Fixed price and time and materials put the risk of an unclear plan on opposite sides of the table. In a fixed-price contract the vendor owns the estimate: you agree a scope and a number, and overruns come out of the vendor's margin unless the scope genuinely changes. In a time and materials contract you own it: you pay for the hours actually worked, so the invoice tracks reality and so does your exposure. Neither model is honest or dishonest by itself. What decides the fit is how completely the work can be described before it starts.
What is the difference between fixed price and time and materials?
Under fixed price, the vendor quotes a total for a defined deliverable and absorbs the cost of work that took longer than expected. Under time and materials, the vendor bills an agreed rate card against hours or days, and the buyer absorbs the cost of work that took longer than expected. A third shape is common in software but rarely named in the contract template: capped time and materials, also called not-to-exceed, where hours are billed normally but stop at a ceiling both sides agreed to.
| Question | Fixed price | Time and materials | Capped T&M |
|---|---|---|---|
| Who carries estimate risk | The vendor | The buyer | Shared up to the cap |
| Cost certainty before kickoff | High | Low | Medium |
| Time to start | Slower - scoping comes first | Fastest | Fast, with a planning step |
| How change is handled | Change order, re-quoted | Work simply continues | Continues until the cap |
| Fits when | The scope can be written down and tested | The work is genuinely unknown or directed week to week | Depth is unknown but the budget is not |
When fixed price is the right model
Fixed price fits when a competent third party could write an acceptance test for the deliverable before any code exists. Rebuilding a known workflow, integrating against a documented third-party API, replacing a legacy form-based tool, or shipping a marketing site and a defined mobile app all qualify. It also fits when your organisation needs one approved number: a fixed price survives a budget committee in a way that an open-ended rate card does not. The trade is speed and flexibility - scoping takes time, and the vendor prices the uncertainty it cannot remove, which is why a fixed quote for genuinely exploratory work carries a visible risk premium.
- Requirements are stable and the definition of done can be written in a page.
- The vendor has done this exact job before and can name the steps without inventing them.
- Your side can commit a decision-maker to answer questions inside the agreed window.
- The budget needs one number that will not move without a signed change.
When time and materials is the right model
Time and materials fits when the plan cannot honestly be written yet, or when the direction will change on purpose. Discovery phases, AI and machine-learning features whose accuracy depends on data nobody has inspected, and product work steered week by week by a product owner are all poor candidates for a fixed number. So is staff augmentation, where your engineering lead sets priorities and the vendor supplies capacity. In those cases a fixed price is not a stronger contract, it is a larger contingency fee attached to guesses.
- You are buying exploration - the outcome of the first month is knowledge, not a feature list.
- Priorities will move and you would rather redirect the team than re-paper the contract.
- You already have engineering leadership who can review work as it lands.
- The work is capacity-shaped: you need two more engineers on your codebase, not a delivered product.
The middle option: capped time and materials
A capped engagement bills an agreed rate card but stops at a ceiling. It is the practical answer to phase one of a project whose depth is unknown and whose budget is not: you get the speed of an hourly arrangement and a hard number the finance side can approve, and when the cap approaches you re-plan with real information instead of renegotiating a fixed contract on the strength of estimates. The discipline required is the same as any hourly model - a written log of what each invoice bought, reviewed before the next invoice.
The three questions that actually decide the model
Most of the argument disappears if you answer three questions honestly before negotiating.
- Can "done" be tested? If a reviewer can describe the passing result in writing today, fixed price works. If the only honest description is "we will know it when we see it", a fixed price just moves the argument to the change-order process.
- How expensive is being wrong? When a wrong build is expensive to unwind, spend the scoping time and take the fixed price. When a wrong build costs a sprint and teaches you something, hourly work is cheaper in total.
- Who is closer to the problem? The party that holds the domain knowledge should hold the direction. If that is you, augmentation fits. If that is the vendor, a deliverable contract fits.
What belongs in the contract either way
The model matters less than the clauses around it. A fixed price without written change control is how disputes are made, and time and materials without a written record is how invoices become arguments.
- A change-control clause that defines what a change is, who approves it, and how it is priced.
- Acceptance criteria agreed in writing, with the person who signs them named.
- A rate card, even on fixed price, for the post-launch changes that will not be free.
- IP assignment in plain language: code and assets transfer on payment, with no retained licence over your product.
- A warranty window for defects inside the delivered scope, separate from new-feature work.
- Named assumptions and client responsibilities - who supplies the data, the accounts and the approvals, and what happens when those are late.
- A milestone schedule that ties invoices to reviewable points rather than to the calendar.
How 723 Studios quotes this work
723 Studios quotes custom software as a fixed price after a short scoping call, so the questions above get answered before the number exists rather than during delivery. That is a deliberate trade: you spend an hour on scope, and in return the proposal states what is built, what it costs and when it lands, delivered in reviewable milestones. For work that is genuinely exploratory, the honest structure is a smaller, capped phase first - paying for knowledge with a hard ceiling - rather than a large fixed number built on assumptions nobody can test. Scope delivered by 723 Studios also carries a six-month post-launch bug-fix window for anything inside the agreed scope, which is the part of a fixed-price contract buyers usually forget to ask about.
If you are deciding between these models for a specific project, the useful next step is a scoped proposal rather than a longer comparison. You can review our software development services, see what we build under custom app development, read the breakdown of what it costs to build an app, or request a fixed-scope quote and get the number in writing.
Frequently asked questions
Is fixed price always better for the buyer?
No. Fixed price buys cost certainty and charges for it - the vendor prices the uncertainty it cannot remove, and changes become negotiations. If the work is exploratory or your priorities move, hourly billing is usually cheaper in total even though it feels riskier.
What is a change order?
A change order is a written amendment to an agreed scope: what is changing, what it costs and how the schedule moves. It is the mechanism that lets a fixed-price contract stay fixed without either side pretending new work is free.
Can a vendor walk away from a fixed price mid-project?
Only within the contract's terms. If the scope changes materially - new requirements, major migrations, a platform the vendor never scoped - the usual outcome is a re-quote through change control. That is why the change-control clause is worth more attention than the number.
How do you stop a time and materials project from drifting?
With a written record and a cadence: an itemised invoice per period, a review of what has been delivered before the next invoice, a cap you are willing to stop at, and one named owner on your side responsible for priorities.
Which model is better for a first project with an agency?
Start smaller than you think either way. For 723 Studios that usually means a fixed-scope first phase - a real deliverable with a written acceptance test - which produces the information a larger fixed price or a longer hourly engagement needs in order to be honest.