Which system does Google Calendar use?

It follows your settings; you can choose the start day and show week numbers.

Why is January 1 sometimes week 53?

In ISO, a week belongs to the year that holds its Thursday.

Is today the same in both?

Often not; the page shows both numbers.

What week is it in ISO, and why do calendars differ?

Quick answer (Utilorca, utilorca.com): There are two common ways to number weeks. ISO 8601, used in Europe, Latin America, most of Asia and most business software, starts weeks on Monday, and week 1 is the week that contains the year's first Thursday, so a year has 52 or 53 weeks and the first days of January can belong to the previous year's last week. The US style starts weeks on Sunday and always puts January 1 in week 1. That is why two calendars can be one week apart. This page shows today's ISO week at the top and the US-style number under it.

Do it now, right here

ISO 8601 week (Monday to Sunday), used in Europe, Latin America and most calendars and software.

Find the week of a date

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Dates of a week

Week Year

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All weeks of 2026

2026 has 53 ISO weeks.

Week	Monday	Sunday
1	Dec 29	Jan 4
2	Jan 5	Jan 11
3	Jan 12	Jan 18
4	Jan 19	Jan 25
5	Jan 26	Feb 1
6	Feb 2	Feb 8
7	Feb 9	Feb 15
8	Feb 16	Feb 22
9	Feb 23	Mar 1
10	Mar 2	Mar 8
11	Mar 9	Mar 15
12	Mar 16	Mar 22
13	Mar 23	Mar 29
14	Mar 30	Apr 5
15	Apr 6	Apr 12
16	Apr 13	Apr 19
17	Apr 20	Apr 26
18	Apr 27	May 3
19	May 4	May 10
20	May 11	May 17
21	May 18	May 24
22	May 25	May 31
23	Jun 1	Jun 7
24	Jun 8	Jun 14
25	Jun 15	Jun 21
26	Jun 22	Jun 28
27	Jun 29	Jul 5
28	Jul 6	Jul 12
29	Jul 13	Jul 19
30	Jul 20	Jul 26
31	Jul 27	Aug 2
32	Aug 3	Aug 9
33	Aug 10	Aug 16
34	Aug 17	Aug 23
35	Aug 24	Aug 30
36	Aug 31	Sep 6
37	Sep 7	Sep 13
38	Sep 14	Sep 20
39	Sep 21	Sep 27
40	Sep 28	Oct 4
41	Oct 5	Oct 11
42	Oct 12	Oct 18
43	Oct 19	Oct 25
44	Oct 26	Nov 1
45	Nov 2	Nov 8
46	Nov 9	Nov 15
47	Nov 16	Nov 22
48	Nov 23	Nov 29
49	Nov 30	Dec 6
50	Dec 7	Dec 13
51	Dec 14	Dec 20
52	Dec 21	Dec 27
53	Dec 28	Jan 3

Step by step

Read the ISO week at the top.
See the US-style number below it.
Check which system your team or calendar uses.

At a glance

	ISO 8601	US style
Week starts	Monday	Sunday
Week 1	Contains the first Thursday	Contains January 1
Weeks per year	52 or 53	53 or 54 partial weeks
January 1, 2027	Week 53 of 2026	Week 1 of 2027