The first Thursday decides
Week 1 is the week with the year's first Thursday, so January 1 can sit in the last week of the year before.
Week numbers 2025
Every ISO week of 2025 with its Monday and Sunday. 52 weeks in all.
Week numbers of 2025
52 weeks, each from Monday to Sunday.
Week 1
Dec 30 to Jan 5
Week 2
Jan 6 to Jan 12
Week 3
Jan 13 to Jan 19
Week 4
Jan 20 to Jan 26
Week 5
Jan 27 to Feb 2
Week 6
Feb 3 to Feb 9
Week 7
Feb 10 to Feb 16
Week 8
Feb 17 to Feb 23
Week 9
Feb 24 to Mar 2
Week 10
Mar 3 to Mar 9
Week 11
Mar 10 to Mar 16
Week 12
Mar 17 to Mar 23
Week 13
Mar 24 to Mar 30
Week 14
Mar 31 to Apr 6
Week 15
Apr 7 to Apr 13
Week 16
Apr 14 to Apr 20
Week 17
Apr 21 to Apr 27
Week 18
Apr 28 to May 4
Week 19
May 5 to May 11
Week 20
May 12 to May 18
Week 21
May 19 to May 25
Week 22
May 26 to Jun 1
Week 23
Jun 2 to Jun 8
Week 24
Jun 9 to Jun 15
Week 25
Jun 16 to Jun 22
Week 26
Jun 23 to Jun 29
Week 27
Jun 30 to Jul 6
Week 28
Jul 7 to Jul 13
Week 29
Jul 14 to Jul 20
Week 30
Jul 21 to Jul 27
Week 31
Jul 28 to Aug 3
Week 32
Aug 4 to Aug 10
Week 33
Aug 11 to Aug 17
Week 34
Aug 18 to Aug 24
Week 35
Aug 25 to Aug 31
Week 36
Sep 1 to Sep 7
Week 37
Sep 8 to Sep 14
Week 38
Sep 15 to Sep 21
Week 39
Sep 22 to Sep 28
Week 40
Sep 29 to Oct 5
Week 41
Oct 6 to Oct 12
Week 42
Oct 13 to Oct 19
Week 43
Oct 20 to Oct 26
Week 44
Oct 27 to Nov 2
Week 45
Nov 3 to Nov 9
Week 46
Nov 10 to Nov 16
Week 47
Nov 17 to Nov 23
Week 48
Nov 24 to Nov 30
Week 49
Dec 1 to Dec 7
Week 50
Dec 8 to Dec 14
Week 51
Dec 15 to Dec 21
Week 52
Dec 22 to Dec 28
How week numbers work
52 or 53 weeks
A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year. 2025 has 52.
Why your calendar may differ
Some calendars, notably in the US, start weeks on Sunday and count the week with January 1 as week 1. That can differ from the ISO number by one.
Other years
More free tools
Frequently asked questions
Ask a questionBy ISO 8601, the standard most calendars and businesses use: a week runs from Monday to Sunday, and week 1 is the week that holds the year's first Thursday (so January 1 can be in week 52 or 53 of the year before, and December 29 to 31 can be in week 1 of the next). A year has 52 or 53 weeks.
52. A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year.
Some calendars, notably in the US, start weeks on Sunday and count the week with January 1 as week 1. That system can differ from the ISO number by one. Check your calendar's settings: most offer ISO weeks.