Estimator inputs
Example data table
| Scenario | Base effort (hours) | Team | Capacity (hrs/day) | Buffer | Workdays | Notes |
|---|---|---|---|---|---|---|
| Website redesign | 120 | 3 people | 12.2 | 10% | 12 | Mon–Fri, few dependencies |
| Mobile feature sprint | 80 | 2 people | 8.2 | 15% | 12 | More reviews and rework |
| Cross-team migration | 200 | 5 people | 18.7 | 25% | 18 | Heavy coordination, lower parallel efficiency |
Formula used
This estimator combines effort, capacity, and calendar constraints. It uses these steps:
- Base effort (hours) is taken from tasks, direct hours, or PERT.
- Adjusted effort = Base × (1 + Overhead%) × (1 + Rework%) ÷ Productivity.
- Effective daily capacity = Team × Hours/day × Focus × Parallel efficiency.
- Workdays needed = Adjusted effort ÷ Effective daily capacity.
- Planned workdays = ceil(Workdays × (1 + Buffer%)).
- End date is found by skipping non-working days and holidays.
How to use this calculator
- Select an estimate method: tasks, direct hours, or PERT.
- Set overhead, rework, productivity, and contingency buffer.
- Enter team size, daily hours, focus, and parallel efficiency.
- Choose start date and working days, then add holiday dates.
- Click Estimate Duration to see start-to-finish dates.
- Use Download CSV or Download PDF for sharing.
Key drivers of project duration
Project duration is driven by effort and how efficiently that effort becomes completed workdays. In this estimator, total effort increases with overhead and rework, then adjusts for productivity. Typical overhead for meetings, reviews, and coordination ranges from 10% to 35%. Rework often adds 5% to 25% depending on ambiguity and quality gates. Productivity below 100% represents blockers or learning curves; 80% to 95% is common for stable teams. Document assumptions so future updates remain consistent always.
Turning estimates into capacity
Capacity converts hours into calendar time using team size and effective daily output. Daily capacity equals team members × hours per day × focus factor × parallel efficiency. Focus captures time available for the project; many teams see 60% to 85% after support work and interruptions. Parallel efficiency reduces capacity when work cannot be fully divided; 70% to 95% fits most knowledge work. Example: 4 people × 6 hrs/day × 0.75 × 0.85 ≈ 15.3 effective hrs/day.
Calendar-aware scheduling
Once planned workdays are known, the end date is calculated by stepping through the calendar and skipping non-working days. Select the working days pattern that matches delivery expectations, then list company holidays to remove them from the schedule. For short projects, a single holiday can shift delivery more than a 5% buffer. Use the start date as the first working day; if kickoff activities happen earlier, reflect them as overhead hours.
Buffers and confidence ranges
Buffers protect milestones from uncertainty, dependencies, and discovery. A contingency buffer of 10% to 20% suits well-defined scopes, while cross-team programs often need 20% to 35%. If you use PERT, the expected effort is (Optimistic + 4×Most Likely + Pessimistic) ÷ 6, and spread is based on (P − O) ÷ 6. Use the confidence range to communicate risk rather than promising a single date.
Reporting and iteration
Duration estimates improve when they are revisited with real progress. Save the latest inputs, update remaining effort weekly, and compare the projected end date with prior runs. When stakeholders ask for “what changed,” point to the specific drivers: capacity, focus, rework, or added scope. Use the CSV export for tracking assumptions in a spreadsheet, and the PDF export for consistent status reports. Small adjustments, applied early, prevent large schedule surprises later.
FAQs
What should I enter for Base Effort hours?
Use the sum of task estimates or time-tracked hours for similar work. If you only know work packages, estimate each in hours, then add them. Keep Base Effort independent of buffers so the calculator can apply risk adjustments cleanly.
How does the Focus factor affect results?
Focus represents the share of each day available for project work. Lower focus reduces daily capacity and extends the schedule. If your team handles support or meetings, start with 0.70 and refine after observing a week of actual time use.
What is Parallel Efficiency and when should it be reduced?
It models how well multiple people can work at the same time. Reduce it when tasks are sequential, require a single reviewer, or depend on one environment. Values of 0.75–0.90 are common for tightly coupled work.
When should I use the PERT method?
Use PERT when uncertainty is high and you can provide optimistic, most likely, and pessimistic hours. It produces an expected effort and a useful range. It’s helpful for new initiatives, integrations, and projects with external dependencies.
How are holidays and non-working days handled?
The end date is computed by counting only selected working days. Any holiday dates you list are skipped even if they fall on a normal workday. This keeps the plan aligned with actual availability and prevents hidden schedule drift.
Can I estimate phases or milestones with this tool?
Yes. Run the calculator separately for each phase using phase-specific effort and capacity assumptions. Then chain the next phase start date to the prior phase end date. This approach highlights where handoffs or constrained resources drive delays.