Week Number Calculator
Find the ISO week number for any date, or see which dates fall in a given week.
Find the ISO week number for any date, and browse every week of the year with its full date range.
What week number is a date?
| Week | Monday (start) | Sunday (end) | Months | Quarter |
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How ISO Week Numbers Are Calculated
What is the ISO 8601 week?
ISO 8601 is the international standard for representing dates and times. Under this standard, weeks run Monday to Sunday, and Week 1 is defined as the week containing the year's first Thursday. This means Week 1 can start in late December of the previous year. For a full calendar view, try the calendar.
Do all years have 52 weeks?
Most years have 52 ISO weeks, but some have 53. A "long year" occurs when January 1 falls on a Thursday, or when January 1 falls on a Wednesday in a leap year. These years have a Week 53 because the extra days push the calendar into a 53rd week.
Week numbering vs US convention
The US commonly numbers weeks differently — Week 1 starts on January 1, and weeks begin on Sunday. This can cause differences of up to several weeks compared to ISO numbering. Most of Europe, and international business and logistics, use the ISO 8601 standard shown here. When scheduling across time zones, the timezone converter handles the date and time offsets.
Why do some weeks span two years?
Because ISO weeks always run Monday–Sunday, the first days of a new year often belong to the final week of the previous year, and the last days of the year can belong to Week 1 of the next year. The ISO week year can therefore differ from the calendar year by one.
Worked example
1 January 2026 is a Thursday — since it is the year's first Thursday, that week is ISO Week 1 of 2026. It starts on Monday 29 December 2025, so those late-December dates carry the 2026 week number. The last week of 2025 is therefore Week 52, ending Sunday 28 December 2025.
Frequently Asked Questions
What is the current week number?
The week number calculator shows today's ISO 8601 week number automatically when the page loads. ISO weeks run Monday to Sunday, and Week 1 is defined as the week containing the year's first Thursday. Most of Europe and international business use this standard.
What is an ISO week number?
ISO 8601 week numbers are the international standard for numbering weeks. Week 1 is the week containing the first Thursday of the year. Weeks run Monday to Sunday. This means Week 1 can start in late December of the previous year — so the ISO week year may differ from the calendar year by one.
Do all years have 52 weeks?
Most years have 52 ISO weeks, but some have 53. A long year (53 weeks) occurs when January 1 falls on a Thursday, or on a Wednesday in a leap year. These extra days push the calendar into a 53rd week before the new year starts.
Why is the ISO week different from the US week number?
The US commonly defines Week 1 as starting on January 1 with weeks beginning on Sunday. ISO 8601 defines Week 1 as containing the first Thursday and weeks beginning on Monday. This can cause differences of up to several weeks in the number for dates in early January or late December.
Evidence & Methodology
How This Page Is Grounded
Method
Calculates ISO week dates, where weeks begin Monday and week 1 contains the year's first Thursday.
Important limitation: ISO week-year numbers can differ from the calendar year near New Year.
Primary Sources
- ISO 8601 date and time format International Organization for Standardization
Quality Checks
Year boundaries, 52/53-week years and known ISO examples are checked.
See how sources are selected and corrections are handled in our Editorial & Calculation Methodology, and which automated checks this page has to pass in How We Test.