Week Number Calculator
Find the ISO week number for any date of the year
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Saturday, October 24, 2026 · 01:49 PM
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About this tool
Find the ISO week number for any date - a standardized way of numbering weeks within a year (1 through 52 or 53), commonly used in business planning, project scheduling, and international contexts where "week 23" is clearer than a specific date range.
How to use this tool
- Enter target datePick any date you want to map to ISO week format.
- Calculate ISO week numberResolve week index using ISO-8601 Monday-based standard.
- Review year/week contextConfirm whether date falls into week 52/53 edge conditions.
- Use in planning systemsApply week number in reports, sprint plans, or logistics schedules.
How ISO week numbers work
ISO weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of the year - which is the same as the week containing 4 January. So the first few days of January can belong to week 52 or 53 of the previous year.
Most years have 52 ISO weeks; a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. Week numbers are widely used in Europe for planning, while US calendars often number weeks differently.
Key features
- Standardizes week-based planning across regions and teams.
- Avoids ambiguity from locale-specific calendar conventions.
- Useful for operations, manufacturing, and sprint reporting.
- Handles edge cases around new-year week boundaries.
Common uses
- Mapping dates to fiscal/operational week dashboards.
- Aligning international project reporting cadence.
- Planning production and shipment windows by ISO week.
- Labeling weekly releases and sprint timelines.
Tips for better results
- Use ISO week convention consistently across all stakeholders.
- Check year transition weeks carefully near January 1st.
- Document week-number standard in reports and templates.
- Validate BI tools use same week-start assumptions.
Mistakes to avoid
- Mixing US week conventions with ISO reporting.
- Assuming every year has exactly 52 ISO weeks.
- Ignoring that week 1 may start in previous calendar year.
- Using week number without corresponding week-year context.
Frequently asked questions
The tool opens on today by default, so the current ISO week number is shown as soon as the page loads. If you have navigated away from today, the Today button jumps straight back to the current week.
The current ISO week number is also the count of weeks elapsed - week 35 means 34 complete weeks have finished and you are in the 35th. Note that ISO weeks start on Monday and week 1 is the week containing the first Thursday of January, so the very start of January can belong to the previous year.
It refers to ISO 8601, an international standard that defines weeks as starting on Monday, with week 1 being the week containing the year's first Thursday - this standardization avoids ambiguity between different regional week-numbering conventions.
52 weeks account for 364 days, one day short of a full year (365 or 366 days) - the leftover day(s) occasionally push the year into having a 53rd week under the ISO definition, depending on which day of the week the year starts and ends on.
Using the ISO 8601 standard specifically, yes - though some non-ISO conventions (like certain US business calendars) may number weeks slightly differently, which is exactly why using the standardized ISO version avoids confusion.
Under ISO 8601, week 1 is the week that contains the first Thursday of the year - equivalently, the week with 4 January in it. So 1 January can belong to week 52 or 53 of the previous year.
Some calendars, especially in the US, count weeks from Sunday and start week 1 on 1 January. ISO weeks start on Monday and use the first-Thursday rule, so the two systems can differ by one around the new year.
Both have a setting to show week numbers: in Google Calendar under Settings > View options, and in Outlook under Calendar options. Make sure the first day of the week is Monday if you want them to match ISO weeks.