What is an ISO week number?

ISO 8601 week numbers divide the year into 52 or 53 weeks, with each week starting on Monday. Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4). The last week of the year ends on the Sunday before the new ISO year begins.

When does ISO week 1 start?

ISO Week 1 starts on the Monday of the week that contains the first Thursday of January. This means Week 1 can start as early as December 29 of the previous year, or as late as January 4 of the current year.

What year does December 31 belong to for ISO week numbering?

December 31 belongs to the ISO year of the week it falls in, not necessarily the calendar year. For example, December 31, 2018 was a Monday — it belonged to ISO week 1 of 2019. This is why the ISO year can differ from the calendar year for dates in late December or early January.

What is the difference between ISO week numbers and US week numbers?

ISO weeks start on Monday and use the first Thursday rule for Week 1. US weeks start on Sunday, and Week 1 is always the week containing January 1. This means the two systems can differ by up to 1 week for the same date.

Week Numbers

All ISO week numbers for any year with Unix timestamps.

ISO weeks(Mon–Sun)

Week	Start	End	Start Timestamp	End Timestamp	Days
W1	Dec 29, 2025	Jan 4, 2026	1766966400	1767571199	7
W2	Jan 5, 2026	Jan 11, 2026	1767571200	1768175999	7
W3	Jan 12, 2026	Jan 18, 2026	1768176000	1768780799	7
W4	Jan 19, 2026	Jan 25, 2026	1768780800	1769385599	7
W5	Jan 26, 2026	Feb 1, 2026	1769385600	1769990399	7
W6	Feb 2, 2026	Feb 8, 2026	1769990400	1770595199	7
W7	Feb 9, 2026	Feb 15, 2026	1770595200	1771199999	7
W8	Feb 16, 2026	Feb 22, 2026	1771200000	1771804799	7
W9	Feb 23, 2026	Mar 1, 2026	1771804800	1772409599	7
W10	Mar 2, 2026	Mar 8, 2026	1772409600	1773014399	7
W11	Mar 9, 2026	Mar 15, 2026	1773014400	1773619199	7
W12	Mar 16, 2026	Mar 22, 2026	1773619200	1774223999	7
W13	Mar 23, 2026	Mar 29, 2026	1774224000	1774828799	7
W14	Mar 30, 2026	Apr 5, 2026	1774828800	1775433599	7
W15	Apr 6, 2026	Apr 12, 2026	1775433600	1776038399	7
W16	Apr 13, 2026	Apr 19, 2026	1776038400	1776643199	7
W17	Apr 20, 2026	Apr 26, 2026	1776643200	1777247999	7
W18	Apr 27, 2026	May 3, 2026	1777248000	1777852799	7
W19	May 4, 2026	May 10, 2026	1777852800	1778457599	7
W20	May 11, 2026	May 17, 2026	1778457600	1779062399	7
W21	May 18, 2026	May 24, 2026	1779062400	1779667199	7
W22	May 25, 2026	May 31, 2026	1779667200	1780271999	7
W23Current	Jun 1, 2026	Jun 7, 2026	1780272000	1780876799	7
W24	Jun 8, 2026	Jun 14, 2026	1780876800	1781481599	7
W25	Jun 15, 2026	Jun 21, 2026	1781481600	1782086399	7
W26	Jun 22, 2026	Jun 28, 2026	1782086400	1782691199	7
W27	Jun 29, 2026	Jul 5, 2026	1782691200	1783295999	7
W28	Jul 6, 2026	Jul 12, 2026	1783296000	1783900799	7
W29	Jul 13, 2026	Jul 19, 2026	1783900800	1784505599	7
W30	Jul 20, 2026	Jul 26, 2026	1784505600	1785110399	7
W31	Jul 27, 2026	Aug 2, 2026	1785110400	1785715199	7
W32	Aug 3, 2026	Aug 9, 2026	1785715200	1786319999	7
W33	Aug 10, 2026	Aug 16, 2026	1786320000	1786924799	7
W34	Aug 17, 2026	Aug 23, 2026	1786924800	1787529599	7
W35	Aug 24, 2026	Aug 30, 2026	1787529600	1788134399	7
W36	Aug 31, 2026	Sep 6, 2026	1788134400	1788739199	7
W37	Sep 7, 2026	Sep 13, 2026	1788739200	1789343999	7
W38	Sep 14, 2026	Sep 20, 2026	1789344000	1789948799	7
W39	Sep 21, 2026	Sep 27, 2026	1789948800	1790553599	7
W40	Sep 28, 2026	Oct 4, 2026	1790553600	1791158399	7
W41	Oct 5, 2026	Oct 11, 2026	1791158400	1791763199	7
W42	Oct 12, 2026	Oct 18, 2026	1791763200	1792367999	7
W43	Oct 19, 2026	Oct 25, 2026	1792368000	1792972799	7
W44	Oct 26, 2026	Nov 1, 2026	1792972800	1793577599	7
W45	Nov 2, 2026	Nov 8, 2026	1793577600	1794182399	7
W46	Nov 9, 2026	Nov 15, 2026	1794182400	1794787199	7
W47	Nov 16, 2026	Nov 22, 2026	1794787200	1795391999	7
W48	Nov 23, 2026	Nov 29, 2026	1795392000	1795996799	7
W49	Nov 30, 2026	Dec 6, 2026	1795996800	1796601599	7
W50	Dec 7, 2026	Dec 13, 2026	1796601600	1797206399	7
W51	Dec 14, 2026	Dec 20, 2026	1797206400	1797811199	7
W52	Dec 21, 2026	Dec 27, 2026	1797811200	1798415999	7
W53	Dec 28, 2026	Jan 3, 2027	1798416000	1799020799	7

