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Convert any date to its ISO 8601 week (1-53) and weekday (Mon=1), or reverse from year+week to Monday-Sunday dates. Matches Excel ISOWEEKNUM exactly.

📘 How to Use

  1. Pick "Date → Week" or "Week → Date" from the tabs at the top
  2. Enter a date, or enter a year and week number
  3. Check the ISO week year, week number and weekday — or the Monday and Sunday dates

ISO Week Number Calculator

Enter as YYYY-MM-DD (ISO format).

ISO Week Year

2026

ISO Week

22 / 53

Denominator is the total weeks (52 or 53) in that ISO year.

ISO Weekday

1 Mon

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ISO Week Number Calculator | Convert Between Dates and W## Notation

Look up the ISO 8601 week number for any date, or reverse a "W##" / "KW##" reference back to a Monday-Sunday range. The tool mirrors Excel's ISOWEEKNUM function (equivalent to WEEKNUM with return_type 21) and displays the ISO week-year separately, so cross-year edge cases like Jan 1 belonging to the previous year's W52 don't catch you off guard.

💡 About this tool

Anyone working with European partners, manufacturing schedules, retail fiscal calendars, or sprint plans keyed to a week number has hit the same friction: someone writes "W35 shipment", "KW22 release", or "fiscal week 14", and the rest of the team has to translate that back into a Monday-Sunday range. The reverse case is just as common — picking a real date for an internal meeting when the upstream partner only deals in week numbers.

This calculator handles both directions on the same page. A segmented control switches between modes, and inputs update results in real time. The "ISO week year" field is shown separately from the calendar year, so dates like 2023-01-01 (which belongs to ISO week-year 2022) are unambiguous.

🧐 Frequently Asked Questions

Q: How does ISO 8601 differ from US/Canadian week numbering? A: ISO weeks start on Monday, and Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4). The US system starts weeks on Sunday and counts the week containing January 1 as Week 1. Results can differ by one or two weeks near year boundaries.

Q: Why does January 1 sometimes show up as W52 of the previous year? A: When the first days of January fall before that year's first Thursday, ISO 8601 assigns them to the last week of the previous ISO year. For example, 2023-01-01 is reported as 2022-W52. The tool surfaces this by showing "ISO Week Year" separately from the calendar year.

Q: How do I know whether a year has 52 or 53 weeks? A: A year has 53 ISO weeks when January 1 or December 31 falls on a Thursday. The "Week → Date" mode of this tool caps the slider at 52 or 53 for the year you enter, and shows the total as a denominator (e.g. "30 / 53").

Q: Does this match Excel's ISOWEEKNUM? A: Yes. The same standard algorithm (shift to the Thursday of the input week, then divide the day-of-year offset by 7) is used. Output should match ISOWEEKNUM and WEEKNUM(date, 21) exactly.

Q: What format should I enter the date in? A: YYYY-MM-DD (the native HTML date input). The browser's date picker handles locale display.

Q: What happens if I enter a week number that doesn't exist for that year? A: An error notice appears and the Monday/Sunday outputs stop updating until a valid week is entered. For example, asking for W53 in 2025 (a 52-week year) is rejected.

📚 Why Monday weeks ended up as the international standard

ISO 8601 codified Monday-starting weeks partly because European retail and finance had already settled on them. Quarterly reporting cycles built on "13 weeks × 4" depend on each week having the same number of business days, which only works cleanly when the week boundary lines up with the work week. The "Thursday rule" for Week 1 was chosen because it produces the most balanced years — at most three days of January can fall in the previous ISO year, and at most three days of December can fall in the next.

In practice, you're most likely to encounter ISO weeks in three places: sprint planning tools that label cycles by week number, manufacturing or logistics schedules that reference "W##", and German-speaking offices that use the abbreviation "KW" (Kalenderwoche) in daily conversation about deadlines.