Fixed Price vs Time & Materials: How to Choose

Every few weeks a founder asks us to quote a project both ways - fixed price and time & materials - hoping the numbers will make the decision for them. They will not, because the two models are not two prices for the same thing. They are two different contracts about who carries which risk, and picking the wrong one costs far more than any rate card difference.

What fixed price actually buys you

Fixed price buys you a number you can put in a board deck. That is genuinely valuable: budget certainty, a hard external deadline, and a vendor who is financially motivated to finish. But be honest about what you are paying for. Any sane studio prices a fixed bid as estimate plus risk premium, because they are absorbing the uncertainty you are offloading. And the mechanism that protects that premium is change control: every "small tweak" becomes a change request, priced and papered.

Fixed price works beautifully when the spec is truly stable - a well-defined integration, a compliance-driven rebuild, a scoped security audit. It works terribly when the product is still discovering itself, because then you are paying a premium for the right to argue about scope.

What T&M actually buys you

Time & materials buys you the right to change your mind at market price. No risk premium, no change-request theater - the backlog is renegotiated every week and the best current idea wins. For products in discovery mode, that flexibility is not a nice-to-have; it is the whole game.

The catch is that T&M transfers delivery risk to you, and it only works with radical visibility: weekly demos on production infrastructure, a burn rate you can read in one glance, and the contractual ability to stop with two weeks notice. A T&M engagement without those is not a pricing model, it is a subscription to hope.

The real variable is spec entropy

Strip away the negotiation posturing and the choice reduces to one variable: how much will the spec change between kickoff and launch? We literally reason about it like a function:

type Engagement = 'fixed' | 'tm' | 'hybrid';

function choose(specEntropy: number, trust: number): Engagement {
  // specEntropy: 0 = frozen spec, 1 = "we'll know it when we see it"
  // trust: 0 = first date, 1 = shipped together before
  if (specEntropy < 0.2) return 'fixed';   // known problem, known shape
  if (trust > 0.6) return 'tm';            // discovery with a proven partner
  return 'hybrid';                         // fixed discovery, then T&M build
}

It is a caricature, but a useful one: fixed price is for low-entropy specs, T&M is for high-entropy specs with trust, and hybrid exists for everything in between. Most first engagements land in the hybrid cell, which is why it is our default recommendation.

The hybrid we actually recommend

1. Fixed-price discovery (1-2 weeks). Small, capped, and finished with artifacts you own: a scoped backlog, an architecture sketch, a real estimate with confidence ranges. Cheap for you, honest for us.
2. T&M build in weekly slices. Each week ends with a demo and a decision: continue, steer, or stop. The estimate from discovery becomes the yardstick the burn is measured against.
3. Optional fixed hardening. Once the surface stops moving - launch checklist, security pass, performance budget - the remaining work is low-entropy again, and fixing its price is finally honest.

This sequence prices each phase according to its actual uncertainty instead of forcing one risk model onto the whole project. It also gives both sides a cheap exit after discovery - which, counterintuitively, is exactly what makes people comfortable enough to continue.

Questions that settle it in ten minutes

• Could you write acceptance criteria today that will still be correct in eight weeks? If yes, fixed price is honest.
• Will user feedback change what you build mid-project? If yes, T&M - anything else taxes your own learning.
• Is this your first project with the studio? Cap the downside with a fixed discovery before committing either way.
• Is the deadline regulatory or market-driven and immovable? Fixed price the critical path, T&M the rest.

Fixed price is renting certainty. T&M is renting flexibility. The expensive mistake is renting the one you do not need.

One last opinion, since you asked a studio: we make more money on badly-chosen fixed bids than on well-run T&M - risk premiums are lucrative. We still push clients toward the model that fits, because the projects that fit are the ones that ship, and shipped projects are the only marketing that compounds.