Week Number Calculator

Find the ISO week number for any date. See which week of the year a specific date falls in.

Week Number Calculator tool

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Example

Input: Date 2026-09-25

Result: ISO Week Number W39 of 2026 (Friday, day 268 of 365; week of Monday, September 21 to Sunday, September 27, 2026)

ISO week 1 of 2026 starts Monday, Dec 29, 2025 (the week with Jan 1, a Thursday); Sep 25 is 270 days later, in week 39.

ISO Weeks per Year, 2024 to 2032

YearJan 1 falls onWeek 1 startsISO weeks
2024MondayMon, Jan 1, 202452
2025WednesdayMon, Dec 30, 202452
2026ThursdayMon, Dec 29, 202553
2027FridayMon, Jan 4, 202752
2028SaturdayMon, Jan 3, 202852
2029MondayMon, Jan 1, 202952
2030TuesdayMon, Dec 31, 202952
2031WednesdayMon, Dec 30, 203052
2032ThursdayMon, Dec 29, 203153
  1. Enter a date using the date picker to find which ISO 8601 week number it falls in.
  2. View the week number along with the Monday start date and Sunday end date of that ISO week.
  3. Check the ISO week-numbering year, which can differ from the calendar year for dates in late December or early January.
  4. Review the extra details - day of the year, day of the week, quarter, and days remaining in the year.
  5. Tap the copy button next to any value, such as the week number in W05 format, to paste it into schedules or reports.

ISO Week Numbers Explained

The week number tells you which week of the year a date falls in. This tool uses ISO 8601, the international standard: weeks start on Monday, and week 1 is the week that holds the first Thursday of the year. September 25, 2026, for example, is in week 39 (written W39).

The Thursday rule means week 1 always has at least four days in the new year. It can also mean the first days of January belong to the last week of the year before. January 1, 2027 is a Friday, so it sits in week 53 of 2026. That is why the tool shows the ISO week year next to the week number. The two can differ from the calendar year in late December and early January.

Most years have 52 ISO weeks, but some have 53. That happens when the year starts on a Thursday, or on a Wednesday in a leap year. 2026 is one of those years.

Next to the week, you also get the Monday and Sunday that bound it. You also see the day of the year, the weekday, the quarter and how many days are left in the year. Europe, much of Asia and many businesses plan in ISO weeks. US calendars often start weeks on Sunday and count them in a different way, so a US planner may be one week off. To check a single date, the Day of the Week Calculator is quicker. To plan by business days, try the Working Days Calculator.

Frequently Asked Questions

Open the tool and it fills in today's date. The ISO week number shows at the top in W05 style, with the Monday and Sunday of that week below it. The answer follows ISO 8601, so weeks start on Monday and week 1 is the one that holds the year's first Thursday.
Yes. January 1, 2026 is a Thursday, so 2026 has 53 ISO weeks. Week 1 runs from Monday, December 29, 2025 to Sunday, January 4, 2026. Week 53 runs from December 28, 2026 to January 3, 2027. The next year with 53 weeks is 2032.
If January 1 falls on a Friday, Saturday or Sunday, most of that week lies in the old year. ISO 8601 gives the whole week to the year that holds its Thursday. So January 1, 2027, a Friday, belongs to week 53 of 2026. Week 1 of 2027 starts on Monday, January 4.
Not always. Many US calendars start the week on Sunday and call the week with January 1 week 1. ISO weeks start on Monday and use the Thursday rule. The two systems often match, but they can be one week apart, mostly around New Year. Check which one a schedule uses.

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