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What Week Number Is It?

The current ISO week number, and the week number for any date you choose.

The current ISO week number is shown above. Under ISO 8601, weeks run Monday to Sunday and week 1 is the week containing the first Thursday of January — which is why the first days of a year sometimes belong to week 52 or 53 of the year before.

Why the first-Thursday rule exists

It guarantees that every week belongs entirely to one year and that week 1 always contains at least four days of the new year. The cost is that 1 January is not always in week 1, which surprises people every January — the full explanation is in ISO week numbers explained.

52 or 53 weeks

Most years have 52 ISO weeks; some have 53. Payroll, manufacturing and retail planning all care, because a 53-week year quietly adds a period. Which convention your software uses depends on whether it follows ISO or a Sunday-start scheme — see why weeks start on Monday or Sunday.

Guides about Week Number

Calendar facts ISO Week Number Explained: The First-Thursday Rule Week numbers look simple until the turn of the year, when 1 January can land in week 52 of the year before. Here is the ISO 8601 rule that decides it. 5 min read Calendar facts Week Start: Why Weeks Begin On Monday Or Sunday Two wall calendars can show the same month with the columns in different orders. Here is why the week start is disputed, who uses which, and what it changes. 5 min read

More datetoday.now pages

Today's Date The date today in full, with the day of year, ISO week, quarter and days left in the year. Tomorrow's Date Tomorrow's date with the same figures — weekday, ISO form, day of year and week. Day of Year Today's ordinal day number, how much of the year has elapsed, and how many days remain. Date Calculator Add or subtract days, weeks, months, years or business days — and count the days between two dates.

Frequently asked questions

What week number is it now?
The current ISO week is shown above, along with how many weeks the current ISO year contains.
Why is 1 January sometimes in week 52?
Because ISO week 1 must contain the first Thursday of January. If 1 January falls late in a week, that week still belongs to the previous year.
Do weeks start on Monday or Sunday?
Under ISO 8601 they start on Monday. Sunday-start numbering is common in the United States and gives different week numbers.