About Week Number Lookup

Find the ISO week number for any date. Part of the DevTools Surf developer suite. Browse more tools in the Date / Time collection.

Use Cases

• Aligning sprint numbers with ISO week numbers for planning
• Generating weekly report headers with correct week identifiers
• Mapping manufacturing batch codes to ISO week timestamps
• Scheduling recurring tasks aligned with ISO week boundaries

Tips

• Enter any date to find its ISO 8601 week number instantly
• Note that ISO weeks start on Monday, not Sunday
• Check which year an ISO week belongs to at year boundaries

Fun Facts

• ISO 8601 defines week 1 as the week containing the first Thursday of January, which can cause the last days of December to fall in week 1 of the next year.
• The ISO week numbering system was standardized in 1988 and is widely used in European business and manufacturing for production planning.
• A year can have either 52 or 53 ISO weeks; years with 53 weeks occur roughly every 5-6 years.

FAQ

Which week numbering?

ISO 8601 by default (week 1 is the week containing the first Thursday; weeks start Monday). Toggle for US-style (week 1 is the week containing January 1; weeks start Sunday).

Does it match Excel's WEEKNUM?

Excel has both — `WEEKNUM` (US default) and `WEEKNUM(date, 21)` (ISO). The tool lets you pick; choose to match your data source.

What's a 'week-year'?

The year the week belongs to, which can differ from the calendar year around January. The tool shows both so you can reference ISO week-year (2023-W52 might be Jan 1 2024).

How many weeks in a year?

Usually 52, sometimes 53. Weeks that straddle year-end belong to one year or the other per the numbering rules.