Fixed price (a set price for an agreed scope) gives predictability and works when the scope is well defined. Time and material (billing for actual time) gives flexibility and fits when the scope will change. In practice it is best to combine the two: a fixed stage to start, a flexible one for further development. Whatever the model, you are entitled to a clear estimate range.
The billing model often matters more for peace of mind than the price itself. A bad choice can turn a good project into constant arguments about scope. This post shows when each model works. For the wider context of choosing a partner, see the guide on how to choose a software house.
Fixed price: how it works
You agree the scope and get a set price and deadline. Upside: full predictability, easy to plan the budget. Downside: every scope change needs an amendment, and for the price to be fair the vendor has to understand the project well upfront. Fixed price suits well-described projects: a simple site, a clearly defined feature, a specific integration.
Time and material: how it works
You pay for time actually worked, usually billed in short periods. Upside: flexibility, easy to change priorities and react to what you learn along the way. Downside: less predictability, requires trust and ongoing oversight. Time and material suits products developed iteratively, where the scope matures with the project, for example an MVP and its further growth.
Mixed models
A combination most often works best. You bill the first, well-defined stage (an MVP, say) as fixed price for a predictable start. Further development, where priorities shift, you run as time and material. You get predictability where you need it and flexibility where it is more valuable.
How to control the budget either way
The model is not everything. You keep the budget in check with: a clear scope in writing, regular previews of a working version, short billing periods and an agreed cap beyond which you talk about priorities. An honest partner proposes these safeguards themselves, rather than hiding the cost until the end.
Frequently asked questions
Neither by default. Fixed price can be more expensive because the vendor prices in the risk of underestimating scope. Time and material can be cheaper but needs ongoing oversight. What matters is the fit to the project, not the label.
When the scope is well defined and unlikely to change much. Then a set price gives peace of mind and easy budget planning.
Yes, and it is a common, sound scenario. A fixed-price first stage gives a predictable start, and further development in time and material gives flexibility as priorities change.