Calculator Inputs
Example Data Table
| Scenario | Items | Hours/item | Complexity | Team | Deadline (w) | Duration (w) | Required FTE |
|---|---|---|---|---|---|---|---|
| Standard backlog | 120 | 1.5 | 1 | 3 | — | 2.94 | 3.00 |
| Scaled delivery | 250 | 2 | 1.2 | 6 | — | 4.90 | 6.00 |
| Tight deadline | 60 | 4 | 1.5 | — | 6 | 6.00 | 2.94 |
Formula Used
TotalHours = BaseHours × (1 + Rework%)
BufferedHours = TotalHours × (1 + Buffer%)
DurationWeeks = BufferedHours ÷ (TeamSize × EffectiveWeekly)
RequiredFTE = BufferedHours ÷ (DeadlineWeeks × EffectiveWeekly)
How to Use This Calculator
- Enter workload inputs: work items, hours per item, and complexity.
- Add risk factors: rework percent and a contingency buffer.
- Set capacity assumptions: weekly hours, utilization, productivity, and overhead.
- Choose a sizing mode: estimate duration from team size, or compute team size for a deadline.
- Review results and export: use CSV or PDF for sharing and tracking.
Calibrate work items with a stable baseline
Start by defining what one “work item” means in your process: a ticket, drawing, test case, or change order. Keep the definition stable, and split oversized items until most finish within one to three days. Pull a recent sample of 30–50 completed items and calculate the median effort per item to avoid outliers. If your baseline is 2.0 hours and you have 120 items, the starting workload is 240 hours before any multipliers.
Use complexity multiplier to reflect variability
Not every backlog is uniform. Apply a complexity multiplier to capture integration effort, cross-team dependencies, safety reviews, or novel technology. Values around 0.8–1.0 fit repeatable work, while 1.2–1.6 fits higher uncertainty. Calibrate by comparing planned versus actual hours from similar past work. For example, multiplying 240 hours by 1.25 produces 300 base hours, aligning the model to the real difficulty.
Separate rework from contingency buffer
Rework percent represents expected iteration: defects, design churn, retesting, or stakeholder changes. Buffer percent represents uncertainty you cannot yet explain, such as requirement gaps or late discovery. Many teams start with 5–15% rework and 10–25% buffer, then tune using historical variance. With 300 hours, 10% rework becomes 330 hours, and a 15% buffer yields 379.5 hours.
Model effective weekly capacity, not calendar hours
Headcount is not capacity unless you correct for how time is actually spent. Effective weekly hours per person combine scheduled hours with utilization, productivity, and overhead. Typical planning bands are 70–90% utilization, 80–110% productivity, and 15–35% overhead, depending on maturity and tooling. Using 40 hours, 85% utilization, 95% productivity, and 20% overhead gives 40×0.85×0.95×0.80 = 25.84 effective hours.
Turn sizing into staffing, duration, and budget decisions
Once you have buffered hours and effective capacity, you can make decisions quickly. Duration mode estimates weeks = hours ÷ (team × effective weekly), supporting milestone planning. Staffing mode estimates FTE = hours ÷ (deadline weeks × effective weekly) and rounds up to a practical team. Add an hourly rate to translate effort into budget, then run baseline and conservative scenarios to understand risk. Review constraints like ramp-up, handoffs, and critical-path tasks that cannot run in parallel across specialized roles.
FAQs
1) Which inputs change the duration the most?
Duration is usually driven by total buffered hours and effective weekly capacity. Large swings come from hours per item, complexity, utilization, and overhead. Test a baseline and a conservative scenario to see sensitivity.
2) How do I pick a realistic utilization percent?
Use historical time allocation if available. Many engineering teams plan 70–90% utilization to cover support, interruptions, training, and leave. If work is frequently reactive, start lower and adjust after tracking for two to four weeks.
3) When should I use rework versus buffer?
Use rework for expected iteration you can explain, like retesting or design changes. Use buffer for unknowns that may appear later, like requirement gaps or supply delays. Keeping them separate helps you learn where variance comes from.
4) Why is my effective capacity unexpectedly low?
High overhead and low utilization compound quickly. Even small meeting load increases reduce effective weekly hours. Check overhead first, then validate productivity and utilization against actual calendars and time tracking.
5) How do I size work that needs multiple specialized roles?
Run separate sizing passes per role group (design, build, verification) using role-specific hours per item and capacity. The schedule is governed by the most constrained role. Summing all hours helps budgeting, but bottlenecks drive duration.
6) Can I use the cost estimate for vendor planning?
Yes. Enter a blended hourly rate that includes vendor fees, internal oversight, and tools. The estimate reflects effort-based cost, not materials or fixed charges. For procurement, export CSV/PDF and attach assumptions used for rework and buffer.