Convert Week Number to Date
Updated: May 2026
Given an ISO year and week number, finding the exact start date (Monday) and end date (Sunday) requires knowing how ISO week 1 is anchored. The calculation is straightforward once you understand the role of January 4 as a fixed reference point.
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The conversion algorithm
To find the Monday of ISO week W in ISO year Y:
- Find January 4 of year Y. January 4 is always in ISO week 1 by definition.
- Find the day of the week of January 4 (using ISO numbering: Monday = 1, Sunday = 7).
- Subtract (day_of_week − 1) days from January 4 to get the Monday of week 1.
- Add (W − 1) × 7 days to reach the Monday of week W.
- Add 6 days to the Monday to get the Sunday (end of week W).
The Monday of week 1 can fall in December of the previous year. This is normal for years where January 1 falls on a Tuesday, Wednesday, or Thursday — in those cases the Monday before January 4 is in late December of the prior year.
Example conversions
| ISO week | Monday (start) | Sunday (end) |
|---|---|---|
| 2026-W01 | December 29, 2025 | January 4, 2026 |
| 2026-W10 | March 2, 2026 | March 8, 2026 |
| 2026-W22 | May 25, 2026 | May 31, 2026 |
| 2026-W52 | December 21, 2026 | December 27, 2026 |
| 2026-W53 | December 28, 2026 | January 3, 2027 |
| 2027-W01 | January 4, 2027 | January 10, 2027 |
Notice that ISO week 1 of 2026 starts on December 29, 2025 — three days before the calendar year begins. And week 53 of 2026 ends on January 3, 2027 — three days into the next calendar year. 2026 is a 53-week year because January 1, 2026 falls on a Thursday.
Common mistake: using the calendar year instead of the ISO year
When converting a week number to a date, always use the ISO year, not the calendar year. The two differ for dates near the year boundary. If someone tells you "week 1 of 2026," they mean ISO year 2026, which starts on December 29, 2025 — not January 1, 2026.
Similarly, if a report is labeled "week 53 of 2026," the dates within it may include days from early January 2027. Any system that treats the calendar year and ISO year as identical will misplace those dates.
Using the online calculator
The Flowfiles ISO week calculator handles this conversion in the "Week to date range" panel. Enter the ISO year and week number and click Calculate. The result shows the Monday and Sunday of that week, formatted as human-readable dates, with a copy button to paste the range directly into a report, email, or scheduling tool.
The year calendar panel also shows all weeks for any year at a glance, with each week's Monday and Sunday, so you can visually locate any week in context.