Fixed Price vs Time and Materials Contracts for Software Projects

Fixed price vs time and materials comes down to who carries the risk of uncertainty. A fixed price contract sets one price for a defined scope, so the vendor carries the risk of overruns but you lose flexibility to change requirements. A time and materials contract pays for actual hours at agreed rates, so you keep flexibility but carry the budget risk. Fixed price works best when the scope is small, clear and stable; time and materials works best when requirements will evolve. Many software projects use a hybrid. This guide explains each model, when to use it and which clauses to insist on.

How each model works

Fixed price. You agree a scope, a price and usually a timeline. The vendor delivers the agreed scope for that price. Changes go through a change request process and are priced separately. The US Federal Acquisition Regulation, which governs federal purchasing, says a firm-fixed-price contract is suitable when fair prices can be set at the outset, for example when there is adequate price competition, realistic cost estimates are possible, and performance uncertainties can be identified and their cost impact reasonably estimated. That is a useful test for private buyers too.

Time and materials (T&M). You pay for the time spent by the vendor's team at agreed hourly or daily rates, plus any agreed expenses. Scope can change as you learn. The same regulation describes a time-and-materials contract as acquiring services on the basis of direct labour hours at specified fixed hourly rates, and states it may be used only when it is not possible at the time of placing the contract to estimate accurately the extent or duration of the work. It also requires a ceiling price, a safeguard private buyers should copy.

Side-by-side comparison

When fixed price works well

• The scope is small and specific, such as a defined integration, a landing page set or a plugin.
• Requirements are documented in detail, ideally after a discovery phase.
• The technology is familiar to the vendor and holds few surprises.
• You have a fixed budget approved in advance and little room to change it.
• The work is a time-boxed phase, such as discovery or a prototype, where the output is defined but the detail is not.

Be aware of the trade-offs. Vendors price risk into fixed quotes, so you may pay more than the work actually costs. Scope must be described tightly, and anything not written down may be treated as a change. Disagreements about whether something was "in scope" are the most common source of conflict in fixed-price projects.

When time and materials works well

• You are building a new product and expect to learn from users as you go.
• Requirements are likely to change because of market, regulation or internal decisions.
• The work is ongoing, such as product development, maintenance or a dedicated team.
• You have a product owner who can prioritise work and review progress regularly.

This model fits naturally with agile ways of working. The Manifesto for Agile Software Development values customer collaboration over contract negotiation and responding to change over following a plan. The Scrum Guide describes Sprints as fixed-length events of one month or less, which gives you regular points to review what was delivered and decide what comes next. With T&M, those reviews are your main budget control.

Hybrid models that balance risk

Many buyers combine elements of both models:

• Fixed-price discovery, then T&M build: a short fixed-price phase produces requirements, designs and an estimate; the build then runs on T&M with a budget cap.
• T&M with a cap (not-to-exceed): you pay for actual time, but the vendor cannot invoice beyond an agreed ceiling without your written approval.
• Fixed price per milestone or sprint: each milestone or sprint has an agreed price and goal; scope for later milestones is refined as you go.
• Capacity-based team: a fixed monthly price for a defined team, with priorities set by you each sprint.
• Target cost with shared savings or overruns: an agreed target where buyer and vendor share the difference, which aligns incentives but needs careful drafting.

Hybrids work best when the contract states clearly how scope, priorities and budget are reviewed, and who can approve changes.

Clauses to insist on in either model

The Scrum Guide's idea of a Definition of Done, a formal description of the state of the work when it meets the required quality measures, is useful in both models. Agree it in writing so "finished" means the same thing to both sides: tested, reviewed, documented and deployed to an agreed environment.

A worked example

Imagine a company that wants a customer portal where clients can view orders, download invoices and raise support requests. The team knows the core features but is unsure how customers will use the portal, and the ERP integration has never been documented.

A sensible structure would be:

1. Fixed-price discovery. A short phase to interview customers, map the ERP data, design the main screens and produce a prioritised backlog with estimates. The output is clear, so a fixed price is fair to both sides.
2. Capped time and materials for the first release. The team builds in sprints against the backlog. The product owner reprioritises as customer feedback arrives, and the vendor cannot exceed the agreed cap without written approval.
3. Fixed price for well-defined extras. Once the portal is live, small, clearly specified additions, such as a new report or an extra payment method, can be quoted individually.
4. A monthly retainer for maintenance. Updates, monitoring and small fixes are covered by an agreed number of hours each month.

Each stage uses the model that matches its level of uncertainty, which keeps risk with the party best placed to manage it.

Red flags in software contracts

• A fixed price for a vague scope. Either the price contains a large risk premium or you will face many change requests.
• T&M with no cap and no reporting. You cannot control what you cannot see.
• No acceptance criteria. Without them, neither side can prove when work is complete.
• IP retained by the vendor or licensed back to you for your own custom code.
• Rates that change without notice, or unclear rules for who can be billed (for example, project managers and senior staff joining calls).
• Heavy penalties or long lock-ins that make it hard to leave if things go wrong.

How to compare proposals that use different models

You will often receive one fixed-price and one T&M proposal for the same brief. To compare them fairly:

1. Ask the T&M vendor for an estimate range for the same scope, with assumptions.
2. Ask the fixed-price vendor to list exclusions and how changes are priced.
3. Estimate how much your requirements are likely to change, and price that into the fixed-price option.
4. Compare team seniority and time allocation, not just totals.
5. Check what each includes for testing, deployment, documentation and warranty.
6. Consider the cost of your own time managing each model.

If two proposals are close, choose the vendor whose working style and communication you trust more. The contract model matters, but the relationship decides how well it works in practice.

Find companies that match your budget and model

On Trusted IT Firms you can filter companies by the smallest projects they take on. There are 97 companies listing a minimum project size of $0 to $10k, 99 listing $10k to $50k and 27 listing $50k to $100k. You can also browse by hourly rate, for example the 133 companies listing $25 to $75 per hour, or start with the software development category. Shortlist three to five companies, ask each which contract model it recommends for your project and why, and compare the answers. Browse all companies on Trusted IT Firms, or list your company for free if you provide software services.

Frequently asked questions

Is fixed price or time and materials cheaper?

Neither is always cheaper. Fixed price includes a margin for the vendor's risk, while time and materials reflects actual effort but can grow if scope expands. The right choice depends on how stable your requirements are.

When should I use a fixed price contract?

When the scope is small, clear and unlikely to change, such as a defined integration or a discovery phase, and when you can describe acceptance criteria in detail.

How do I control costs on a time and materials contract?

Set a budget cap, require regular spend reports, review progress at the end of each sprint or milestone and keep a product owner responsible for priorities.

What is a hybrid contract model?

A combination of models, for example a fixed-price discovery followed by a capped time and materials build, or a fixed price per milestone with later scope refined as you go.

Who should own the code under each model?

You should, in both cases. The contract should transfer ownership of custom code and other deliverables to you as they are paid for.

Sources

• FAR 16.202-2 Application (firm-fixed-price contracts), Acquisition.gov
• FAR 16.601 Time-and-materials contracts, Acquisition.gov
• Manifesto for Agile Software Development
• The 2020 Scrum Guide