Date & Time · Weeks
Week number calendar.
Enter a date to get its ISO 8601 week and the US week count, the dates the week runs from and to, and every week of that year.
Short answer: Monday, October 26, 2026 is ISO week 44 of 2026. January 1, 2027 is still ISO week 53 of 2026, and ISO week 1 of 2027 starts on Monday, January 4.
Two ways to number weeks
Microsoft's documentation for the WEEKNUM function describes the two systems in use. In System 1 the week containing January 1 is week 1, and by default weeks begin on Sunday. In System 2 the week containing the first Thursday of the year is week 1, and Microsoft states that this is the methodology specified in ISO 8601. ISO weeks begin on Monday. This tool shows both: the ISO week, which is the one used on most European calendars and in payroll and shipping systems, and the US week, which counts the calendar year only.
How the ISO week is worked out
Take the Thursday of the week the date is in. The year that Thursday belongs to is the ISO year, and the week number is the count of weeks from January 1 of that year to that Thursday. Because the Thursday decides the year, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year; otherwise it has 52.
Worked example
Monday, October 26, 2026 is the Monday of ISO week 44. It is day 299 of the year with 66 days left. Now take Friday, January 1, 2027: its Thursday is December 31, 2026, so it belongs to ISO week 53 of 2026, which runs 2026-12-28 to 2027-01-03. The US count calls the same day week 1. ISO week 1 of 2027 starts on Monday, January 4, 2027 (the US count shows week 2 that day).
Where ISO week 1 starts, 2025 to 2030
| ISO year | Week 1 starts | ISO weeks in year | Last week ends |
|---|---|---|---|
| 2025 | 2024-12-30 (Monday) | 52 | 2025-12-28 |
| 2026 | 2025-12-29 (Monday) | 53 | 2027-01-03 |
| 2027 | 2027-01-04 (Monday) | 52 | 2028-01-02 |
| 2028 | 2028-01-03 (Monday) | 52 | 2028-12-31 |
| 2029 | 2029-01-01 (Monday) | 52 | 2029-12-30 |
| 2030 | 2029-12-31 (Monday) | 52 | 2030-12-29 |
What this page does not cover
Some calendars start the week on a different day or define week 1 their own way, and fiscal calendars such as a 4-4-5 retail calendar have their own week numbers. This tool covers the ISO system and the Sunday-start calendar-year count only.
Frequently asked questions
What week is it today?
Open this page and the tool is set to today's date. It shows the ISO 8601 week number, the US week number, and the Monday-to-Sunday dates of the current ISO week.
How many weeks are in a year?
An ISO year has 52 or 53 weeks. It has 53 when January 1 is a Thursday, or a Wednesday in a leap year. 2026 has 53 and 2027 has 52.
Why is January 1, 2027 in week 53?
Under ISO 8601 the week belongs to the year of its Thursday. The week of Monday, December 28, 2026 to Sunday, January 3, 2027 has its Thursday on December 31, 2026, so the whole week is ISO week 53 of 2026.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday and week 1 holds the first Thursday of the year. The US count used here starts weeks on Sunday and makes the week containing January 1 week 1, so it never uses the previous or next year's number.
When does week 1 of 2027 start?
Monday, January 4, 2027 under ISO 8601.
Is my date sent anywhere?
No. The week is worked out in your browser and nothing is stored.
Sources
Retrieved
Runs entirely in your browser. Nothing you type is sent or stored.
Check payroll and shipping rules
Employers, carriers and fiscal calendars sometimes define their own week 1. Use this tool for the ISO and calendar-year counts and confirm any contract or reporting calendar separately.