Date & Time

ISO Week Number Calculator

Gives the ISO 8601 week number, weekday, day of year and quarter for every date you paste.

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How to use this tool

  1. Paste your dates, one per line.
  2. Select Calculate.
  3. Read the ISO week column, not just the number — around New Year the year part differs from the calendar year, and that is where week-based reporting breaks.

What iso week calculator does

ISO week numbering has one rule that surprises people every January: a week belongs to whichever year its Thursday falls in. That is why 1 January 2021 sits in week 53 of 2020, and why reporting that groups by week and by calendar year at the same time double-counts the boundary. This page shows the ISO year alongside the week number so the mismatch is visible.

The other columns are the ones that go into a date dimension — weekday, day of year and quarter. Building or checking a calendar table is the usual reason to want all of these at once for a list of dates, and having them in one grid makes a generated dimension easy to spot-check against.

Frequently asked questions

Because an ISO week belongs to whichever year its Thursday falls in. 1 January 2021 was a Friday, so its week — Monday 28 December to Sunday 3 January — had its Thursday in 2020 and is week 53 of 2020. The ISO year is shown alongside the number for exactly this reason.

Monday, because that is what ISO 8601 specifies. North American reporting conventions often start on Sunday and number weeks differently; if your reports use that convention, this page’s numbers will not match them and neither is wrong — they are different standards.

Because grouping by ISO week and by calendar year at the same time splits a single week across two years. Group by the ISO year and ISO week together, or accept that one week each January is reported twice at partial size.