last project's priorities change?
If they moved monthly, fixed price will fight you. Re-scoping every change costs more than it protects.
Do you need a number for approval?
A legitimate constraint. A hybrid works — fixed price for a defined first phase, then time and materials once you are learning from real usage.
Will you review progress every two weeks?
If yes, time and materials is safe. If nobody will look until month three, take the fixed price and its premium.
What people ask next.
Only when the scope is genuinely definable. On a vague scope it produces the opposite of safety: the supplier pads the estimate to cover uncertainty, then defends the scope line at every change because their margin depends on it. You end up paying more and arguing about whether something was included. Fixed price is a good instrument on a well-understood build and a poor one otherwise.
With a cap and a cadence. Agree a monthly ceiling, and review what was delivered every two weeks so you can stop or redirect early. The risk in time and materials is not the rate — it is discovering three months in that progress did not match spend. Seeing working software every fortnight is what makes it safe.
Whichever fits, and we will say which we think that is rather than defaulting to whatever suits us. Fixed price is genuinely better for us on a scope we understand well, because we can be efficient. Time and materials suits ongoing work where a fixed price would mean pricing in uncertainty you would rather not pay for.
Common and often sensible. A fixed price for a defined first phase — where the scope is clear and you want budget certainty — then time and materials for the ongoing work once you are learning from real usage. It gives you the certainty where it matters and the flexibility where it matters.
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