WatchTimeWhat Week Number Is It?
The current ISO 8601 week number, plus a lookup for any date. ISO weeks start on Monday and are the standard in European business, manufacturing, and logistics.
Week number for a date
Uses the ISO 8601 definition: weeks run Monday–Sunday, and week 1 is the week containing the year's first Thursday.
How ISO week numbers are defined
ISO 8601 numbers the weeks of the year 1 to 52 or 53, with two rules. Weeks run Monday through Sunday. And week 1 is the week that contains the first Thursday of January — equivalently, the week containing 4 January. The consequence: the first few days of January can belong to week 52 or 53 of the previous ISO year, and the last days of December can belong to week 1 of the next. When 1 January falls on a Friday, for example, that Friday is still part of the old year's final week.
Because of this, ISO defines a "week-based year" that can differ from the calendar year by a few days at each end. Date formats write it with a W: 2026-W33 means the 33rd ISO week of week-year 2026. Software that mixes up the calendar year and the week-year produces the classic bug where late-December dates print with the wrong year.
Why some years have 53 weeks
52 weeks is only 364 days, so one or two days a year are left over, and every five to six years they accumulate into an extra week. A year has 53 ISO weeks when it starts on a Thursday, or when it's a leap year starting on a Wednesday. Recent and upcoming long years: 2015, 2020, 2026, 2032. If your company runs weekly reporting, week 53 is worth planning for — annual comparisons of "week 40 this year vs last year" silently misalign around long years.
Not everyone counts weeks the same way
North American calendars traditionally start the week on Sunday, and some US systems number weeks from whichever partial week contains 1 January ("week 1" even if it's two days long). That scheme can disagree with ISO by one week for most of the year. The Middle East commonly uses Saturday or Sunday week starts. When someone quotes a week number across borders — factory delivery in "week 47", a rental changeover in "week 28" — confirm the standard; European industry almost always means ISO.
Frequently asked questions
How many weeks are in a year?
52 full weeks plus one day (two in leap years). In ISO numbering, most years have 52 weeks; roughly one year in five or six has 53.
What week is a given date in Excel or Sheets?
Use ISOWEEKNUM(date) in both. Plain WEEKNUM(date) defaults to the US Sunday-start scheme and will often be one off from ISO.
Which countries actually use week numbers day-to-day?
Germany, the Nordics, and the Netherlands most of all — meeting invitations, factory schedules, and school calendars routinely say "KW 33" (Kalenderwoche) or "vecka 33" rather than a date range.
Planning around a week-numbered deadline? Convert it to exact dates here, then count working days with the date calculator.