📅 Week Number Converter (ISO week)

Convert dates to ISO week numbers (YYYY-Www), or convert an ISO week (year + week) into the date range (Monday → Sunday) or the Monday date. Batch CSV mode supported.

Date → Week Number



Result: -

Week Number → Date Range





Result: -

Batch Mode (CSV Upload)

Upload CSV lines in either format (header optional):

  • date (YYYY-MM-DD) — returns YYYY-Www (ISO week)
  • year,week — returns Monday date and date range

Examples:

2025-03-18
2024-12-31
2025,12

ISO Week Numbering — rules, history and uses

Week numbers are commonly used in business planning, scheduling, software versioning, logistics, and reporting. The ISO-8601 standard defines week numbering used by many countries and organizations worldwide. Unlike simple "week of month" schemes, ISO weeks are tied to years and guarantee that each week begins on Monday and belongs to exactly one ISO year-week label (for example, 2025-W01).

1. ISO Week Rules (brief)

  • Weeks start on Monday.
  • The first week of an ISO year (week 01) is the week with the year's first Thursday in it, equivalently the week that contains January 4th.
  • ISO years may have 52 or 53 weeks. A year has 53 ISO weeks if it contains 53 Thursdays (equivalently if Jan 1 is Thursday in a non-leap year or Wednesday in a leap year), otherwise 52.
  • Dates near the start or end of a calendar year may belong to the previous or next ISO year. Example: 2014-12-29 is ISO week 2015-W01.

2. Why this matters

ISO week numbers avoid ambiguity in cross-border schedules and are especially useful for repeating weekly reports, manufacturing planning, and ISO-compliant documentation. They are used by ERP systems, payroll, retail promotions, and project sprints.

3. Examples

  • 2025-01-01 (Wednesday) is in ISO week 2025-W01 because the week contains Jan 4.
  • 2020-12-31 (Thursday) is in ISO week 2020-W53 (2020 had 53 weeks).
  • A date like 2015-01-01 (Thursday) is in ISO week 2015-W01.

4. Worked conversions

Date → ISO week: 2025-03-18 → Monday is 2025-03-17 → week number 2025-W12 (example).

ISO week → range: 2025-W12 → Monday 2025-03-17, Sunday 2025-03-23.

5. Edge cases

Because ISO weeks can cross calendar boundaries, be careful when grouping data by week near Jan 1 or Dec 31. Always include the ISO year when you store week numbers (YYYY-Www), never just the week number alone.

6. Batch usage & automation

Batch conversion is useful when you have many timestamps to map into weekly buckets for analytics. The CSV mode on this page accepts either single-date lines or year+week lines for conversion both ways; the output keeps a clear result field so you can import it into spreadsheets or databases.

7. Frequently asked questions

Is ISO week used everywhere?

Not everywhere — the US commonly uses "week of month" or Sunday-start weeks. But many international organizations and European countries use ISO weeks.

Does ISO week always start Monday?

Yes. ISO defines Monday as the first day of the week.

How can a year have 53 weeks?

If the year contains 53 Thursdays (or equivalently Jan 1 is Thursday in a non-leap year or Wednesday in a leap year), ISO assigns 53 weeks that year.

Should I store ISO week as text or number?

Store both year and week (e.g., fields 'iso_year' and 'iso_week') or a combined string 'YYYY-Www' to avoid ambiguous sorting.

How do I convert timezones with weeks?

Determine date in target timezone first, then compute ISO week — timezone shifts can move a timestamp across a date boundary and thus change the week.

8. Conclusion

Week numbers — when used with the ISO standard — provide a robust, unambiguous way to reference weekly intervals across systems and geographies. This converter helps you map dates to weeks and weeks back to date ranges for reporting, scheduling, and automation.