Week Number Calculator
Find which week of the year (ISO 8601) and week of the month any date falls in.
Week starts on
Defaults to today.
Week
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Week of month-
This week-
Weeks in this year-
Weeks remaining-
Find dates from a week number
Dates of that week
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What is the Week Number Calculator?
Enter a date to see which week of the year it falls in, which week of the month it is, and the first and last day of that week. Enter a year and a week number to get the dates the other way round. It is handy for shift schedules, project and school calendars, and weekly billing cycles.
How it works
The default is ISO 8601: weeks start on Monday and week 1 is the week with the first Thursday of the year, so dates in early January or late December can belong to a neighboring year's week. The week of the month is based on the month that contains the week's Thursday (the Thursday's day of month divided by 7, rounded up).
The US style option starts weeks on Sunday and treats the week containing January 1 as week 1; the week of the month follows the same rule. All date math uses UTC day counts, so your time zone does not matter.
Weekly calendar by year
| Week | Dates (start ~ end) | Week of month |
|---|---|---|
| 1 | 2025.12.29 ~ 2026.01.04 | Week 1 of Jan |
| 2 | 2026.01.05 ~ 2026.01.11 | Week 2 of Jan |
| 3 | 2026.01.12 ~ 2026.01.18 | Week 3 of Jan |
| 4 | 2026.01.19 ~ 2026.01.25 | Week 4 of Jan |
| 5 | 2026.01.26 ~ 2026.02.01 | Week 5 of Jan |
| 6 | 2026.02.02 ~ 2026.02.08 | Week 1 of Feb |
| 7 | 2026.02.09 ~ 2026.02.15 | Week 2 of Feb |
| 8 | 2026.02.16 ~ 2026.02.22 | Week 3 of Feb |
| 9 | 2026.02.23 ~ 2026.03.01 | Week 4 of Feb |
| 10 | 2026.03.02 ~ 2026.03.08 | Week 1 of Mar |
| 11 | 2026.03.09 ~ 2026.03.15 | Week 2 of Mar |
| 12 | 2026.03.16 ~ 2026.03.22 | Week 3 of Mar |
| 13 | 2026.03.23 ~ 2026.03.29 | Week 4 of Mar |
| 14 | 2026.03.30 ~ 2026.04.05 | Week 1 of Apr |
| 15 | 2026.04.06 ~ 2026.04.12 | Week 2 of Apr |
| 16 | 2026.04.13 ~ 2026.04.19 | Week 3 of Apr |
| 17 | 2026.04.20 ~ 2026.04.26 | Week 4 of Apr |
| 18 | 2026.04.27 ~ 2026.05.03 | Week 5 of Apr |
| 19 | 2026.05.04 ~ 2026.05.10 | Week 1 of May |
| 20 | 2026.05.11 ~ 2026.05.17 | Week 2 of May |
| 21 | 2026.05.18 ~ 2026.05.24 | Week 3 of May |
| 22 | 2026.05.25 ~ 2026.05.31 | Week 4 of May |
| 23 | 2026.06.01 ~ 2026.06.07 | Week 1 of Jun |
| 24 | 2026.06.08 ~ 2026.06.14 | Week 2 of Jun |
| 25 | 2026.06.15 ~ 2026.06.21 | Week 3 of Jun |
| 26 | 2026.06.22 ~ 2026.06.28 | Week 4 of Jun |
| 27 | 2026.06.29 ~ 2026.07.05 | Week 1 of Jul |
| 28 | 2026.07.06 ~ 2026.07.12 | Week 2 of Jul |
| 29 | 2026.07.13 ~ 2026.07.19 | Week 3 of Jul |
| 30 | 2026.07.20 ~ 2026.07.26 | Week 4 of Jul |
| 31 | 2026.07.27 ~ 2026.08.02 | Week 5 of Jul |
| 32 | 2026.08.03 ~ 2026.08.09 | Week 1 of Aug |
| 33 | 2026.08.10 ~ 2026.08.16 | Week 2 of Aug |
| 34 | 2026.08.17 ~ 2026.08.23 | Week 3 of Aug |
| 35 | 2026.08.24 ~ 2026.08.30 | Week 4 of Aug |
| 36 | 2026.08.31 ~ 2026.09.06 | Week 1 of Sep |
| 37 | 2026.09.07 ~ 2026.09.13 | Week 2 of Sep |
| 38 | 2026.09.14 ~ 2026.09.20 | Week 3 of Sep |
| 39 | 2026.09.21 ~ 2026.09.27 | Week 4 of Sep |
| 40 | 2026.09.28 ~ 2026.10.04 | Week 1 of Oct |
| 41 | 2026.10.05 ~ 2026.10.11 | Week 2 of Oct |
| 42 | 2026.10.12 ~ 2026.10.18 | Week 3 of Oct |
| 43 | 2026.10.19 ~ 2026.10.25 | Week 4 of Oct |
| 44 | 2026.10.26 ~ 2026.11.01 | Week 5 of Oct |
| 45 | 2026.11.02 ~ 2026.11.08 | Week 1 of Nov |
| 46 | 2026.11.09 ~ 2026.11.15 | Week 2 of Nov |
| 47 | 2026.11.16 ~ 2026.11.22 | Week 3 of Nov |
| 48 | 2026.11.23 ~ 2026.11.29 | Week 4 of Nov |
| 49 | 2026.11.30 ~ 2026.12.06 | Week 1 of Dec |
| 50 | 2026.12.07 ~ 2026.12.13 | Week 2 of Dec |
| 51 | 2026.12.14 ~ 2026.12.20 | Week 3 of Dec |
| 52 | 2026.12.21 ~ 2026.12.27 | Week 4 of Dec |
| 53 | 2026.12.28 ~ 2027.01.03 | Week 5 of Dec |
Frequently asked questions
Which standard does the week number follow?
The default is ISO 8601 (Korean standard KS X ISO 8601): weeks start on Monday, and week 1 is the week with the first Thursday of the year (the week containing January 4). The US style option starts weeks on Sunday and makes the week containing January 1 week 1.
Why can January 1 fall in week 52 or 53 of the previous year?
An ISO week belongs to the year that contains its Thursday. When January 1 is a Friday, Saturday or Sunday, that week's Thursday is still in the previous year, so it counts as the previous year's last week. For example, 2027-01-01 (Friday) is week 53 of 2026, while 2025-12-29 (Monday) is week 1 of 2026.
Which years have 53 weeks?
A year has 53 ISO weeks when January 1 is a Thursday, or when it is a leap year starting on a Wednesday. 2015, 2020 and 2026 are such years; 2026 starts on a Thursday and has 53 weeks. All other years have 52.
How is the "nth week of the month" decided?
This tool uses the month that contains the week's Thursday. October 1, 2026 is a Thursday, so the week of Sep 28 - Oct 4 is week 1 of October, and Sep 30 is shown as week 1 of October too. Some organizations count the week containing the 1st as week 1, so check the rule of whoever asks. The US style option does exactly that.
Why do Excel's ISOWEEKNUM and WEEKNUM give different results?
ISOWEEKNUM returns the ISO 8601 week, while WEEKNUM's default type (1) starts weeks on Sunday and treats the week containing January 1 as week 1. This tool's ISO (Monday) and US (Sunday) options follow those two rules, so dates near the start or end of a year can get different numbers.
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