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 2027
Every ISO week of 2027 with its Monday and Sunday. 52 weeks in all.
Week numbers of 2027
52 weeks, each from Monday to Sunday.
Week 1
Jan 4 to Jan 10
Week 2
Jan 11 to Jan 17
Week 3
Jan 18 to Jan 24
Week 4
Jan 25 to Jan 31
Week 5
Feb 1 to Feb 7
Week 6
Feb 8 to Feb 14
Week 7
Feb 15 to Feb 21
Week 8
Feb 22 to Feb 28
Week 9
Mar 1 to Mar 7
Week 10
Mar 8 to Mar 14
Week 11
Mar 15 to Mar 21
Week 12
Mar 22 to Mar 28
Week 13
Mar 29 to Apr 4
Week 14
Apr 5 to Apr 11
Week 15
Apr 12 to Apr 18
Week 16
Apr 19 to Apr 25
Week 17
Apr 26 to May 2
Week 18
May 3 to May 9
Week 19
May 10 to May 16
Week 20
May 17 to May 23
Week 21
May 24 to May 30
Week 22
May 31 to Jun 6
Week 23
Jun 7 to Jun 13
Week 24
Jun 14 to Jun 20
Week 25
Jun 21 to Jun 27
Week 26
Jun 28 to Jul 4
Week 27
Jul 5 to Jul 11
Week 28
Jul 12 to Jul 18
Week 29
Jul 19 to Jul 25
Week 30
Jul 26 to Aug 1
Week 31
Aug 2 to Aug 8
Week 32
Aug 9 to Aug 15
Week 33
Aug 16 to Aug 22
Week 34
Aug 23 to Aug 29
Week 35
Aug 30 to Sep 5
Week 36
Sep 6 to Sep 12
Week 37
Sep 13 to Sep 19
Week 38
Sep 20 to Sep 26
Week 39
Sep 27 to Oct 3
Week 40
Oct 4 to Oct 10
Week 41
Oct 11 to Oct 17
Week 42
Oct 18 to Oct 24
Week 43
Oct 25 to Oct 31
Week 44
Nov 1 to Nov 7
Week 45
Nov 8 to Nov 14
Week 46
Nov 15 to Nov 21
Week 47
Nov 22 to Nov 28
Week 48
Nov 29 to Dec 5
Week 49
Dec 6 to Dec 12
Week 50
Dec 13 to Dec 19
Week 51
Dec 20 to Dec 26
Week 52
Dec 27 to Jan 2
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. 2027 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.