Professional Services Intelligence Brief
How to Compare Fixed-Fee and Time-Based Engagements
Neither model is better everywhere. The right choice depends on how predictable the work is, who bears the risk and how well you can scope and estimate. Here is how to compare them with your own data.
Short answer
Compare fixed-fee and time-based engagements on four tests: predictability of scope, quality of your estimating, who should bear the risk, and what each model does to margin and cash flow. In a fixed fee the firm bears the risk of overruns but keeps the benefit of efficiency; under hourly billing the client bears the risk and, as one practitioner put it, “all the efficiencies go to the client.”[1] Fixed-fee pricing is typically simpler than value pricing but may not offer the same margins,[2] and it only works if scope is defined and tracked.[2][3]
How do fixed-fee and time-based engagements differ?
In a time-based engagement the client pays for the hours worked at agreed rates, so the client carries the risk that the work takes longer than expected. In a fixed-fee engagement the client pays an agreed price, so the firm carries that risk and also keeps any saving if it delivers faster. Thomson Reuters describes fixed-fee pricing as a set price agreed before work begins and often presented as a menu of services, which simplifies pricing and billing but does not give the same margins as value-based pricing.[2]
The practical consequence is a shift in incentives. A Journal of Accountancy report quotes a CPA who said that when you bill by the hour, all the efficiencies go to the client, whereas his firm found after switching to value pricing that the time staff spent on the work went down and quality went up.[1] Under a fixed fee, the firm wants to finish efficiently, but it also has an incentive to do less, so the scope and quality standards must be explicit.
When does each model fit best?
Use the nature of the work as your guide. Work with a stable scope and a repeatable process, such as compliance filings, standard reviews, recurring reports or defined implementations, suits a fixed fee because the firm can estimate the effort from experience and improve it over time. Work with an uncertain scope, such as investigations, disputes, early-stage advisory work or projects where the client’s requirements will emerge, suits time-based billing, or a hybrid, because neither side can estimate the effort reliably.
| Factor | Favors fixed fee | Favors time-based |
|---|---|---|
| Scope | Well defined and stable | Uncertain or likely to change |
| Estimating history | You have done similar work many times | Little history, novel work |
| Client preference | Wants budget certainty | Wants flexibility and control of scope |
| Risk appetite | Firm can absorb overruns | Firm cannot absorb overruns |
| Efficiency opportunity | Process or technology can cut hours | Little scope to speed up |
Hybrids work well in between: a fixed fee for a defined phase followed by time-based billing for extras, a capped fee, a fixed monthly fee for a defined scope with an explicit rate for additional work, or a budget with an agreed change process. Some firms combine value pricing with monthly fixed-price agreements, which can also make budgeting easier for clients.[1]
How do you test the choice with your own data?
Take your last twenty to fifty completed engagements and compute, for each, the fee or time-based revenue, the hours worked, the realized rate per hour (fee collected divided by hours) and the margin after labor cost. Group them by model and by type of work. Then ask three questions. What was the average realized rate under fixed fee compared with time-based billing for similar work? How wide was the spread: fixed-fee work often has a few large overruns that erase the margin on many successful jobs. And which work types had stable results?
Next, measure estimate accuracy. SPI Research finds that project overruns above 10 percent harm client satisfaction, project margins and future bookings, and that Level 5 firms hold overruns to single digits.[3] If your fixed-fee projects regularly overrun by more than 10 percent, the problem is the estimate, the scope control or both, not the model. Calculate the average overrun and the share of jobs above 10 percent, and use them to set a contingency in future fixed fees.
How do you protect margin in a fixed-fee engagement?
Control scope. Thomson Reuters calls the lack of a defined scope the biggest problem for many firms and recommends educating the client, before work begins, about what is included and what costs extra.[2] Write the scope, assumptions, deliverables, client responsibilities and change process into the engagement letter, and treat every request outside it as a change to be priced. Scope creep is common: PMI’s 2018 Pulse of the Profession found that 52 percent of projects completed in the prior twelve months experienced scope creep or uncontrolled changes.[4]
Track hours even when you do not bill by them. Without time data you cannot measure estimate accuracy or the realized rate. Build a contingency into the price based on your historical overrun, set check points when 50 and 80 percent of budgeted hours are consumed, and make someone accountable for scope. If you are a lawyer, remember that ABA Model Rule 1.5 requires that the scope of the representation and the basis of the fee be communicated to the client, preferably in writing, before or soon after the work begins.[5]
What should leaders watch after the switch?
