Week Number Calculator

What this tool does

Week Number Calculator applies the ISO-8601 rule, weeks start on Monday and week 1 is the week containing the year’s first Thursday, to find the week number and week-numbering year for any date, along with the Monday-to-Sunday date range covered by that week.

Several industries, especially manufacturing, logistics and payroll, refer to dates by week number rather than calendar date, but "week number" is genuinely ambiguous: different systems disagree on whether weeks start on Sunday or Monday, and on exactly which week counts as week 1 of the year.

This calculator uses the internationally standardised ISO-8601 definition: weeks always start on Monday, and week 1 of a year is specifically the week containing that year’s first Thursday. Because of that rule, the first few days of January can sometimes belong to week 52 or 53 of the previous ISO year, and the ISO week-numbering year can differ from the plain calendar year right at the boundary, both of which this calculator reports explicitly.

How to use Week Number Calculator

1. Enter the Date you want the week number for.
2. Read the ISO week number result.
3. Check the ISO week-numbering year, especially near 1 January, where it can differ from the calendar year.
4. Check the Week range (Mon–Sun) to see exactly which dates that week covers.

Tips for better results

• If a date in early January shows a week number in the 52s or 53s, that is correct ISO-8601 behaviour, not an error: it means that date belongs to the last week of the previous ISO year.
• The "Week of month" figure shown is a simpler, non-ISO approximation for quick reference; it is not the standardised figure and will sometimes disagree with the ISO week number.
• ISO weeks always run Monday to Sunday; if your organisation uses Sunday-to-Saturday weeks, this tool’s week boundaries won’t match that convention.
• Some years have 53 ISO weeks instead of the usual 52, which happens whenever 1 January falls on a Thursday (or on a Wednesday in a leap year).

Common use cases

• Converting a calendar date into the ISO week number used by manufacturing, logistics or payroll date codes.
• Checking the date range covered by a specific week number mentioned in a report or production schedule.
• Resolving the ISO week-numbering year ambiguity for dates right around 1 January.
• Cross-checking a "week of year" figure reported by another system against the standardised ISO-8601 definition.

Common mistakes to avoid

• Assuming the ISO week-numbering year always matches the calendar year; near 1 January the two can legitimately differ by one.
• Expecting weeks to start on Sunday; ISO-8601 weeks always start on Monday, which differs from some regional or software conventions.
• Treating the "Week of month" approximation shown alongside as equivalent to the standardised ISO week number; they use different, incompatible counting rules.