ISO-8601 and US week numbers for any date, the Monday to Sunday range, and every week's start date in the year.
| ISO week code | – |
| Week runs | – |
| Weekday | – |
| Weeks left in ISO year | – |
| Week 1 of this ISO year began | – |
| Week | Monday | Sunday | Months |
|---|
The Week Number Calculator gives the ISO-8601 week number for any date, together with the ISO week-based year (which can differ from the calendar year around New Year), the Monday and Sunday that bound that week, and the number of weeks in the year, 52 or 53. It also shows the US-style week number, where weeks start on Sunday and week 1 is the week containing January 1, since many US calendars and spreadsheets use that convention instead. A reverse lookup turns a year and week number back into dates, and a table lists the start and end of every ISO week in the year. Useful for payroll, project plans, retail and manufacturing calendars, and anyone reading a European diary or a WEEKNUM cell.
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of January, equivalently the week containing January 4. The page computes the week as floor((dayOfYear - weekday + 10) / 7) with Monday as weekday 1; a result of 0 means the date belongs to the last week of the previous ISO year, and a result of 53 belongs to week 1 of the next year when December 31 falls on a Monday, Tuesday, or Wednesday. A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year. The US week number is floor((dayOfYear - 1 + weekdayOfJan1) / 7) + 1 with Sunday as weekday 0. All dates are parsed as local calendar dates at midnight so daylight-saving changes cannot shift a day.
Because ISO 8601 assigns whole weeks to one year, based on where the Thursday falls. If January 1 is a Friday, Saturday, or Sunday, most of that week lies in the old year, so those days are counted as the last week of the previous ISO year. January 1, 2021 was a Friday, so it and January 2 and 3 belong to week 53 of 2020.
Years that start on a Thursday, and leap years that start on a Wednesday. Recent and upcoming examples are 2015, 2020, 2026, and 2032. All other years have 52 weeks, which is why a plan built on 52 weeks drifts by one week in those years.
Excel's default WEEKNUM(date) uses the US system: weeks start on Sunday and week 1 is whichever week contains January 1, so it can return 53 or 54 and does not match ISO. Excel's WEEKNUM(date, 21) and ISOWEEKNUM(date) return the ISO week that this page shows. Google Sheets follows the same pattern.
It is the year the ISO week belongs to, written like 2020-W53. It matters when you store or sort dates by week: labelling January 2, 2021 as week 53 of 2021 would be wrong, because 2021 has only 52 weeks. Always pair an ISO week number with its week-based year, not the calendar year of the date.
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