Compare the same measures before and after: realized rate, margin, overruns, write-offs and client satisfaction. Watch staff behavior too. Moving from billable hours to fixed fees changes how people think about time; the Journal of Accountancy report noted that staff who take part in the pricing process learn to estimate scope accurately and see the link between their effort and the firm’s profit.[1] Invite staff into the estimating, and review completed jobs together to improve the next estimate. The best fixed-fee firms treat every engagement as data for pricing the next one.
Frequently asked questions
Is fixed-fee pricing more profitable than hourly billing?
Not automatically. Thomson Reuters says fixed fees simplify pricing but do not offer the same margins as value-based pricing, and the result depends on how well scope and effort are estimated.[2]
Who bears the risk in each model?
In time-based billing the client bears the risk of overruns, and efficiency gains go to the client; under a fixed fee the firm bears the risk and keeps any saving.[1]
How do I avoid losing money on fixed-fee work?
Define scope in writing, track hours, add a contingency based on your past overruns and bill changes outside scope. SPI finds overruns above 10 percent damage margins and client satisfaction.[2][3]
What is a hybrid fee arrangement?
A hybrid combines models, for example a fixed fee for a defined phase with time-based billing for extras, or a monthly fixed price with an agreed rate for additional work; some firms pair value pricing with monthly fixed installments.[1]
Do professional rules affect fee arrangements?
Yes. For lawyers, ABA Model Rule 1.5 requires reasonable fees and says the scope and basis of the fee should be communicated, preferably in writing.[5]
What to do next
- Pull the last 20 to 50 completed engagements and compute realized rate and margin by fee model.
- Calculate average overrun and the share of jobs above 10 percent.
- Add scope, assumptions and a change process to your engagement letter template.
- Pilot a hybrid structure on one uncertain-scope service.
Optional · interactive example
See it in an interactive example.
Firm Economics Dashboard: a service-line dashboard with utilization and realization, a scenario simulator for hours, rate and cost, evidence-linked answers from APIS, and a sample brief, all on synthetic data.
The experience below is built on a synthetic firm, “Demo Advisory Partners,” analyzed by service line. It has no client-level or engagement-level records, so service lines are used to show the method. Nothing in it is real.
- Select Technology Consulting. Its hours grew far faster than its revenue. What might that mean for fees?
- In the Scenario tab, lower realized rate by 5%. This is the effect of a fixed fee overrunning.
- Ask APIS why a busy firm may have weak margins.
Understand Your Firm’s Economics and Growth Opportunities
Two Pillars helps professional services firms investigate profitability, capacity, pricing and growth using the firm’s own data and a structured assessment process. A good first step is a firm economics review that follows your own hours from capacity to cash, by client, service and person, and shows where margin is lost.
Sources
- Journal of Accountancy (AICPA), How implementing value pricing can help your staff get more done (2017). Accessed October 2026.
- Thomson Reuters Tax & Accounting, Strategically pricing accounting services. Accessed October 2026.
- Deltek, 5 KPIs from the SPI Professional Services Maturity Benchmark Report Every PSO Must Track for Sustainable Growth (July 15, 2026). Accessed October 2026.
- Project Management Institute (PM Network), Scope Patrol: Scope Creep Is on the Rise as Stakeholder Expectations Increase (Elton, July 2018). Accessed October 2026.
- American Bar Association, Model Rules of Professional Conduct, Rule 1.5: Fees. Accessed October 2026.
Sources checked October 2026. This guide is general information for the leaders of professional services firms and is not legal, accounting, tax or financial advice. Figures and rules are cited to the sources below and may have changed since they were accessed, so confirm anything you rely on against the current source. Two Pillars & Co is independent of the organizations cited. See our editorial policy.