ISO Week and Date Converter

Overview

Production planning, shift rosters, financial reporting weeks and weekly report headings all ask for a week number, yet the week number people say out loud is often not the ISO 8601 week number at all. The ISO rule rests on two hard requirements: a week begins on Monday, and the first week of the year is the one containing the fourth of January. The consequence is that 31 December can belong to week one of the following year while 1 January can still sit in week 52 or 53 of the year before. This converter applies both rules in either direction. Given a date it reports the ISO year, the week number, the weekday, the Monday and Sunday of that week and the day of the year. Given a year and a week it reports the date range, and when the requested week exceeds the actual weeks in that year it says so instead of inventing a date.

Features

• Date to week: ISO year, week number, weekday, week start and end, and the day of the year
• Week to date: enter a year and week to get the Monday and Sunday, filling a copyable range
• Cross-year weeks detected automatically, so late December can resolve to week one of the next year
• Year length validation reports the true week count when 53 is requested from a 52 week year
• Weeks always start on Monday and end on Sunday, matching ISO 8601 and common production calendars
• Copy the range as 2026-W01 (2025-12-29 to 2026-01-04), or export a single CSV row

How to use

1. Open the date to week tab and pick a date
2. Read the four headline rows: ISO year, week number, weekday and the week range
3. Switch to the year and week tab when working backwards from a week number
4. Check the week count for that year, since 53 appears only in specific years, and note any out-of-range warning
5. Copy the range text or export CSV for a planning sheet

Notes

• The ISO year is not the calendar year: 31 December can fall in week one of the next year, and 1 January can belong to the last week of the previous one
• Weeks start on Monday, which matches European and Chinese planning practice but differs from US calendars that open the week on Sunday
• Week one is the week containing 4 January, not the week containing 1 January, and the two conventions can differ by a full week at the start of a year
• Years with 53 weeks are irregular: 2015, 2020 and 2026 all have one, so the count cannot be derived from the leap-year cycle
• An out-of-range week is reported rather than silently corrected, because quietly rewriting week 53 into week one would shift an entire production schedule
• The tool covers Gregorian dates and ISO weeks only; lunar calendars, retail 4-5-4 weeks and custom fiscal weeks are out of scope
• Ranges are inclusive on both ends, so a week listed as 2025-12-29 to 2026-01-04 spans seven days as expected
• ISO weeks start on Monday and week one is the week containing the first Thursday, which is why late December dates can belong to the next ISO year; the tool reports both the ISO year and the calendar year so that overlap is visible.