ISO 8601 Week Numbering Explained

The ISO 8601 standard defines a week numbering system where:

Each week starts on Monday
Week 1 is the week containing the first Thursday of January
A year has either 52 or 53 ISO weeks
Dates in late December can belong to ISO week 1 of the next year
Dates in early January can belong to ISO week 52/53 of the previous year

This system is used in European business contexts, project management tools, and many database systems. It ensures every week is "whole" — no week is split across two years from a working-week perspective.

ISO vs US Week Numbering

	ISO 8601	US / Sunday-based
Week starts on	Monday	Sunday
Week 1 definition	Week containing first Thursday of Jan	Week containing January 1
Weeks per year	52 or 53	52 or 53
Late-Dec / early-Jan	May belong to adjacent ISO year	Always in calendar year
Used by	Europe, ISO standards, PostgreSQL	USA, many apps (Moment.js default)

Week Numbers in Code

Python

datetime.now().isocalendar().week # ISO week number

JavaScript

// No built-in ISO week — use date-fns: getISOWeek(new Date())

PostgreSQL

SELECT EXTRACT(WEEK FROM NOW()); -- ISO week number

_, week := time.Now().ISOWeek() // returns ISO year and week

Frequently Asked Questions

What is the current ISO week number?▼

Select the current year in the week numbers tool above and the current week is highlighted. The ISO week number changes every Monday at midnight.

Can a year have 53 ISO weeks?▼

Yes. A year has 53 ISO weeks when January 1 falls on Thursday, or when it is a leap year and January 1 falls on Wednesday. This occurs roughly 5–6 times per decade.

What is the Unix timestamp for the start of ISO Week 1, 2025?▼

ISO Week 1 of 2025 starts on Monday December 30, 2024 at 00:00:00 UTC, which is Unix timestamp 1735516800.

How do I group data by ISO week in SQL?▼

In PostgreSQL: GROUP BY DATE_TRUNC('week', created_at::timestamptz) or GROUP BY EXTRACT(ISOYEAR FROM created_at), EXTRACT(WEEK FROM created_at). In MySQL: GROUP BY YEARWEEK(created_at, 3) where mode 3 is ISO 8601.

Week Numbers

ISO 8601 Week Numbering Explained

ISO vs US Week Numbering

Week Numbers in Code

Frequently Asked Questions

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