Calculate the calendar dates covered by a given ISO-8601 week number and week-year. The calculator runs entirely in your browser and pairs a focused answer with the method, assumptions and the related tools worth checking next.
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Enter an ISO week-year, a week number (1 to 53) and a day of the week. The calculator finds the Monday of ISO week 1 for that week-year, adds the requested whole weeks, and reports the chosen weekday’s date plus the Monday and Sunday that bound the week.
ISO week 1 is the week containing 4 January, and every ISO week runs Monday to Sunday. The conversion counts forward in whole weeks from that week’s Monday, so a week-year can start in late December of the previous calendar year.
ISO week 2026-W43 runs from Monday 19 October to Sunday 25 October 2026, so its Friday is 23 October 2026, with the ISO designation 2026-W43-5.
Only some week-years have 53 weeks: 2026 does, but 2025 does not, so asking for 2025-W53 returns a message that the week does not exist. Week 1 of 2026 starts on 29 December 2025, so early ISO dates often sit in the previous calendar year.
Enter the ISO week-year, week number and day of the week, then select Calculate. Each step below is a short action rather than a wall of explanation.
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Find the Monday of ISO week 1 for the week-year, add the requested number of whole weeks, then offset to the chosen weekday.
Similar tools answer different questions. Match the inputs and the reference point to the answer you actually need.
The numbers come from your inputs; this shows the shape of the calculation.
Enter the ISO week-year — week 1 is the week containing 4 January, so a week-year can start in late December of the previous calendar year.
Enter the week number, 1 to 53, and the Day of week within it; every ISO week runs Monday to Sunday.
The chosen weekday's date, with the Monday and Sunday that bound the week: ISO week 2026-W43 runs 19 to 25 October 2026, so its Friday is 23 October 2026 — 2026-W43-5.
Use exact source inputs, keep the reference date visible and verify any official rule separately.
Set the reference date to the event that actually controls the decision, not to today.
Compare the stated method here against the other tool's convention before assuming an error.
For a neighbouring calculation, open the Days Between Dates Calculator or the Date Difference Calculator. To understand the reasoning first, read How to Add Months to a Date Safely.
Choose another focused tool when the underlying question changes.
Understand the calendar rules, edge cases and next steps behind this calculator.
Answers about method, inputs, privacy, boundaries and responsible interpretation.
ISO week 1 is the week containing 4 January, and every ISO week runs Monday to Sunday. The converter finds the Monday of week 1 for the week-year entered, adds the requested whole weeks, and reports the chosen weekday's date plus the Monday and Sunday that bound the week.
It runs from Monday 19 October to Sunday 25 October 2026. Its Friday is 23 October 2026, which carries the ISO designation 2026-W43-5.
Because week 1 is anchored on the week containing 4 January rather than starting on 1 January itself. Week 1 of 2026 starts on Monday 29 December 2025, so a week-year can begin in late December of the previous calendar year and early ISO dates often sit there.
No. Only some week-years have 53 weeks: 2026 does, but 2025 does not, so asking for 2025-W53 returns a message that the week does not exist.
The ISO week-year, which is not always the calendar year of the date you end up with. Around New Year a date can belong to the neighbouring week-year, which is why the converter takes the week-year, the week number (1 to 53) and the day of the week as separate inputs.
Navjeet Kamboj is the author and editor of Age Calculator Lab. He focuses on making age, date, milestone and planning calculations easier to understand through clearly explained methods, practical examples and responsible limitations.