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CALCPARK

Week Number Calculator

See the current week number with its Monday-to-Sunday dates, find the ISO week of any date, switch to US Sunday-start numbering and list every week of the year.

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How to use it

  1. The current week number is shown for today as soon as the page opens, with its Monday-to-Sunday dates.
  2. Pick any other date to get its week number; use “Back to today” to return.
  3. Switch the numbering to US / Sunday start if your planner or employer counts weeks that way.
  4. Scroll the list of all weeks of a year to see the date range of every week; click a week number to select it.

Example

Monday, 19 October 2026 is in ISO week 43 of 2026, which runs from Monday 19 October to Sunday 25 October. It is an odd week; 2026 has 53 ISO weeks, so 10 weeks follow. In US numbering the same day is also week 43, but the week runs Sunday 18 to Saturday 24 October. On the other hand, Friday 1 January 2027 is still in ISO week 53 of 2026, while the US system calls it week 1 of 2027.

How week numbers are calculated

ISO 8601: find the Thursday of the week containing the date (Monday-based). The year of that Thursday is the week-based year, and the week number is the number of whole weeks between the Monday on or before 4 January of that year and the date, plus one.

ISO week = ⌊(thursdayOfWeek − mondayOfWeek1) / 7⌋ + 1
mondayOfWeek1 = the Monday on or before 4 January
weeks in year = 53 if 1 Jan is Thursday (or Wednesday in a leap year), else 52

US / Sunday-start numbering: week = ⌊(dayOfYear − 1 + weekday(1 Jan)) / 7⌋ + 1with Sunday = 0. Week 1 may be a single day, and the first and last weeks are cut at the year boundary, so a year has 53 weeks – or 54 when a leap year begins on a Saturday (2028 is the next such year).

Neither system depends on the time of day or the time zone; dates are handled as plain calendar dates. ISO numbering is the standard in Europe, in most business software and in the %V / II date formats; US and Canadian wall calendars usually print the Sunday-start numbers.

Sources: ISO 8601 week date, ISO 8601-1:2019 – Date and time representations.

Questions

What week number is it right now?
The result panel shows the ISO week of today together with the Monday and Sunday that bound it. Dates in the first or last days of a year can belong to the neighbouring ISO year – the panel says so when that happens.
Why does my calendar show a different week number?
Many North American calendars start weeks on Sunday and number the week of 1 January as week 1. Switch to “US / Sunday start” to match them. Outlook, Google Calendar and most European systems default to ISO 8601.
How many weeks does the year have?
52 ISO weeks in most years, 53 in years that start on a Thursday or are leap years starting on a Wednesday (2015, 2020, 2026, 2032). The “All weeks of a year” table lists them with dates.
Is this week odd or even?
Look at the “Odd or even week” row – it uses the ISO (or chosen) week number, which is what school and shift timetables normally refer to.