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Calendar Weeks.

The current ISO calendar week at a glance — plus every week of any year, and which week any date falls into.

Developed and hosted in the EU
Calendar Weeks
ISO 8601 · weeks start Monday
Runs entirely in your browser
Current calendar week
W–

All weeks of
Computed from your device's clock, entirely in your browser — nothing is transmitted or stored.

How ISO calendar weeks work

ISO 8601 — the standard across Europe — defines weeks that start on Monday and numbers them from the week containing the year's first Thursday (the week of January 4). Every date belongs to exactly one week, written like 2026-W28. Because weeks do not align with month or year boundaries, the days around New Year can belong to the other year's first or last week.

52 weeks — usually

Most years have 52 ISO weeks, but a year that starts on a Thursday — or a leap year starting on a Wednesday — squeezes in a week 53. 2026 is such a long year; the next ones are 2032 and 2037. Software that hard-codes 52 weeks per year quietly breaks in those years, so treat the week count as data, not as a constant.

Week numbers as version numbers

Calendar-based versioning (CalVer) uses the year and week as a release number — 2026.28 or 26w28 — so every artifact instantly tells you when it was built and how old it is. Ubuntu, JetBrains and many internal platform teams version this way. The copy buttons above produce exactly these formats.

Careful with US week numbers

The North American convention starts weeks on Sunday and calls the week containing January 1 "week 1" — which often runs one number ahead of ISO. The same trap exists in software: Excel's WEEKNUM() uses the US system by default, while ISOWEEKNUM() matches this page. When exchanging week numbers across teams, always say which system you mean.

Week numbers in your code

JavaScript
no built-in — use a helper
function isoWeek(date) {
  const d = new Date(Date.UTC(
    date.getFullYear(), date.getMonth(), date.getDate()));
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil(((d - yearStart) / 86400000 + 1) / 7);
}
Python
built-in isocalendar()
from datetime import date

today = date.today()
year, week, weekday = today.isocalendar()

print(f"{year}-W{week:02d}")  # e.g. 2026-W28

Frequently asked questions

What calendar week is it right now?
The tool at the top of this page always shows the current ISO calendar week, computed live from your device's clock — including the exact Monday-to-Sunday date range it covers.
When does week 1 of a year begin?
Under ISO 8601, week 1 is the week that contains the year's first Thursday — equivalently, the week containing January 4. This means week 1 can start in late December, and the last days of December can already belong to week 1 of the next year.
Why does my calendar app show a different week number?
There are several numbering systems. ISO 8601 (used across Europe and in this tool) starts weeks on Monday. The US system starts weeks on Sunday and calls the week containing January 1 week 1 — so US week numbers often run one ahead of ISO numbers. Check your calendar app's week-numbering setting.
How many weeks does a year have?
52 or 53 ISO weeks. A year has 53 weeks when it starts on a Thursday — or when it is a leap year starting on a Wednesday. 2026 is such a "long year" with 53 weeks; the next ones are 2032 and 2037.
Can a date belong to a week of a different year?
Yes. December 29–31 can fall into week 1 of the following year, and January 1–3 can belong to week 52 or 53 of the previous year. That is why the ISO standard pairs the week number with a "week-based year", written like 2026-W53.
How do I get the ISO week number in Excel?
Use =ISOWEEKNUM(A1). Note that the older =WEEKNUM(A1) defaults to the US system (weeks start Sunday, week 1 contains January 1) and often returns different numbers.

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