| Week 1 | January 4 – January 10, 2021 |
| Week 2 | January 11 – January 17, 2021 |
| Week 3 | January 18 – January 24, 2021 |
| Week 4 | January 25 – January 31, 2021 |
| Week 5 | February 1 – February 7, 2021 |
| Week 6 | February 8 – February 14, 2021 |
| Week 7 | February 15 – February 21, 2021 |
| Week 8 | February 22 – February 28, 2021 |
| Week 9 | March 1 – March 7, 2021 |
| Week 10 | March 8 – March 14, 2021 |
| Week 11 | March 15 – March 21, 2021 |
| Week 12 | March 22 – March 28, 2021 |
| Week 13 | March 29 – April 4, 2021 |
| Week 14 | April 5 – April 11, 2021 |
| Week 15 | April 12 – April 18, 2021 |
| Week 16 | April 19 – April 25, 2021 |
| Week 17 | April 26 – May 2, 2021 |
| Week 18 | May 3 – May 9, 2021 |
| Week 19 | May 10 – May 16, 2021 |
| Week 20 | May 17 – May 23, 2021 |
| Week 21 | May 24 – May 30, 2021 |
| Week 22 | May 31 – June 6, 2021 |
| Week 23 | June 7 – June 13, 2021 |
| Week 24 | June 14 – June 20, 2021 |
| Week 25 | June 21 – June 27, 2021 |
| Week 26 | June 28 – July 4, 2021 |
| Week 27 | July 5 – July 11, 2021 |
| Week 28 | July 12 – July 18, 2021 |
| Week 29 | July 19 – July 25, 2021 |
| Week 30 | July 26 – August 1, 2021 |
| Week 31 | August 2 – August 8, 2021 |
| Week 32 | August 9 – August 15, 2021 |
| Week 33 | August 16 – August 22, 2021 |
| Week 34 | August 23 – August 29, 2021 |
| Week 35 | August 30 – September 5, 2021 |
| Week 36 | September 6 – September 12, 2021 |
| Week 37 | September 13 – September 19, 2021 |
| Week 38 | September 20 – September 26, 2021 |
| Week 39 | September 27 – October 3, 2021 |
| Week 40 | October 4 – October 10, 2021 |
| Week 41 | October 11 – October 17, 2021 |
| Week 42 | October 18 – October 24, 2021 |
| Week 43 | October 25 – October 31, 2021 |
| Week 44 | November 1 – November 7, 2021 |
| Week 45 | November 8 – November 14, 2021 |
| Week 46 | November 15 – November 21, 2021 |
| Week 47 | November 22 – November 28, 2021 |
| Week 48 | November 29 – December 5, 2021 |
| Week 49 | December 6 – December 12, 2021 |
| Week 50 | December 13 – December 19, 2021 |
| Week 51 | December 20 – December 26, 2021 |
| Week 52 | December 27, 2021 – January 2, 2022 |
2021 has 52 ISO weeks. Most years have 52; a year has 53 when it starts on a Thursday (or on a Wednesday in a leap year).
Week 1 of 2021 starts on Monday, January 4, 2021 — ISO week 1 is the week containing the year's first Thursday, so it can start in late December.
=ISOWEEKNUM(A1) returns the ISO week number of the date in A1 (the numbering used on this page); =WEEKNUM(A1) uses the US Sunday-start convention. Both work in Excel and Google Sheets.