Decision in One Paragraph
A fixed fee can attach to specified deliverables; time-based charging attaches fees to verified time under agreed terms. Choose with the actual service uncertainty, evidence and expense rules visible. A time ceiling limits charges under its terms; it does not turn time-based charging into a fixed price. Both structures still need quality and authority checks.
Comparison Table
| Question | Fixed price | Time-based |
|---|---|---|
| Effort uncertainty | Agreed fee need not rise merely because unchanged work takes longer | Verified time affects the fee within actual limits |
| Payment evidence | Agreed deliverables/milestones and required acceptance | Verified time, applicable rate and required service evidence |
| Expenses | Separate permitted categories, support and cap if agreed | Same questions; do not assume expenses are inside the day rate |
| Change handling | Actual scope changes require the contract's decision route | More days or a ceiling breach does not itself approve change |
| Quality checks | Fixed price does not prove acceptable work | Verified hours do not prove acceptable work |
Fictional Service With Stated Units
All money is invented USD. The same two-workshop facilitation service has identical named deliverables, scope and authorized quality criteria in all three outcomes; no scope change is agreed.
Fixed terms: fee 6,000, plus receipt-supported permitted expenses up to 600. Time-based terms: 750 per verified eight-hour day, estimate eight days, fee cap 7,500, plus receipt-supported permitted expenses up to 600. The supplied terms limit fees to 7,500 absent a separately authorized written change. These are exercise terms, not recommended rates or a legal working-day definition.
Three Outcome Cases With Totals
The last two columns assume the maximum 600 of permitted expenses, not an expense automatically owed.
| Verified days | Nominal time fee | Time fee under cap | Time plus maximum expenses | Fixed plus maximum expenses |
|---|---|---|---|---|
| 8 | 8 x 750 = 6,000 | 6,000 | 6,600 | 6,600 |
| 10 | 10 x 750 = 7,500 | 7,500 | 8,100 | 6,600 |
| 12 | 12 x 750 = 9,000 | 7,500 | 8,100 | 6,600 |
At twelve days, nominal fees exceed the cap by 9,000 - 7,500 = 1,500. The contract owner must resolve any disputed work/cost question; no universal rule about who bears that difference is supplied. The maximum-expense totals differ by 8,100 - 6,600 = 1,500. The fixed fee remains 6,000 here because scope explicitly stays unchanged, not because fixed-price contracts can never change.
Tradeoffs and Limits
Clear outputs may make a deliverable price easier to assess; uncertain duration may make verified time more useful. Neither structure removes the need to define quality, permitted expenses, limits and decision rights. Do not rebuild the effort estimate here; use the effort-estimation tutorial for that task.
Sources and Scope
World Bank Contract Management (first edition September 2018), Annex1 printed pages 86-87, contrasts lump-sum deliverables and verified time/expenses in Bank-financed procurement examples. CC8 (November 27, 2024), CC26 (September 16, 2026) and decision guidance (November 2, 2020) inform financial checks, contract risks and documented choices in England and Wales. None prescribes these rates, caps, terms or current procurement thresholds. Checked November 1, 2026.