Fixed-Price vs Time-and-Materials Software Contracts
Use a fixed-price contract when the scope is small, stable and fully specified, because it caps your budget and puts delivery risk on the vendor. Use time-and-materials when requirements will evolve, which is true of most new apps, because you pay for actual work and can change priorities without renegotiating. A common middle ground is a fixed-price discovery phase followed by time-and-materials sprints with a budget cap.
- Fixed-price moves cost risk to the vendor, who prices that risk in, so the quote is often higher than the likely effort.
- Time-and-materials keeps flexibility but leaves budget risk with you unless you add caps and regular reporting.
- Fixed-price only works with detailed requirements; every change after signing becomes a change order.
- Hybrids, such as fixed-price discovery then capped sprints, often give the best balance for new products.
- Whatever the model, define acceptance criteria, payment milestones and code ownership in writing.
At a Glance
| Factor | Fixed-price | Time-and-materials |
|---|---|---|
| Who carries cost overrun risk | Mostly the vendor | Mostly you |
| Budget predictability | High, if scope holds | Lower, unless capped |
| Flexibility to change scope | Low, change orders | High, reprioritize each sprint |
| Requirements needed up front | Detailed specification | Clear goals and a prioritized backlog |
| Pricing tendency | Includes a risk contingency | Pays for actual effort |
| Your oversight effort | Lower during build, high at acceptance | Ongoing sprint reviews |
| Best fit | Small, stable, well-defined work | New products and evolving scope |
Choose fixed-price if the scope is small, stable and fully written down, and you value a known total above flexibility. Choose time-and-materials, ideally with a budget cap and sprint reporting, if you are building something new and expect to learn and adjust as you go.
How Each Contract Type Works
In a fixed-price contract, the vendor agrees to deliver a defined scope for a set total, usually paid in milestones. If the work takes longer than estimated, the vendor absorbs the cost. If you want something not in the specification, it becomes a change order with its own price.
In a time-and-materials (T&M) contract, you pay for the hours actually worked at agreed rates, plus any direct costs such as third-party services. Scope can shift as you learn, and you decide each sprint what matters most. The final cost depends on how much work you ask for and how efficiently it is done.
These are not new ideas. US federal contracting rules (FAR Part 16) describe both families in detail, and note that a firm-fixed-price contract suits work where requirements are reasonably definite and costs can be estimated with confidence, while T&M fits when the extent or duration of the work cannot be estimated accurately at the start.
In practice, the model shapes behavior on both sides. Under fixed-price, the vendor's incentive is to hold scope tightly and finish efficiently. Under T&M, the vendor's incentive is to keep you satisfied so the engagement continues, while your incentive is to prioritize ruthlessly. Neither model replaces trust and a good working relationship, but each creates different pressure points worth understanding before you sign.
When Fixed-Price Works, and When It Backfires
Fixed-price is a good fit for a marketing website, a small app with clear screens and flows, a well-specified integration, or the next phase of an existing product where the team knows the codebase. It gives you a number for your budget and a clear deliverable.
The problems show up when the scope is uncertain:
- Risk premium. Experienced vendors add a contingency for unknowns, so you may pay more than the work actually takes.
- Change friction. New apps reveal better ideas once users touch them. Under fixed-price, every change is a negotiation.
- Quiet corner-cutting. If the vendor is running over budget, testing, documentation and code quality are the easiest things to trim. Acceptance criteria and code review rights protect you.
- Spec effort up front. You need a detailed requirements document before signing. Our guide on writing an app requirements document shows what that involves.
When Time-and-Materials Works, and How to Control It
T&M suits new products, MVPs, ongoing development and anything with significant unknowns, such as integrating with a legacy system or adding AI features. You pay for real effort instead of a padded estimate, and you can reprioritize every sprint based on what you learn. It aligns naturally with agile delivery, where working software is reviewed often.
The risk is an open-ended budget. Control it with:
- A not-to-exceed cap per phase or month that the vendor cannot pass without written approval.
- Weekly or per-sprint reporting: hours by person, work completed, and burn against the budget.
- A prioritized backlog, so the most valuable features get built first if money runs short.
- Demos of working software every sprint, not status slides.
- A termination clause that lets you stop at any sprint boundary and keep everything built so far.
Estimates under T&M are still useful. Ask for a ranged estimate, such as a low and high figure for each major feature, with the assumptions behind it. That lets you plan a budget, see which features carry the most uncertainty, and decide which to build first or simplify.
Change is not a failure in either model. On a new app, learning that a feature should work differently after users try it is the process working. The contract should make that kind of change cheap to act on, which is the strongest argument for keeping fixed scope small and short.
Hybrid Models That Often Work Best
- Fixed-price discovery, then T&M build. A short paid discovery phase produces requirements, designs, architecture and a ranged estimate. The build then runs as capped T&M sprints.
- Fixed price per sprint or milestone. Each sprint has a fixed fee and an agreed goal. You get budget predictability in small pieces while keeping the ability to change direction.
- Fixed-price MVP, T&M afterward. Lock the smallest launchable version, then move to a monthly retainer for improvements.
Whatever you choose, read the contract for acceptance criteria, warranty period for defects, payment milestones and intellectual property assignment. Have an attorney review anything significant; this page is general guidance, not legal advice. For budgeting either model, see how to budget for app development and our pricing page.
Sources and Further Reading
We link to the primary documentation so you can verify the details yourself.
- FAR Part 16: Types of Contracts, Acquisition.gov (U.S. General Services Administration)
- Manifesto for Agile Software Development, Agile Alliance authors
- Works Made for Hire (Circular 30), U.S. Copyright Office
Related Questions
Why did my fixed-price quote come in higher than a T&M estimate?
The vendor is taking on the risk of overruns and prices in a contingency for unknowns. If your scope is truly fixed, that premium buys certainty. If it is likely to change, you may pay the premium and change-order fees on top.
What is a not-to-exceed clause?
It is a ceiling on a time-and-materials engagement, per phase or per month. The vendor bills actual hours up to that limit and must get written approval before going beyond it, which gives you T&M flexibility with a firm budget guardrail.
How should payments be structured on a fixed-price project?
Tie payments to accepted deliverables rather than dates, such as approved designs, a working beta and launch, with a modest deposit up front. Holding back a small final payment until a defect warranty period ends also encourages quality.
Does agile development require time-and-materials?
No, but they fit naturally. Agile assumes priorities change as the team learns. You can run agile under fixed-fee sprints, where each sprint has a set price and a goal, which many businesses find easier to budget.