Effort and elapsed time are different quantities, and confusing them is the most common way a plan slips. 45 working days of effort from 10 August 2026 does not land 45 calendar days later — it lands on Monday 12 October 2026, 63 calendar days on, because weekends are skipped along the way.
The contingency buffer is added to the effort before the date is worked out, which is the conservative order. Adding 20% to a 45-day estimate makes it 54 working days, and applying the percentage to elapsed calendar time instead would understate the buffer by every weekend it happens to contain.
Public holidays are entered as a count and added to the working days required, since holiday calendars differ by country. The result is a date you can defend: derived from a stated effort, a stated buffer and the actual shape of the working week rather than from optimism.
The formula
S- Start date
E- Effort in working days
b- Contingency buffer
H- Public holidays in the window
How it works, step by step
- Enter the date work starts.
- Enter the effort estimate in working days.
- Set a contingency buffer as a percentage of the estimate.
- Add any public holidays that fall inside the window.
Worked examples
45 working days from 10 August 2026
Ignoring any buffer, the work finishes on Monday 12 October 2026 — 63 calendar days later, of which 18 are weekend days. The elapsed time is 1.40 times the effort.
Adding a 20% buffer
45 days plus 20% is 54 working days, which moves delivery to Friday 23 October 2026 — 11 calendar days later than the unbuffered date and 74 days from the start.
How to read your score
Frequently asked questions
Why apply the buffer to effort rather than to the date?
Because a percentage of elapsed calendar time silently includes weekends that were never working time. Adding contingency to the effort and then converting to a date keeps the buffer as real working capacity.
What buffer should I use?
Twenty per cent is a common default for work with a clear scope. Novel or dependency-heavy work often needs 30% to 50%, and the honest test is whether past projects of the same type finished inside the buffer you chose.
Does this account for more than one person?
No — the effort figure is elapsed working days for the team as it stands. If two people halve the duration, enter the halved figure; work rarely divides that cleanly.
Are public holidays included automatically?
No. Only Saturdays and Sundays are skipped. Enter the number of public holidays falling in the window and they are added to the working days required.
Effort to delivery, from 10 August 2026
| Effort (working days) | Delivery date | Weekday | Calendar days | Elapsed ÷ effort |
|---|---|---|---|---|
| 5 | 17 August 2026 | Monday | 7 | 1.40 |
| 10 | 24 August 2026 | Monday | 14 | 1.40 |
| 20 | 7 September 2026 | Monday | 28 | 1.40 |
| 30 | 21 September 2026 | Monday | 42 | 1.40 |
| 45 | 12 October 2026 | Monday | 63 | 1.40 |
| 60 | 2 November 2026 | Monday | 84 | 1.40 |
| 90 | 14 December 2026 | Monday | 126 | 1.40 |
| 120 | 25 January 2027 | Monday | 168 | 1.40 |
The elapsed-to-effort ratio settles near 1.4 as the span grows, since five working days consume seven calendar days.