Fixed price, retainers, and T&M: two contracts, two logics

In my experience, one of the topics that creates the most misunderstanding between digital agencies or software shops and clients is fixed-price proposals versus hour buckets / time-and-materials.

I get it: from the outside they look like two equivalent ways to reach the same result. In practice they are two different contracts, with different economics and different risks.

Fixed price: a closed number

A fixed-price quote is, by definition, a closed estimate: the client wants to know what it will cost, full stop. That’s fair.

What’s often invisible is what sits behind that number. It isn’t pulled out of thin air: it’s an estimate of scope, person-days, complexity, roles involved, hourly rates, overheads, plus a safety margin.

That margin isn’t agency “cleverness”: it’s the price of risk.

When you commit to a fixed price, you take on estimation risk — the risk of having underestimated time, of surprises, of context shifting mid-flight. Fixed price means risk, with guardrails (for both sides, when the perimeter is clear).

Hours and T&M: the other logic

Hour packages or time-and-materials are the opposite. They are not automatically cheaper.

They rely on trust between client and vendor.

They work when the relationship is solid: work is tracked, hours are real, and you bill what you actually do.

Here the risk isn’t only on the agency: it’s shared, which is why the model is more mature — it assumes mutual trust.

When the estimate becomes a cap

Problems start when the client asks for an hour package to deliver something specific, but then treats the estimate like a hard ceiling.

Asking for an estimate is fine: it sets order of magnitude, priorities, budget.

But if that estimate becomes a rigid limit, you get a distortion.

Working on hours (especially prepaid, maybe at a competitive rate) doesn’t include room to absorb a bad estimate.

If more hours are needed, the agency shouldn’t be expected to donate extra work — otherwise you might as well have used a fixed quote.

A simple rule

For me the rule should be simple.

If the client wants a fixed number that doesn’t move, you go fixed price — but you need clear specs and often a proper discovery phase.

If they want flexibility, adaptation, evolution as you go, and a moving target, you go hours — and the estimate stays a compass, not a wall.

Closing

They’re two valid ways of working.

What matters is being explicit before signing, so nobody ends up frustrated halfway through.