Week Number Calculator

Find the ISO week number of any date, with the week's Monday to Sunday range, the day of the year, and the weeks left in the year.

Date

The date defaults to today, so the answer on load is the current week number. Weeks use the ISO 8601 system: weeks start on Monday, and week 1 is the week with the first Thursday of the year.

Guide updated November 2026.

How ISO week numbers work

The ISO week system counts weeks from Monday to Sunday, and week 1 is the week that contains the first Thursday of the year. January 4 always falls in week 1, because the first Thursday can be no later than January 7 and no earlier than January 1. The week number of a date is found by taking the Thursday of its week and counting Thursdays from the first Thursday of the year.

That Thursday rule is why the year of a week can differ from the year of the date. December 29, 2025 is a Monday, but its Thursday is January 1, 2026, so it belongs to ISO week 1 of 2026, not to a week 53 of 2025. The same thing happens at the end of 2026: December 31, 2026 is a Thursday in week 53 of 2026, and the week runs to Sunday, January 3, 2027.

Worked examples

Date	Weekday	ISO week	Week runs	Day of year
November 13, 2026	Friday	Week 46 of 2026	Monday, November 9 to Sunday, November 15, 2026	Day 317 of 365
January 1, 2026	Thursday	Week 1 of 2026	Monday, December 29, 2025 to Sunday, January 4, 2026	Day 1 of 365
December 31, 2026	Thursday	Week 53 of 2026	Monday, December 28, 2026 to Sunday, January 3, 2027	Day 365 of 365
December 29, 2025	Monday	Week 1 of 2026	Monday, December 29, 2025 to Sunday, January 4, 2026	Day 363 of 365
February 29, 2028	Tuesday	Week 9 of 2028	Monday, February 28 to Sunday, March 5, 2028	Day 60 of 366

How many weeks are in 2026?

2026 has 53 ISO weeks. A year gets a 53rd week when it starts on a Thursday, and January 1, 2026 is a Thursday. The extra week sits at the end of the year: week 53 runs Monday, December 28, 2026 to Sunday, January 3, 2027. Most years have 52 ISO weeks. Counted from November 13, 2026, which is week 46, there are 8 weeks left in 2026 including the current week.

Where week numbers get used

Work planning: sprints, releases, and payroll periods are often named by week number, like “week 46”, instead of by date.
School terms: term dates and exam weeks are published as week numbers in many US school and college calendars.
Project deadlines: “deliver in week 48” is unambiguous once everyone agrees weeks start on Monday.
Payroll and timesheets: weekly pay periods run Monday to Sunday in most US workplaces, and the week number labels the period.
Travel and events: conferences and tours are scheduled by week number months ahead, and the date range follows from it.

To count the days between two dates instead of finding a week number, use the Date Difference Calculator. To find what date falls a number of days from today, use the Days From Today Calculator.

Frequently asked questions

What week is it now?

It depends on the date. Counted from Friday, November 13, 2026, this is ISO week 46 of 2026, running Monday, November 9 to Sunday, November 15, 2026. Enter any date above to find its week number.

How many weeks are in 2026?

2026 has 53 ISO weeks. A year has 53 ISO weeks when it starts on a Thursday, and January 1, 2026 is a Thursday. The last week of 2026, week 53, runs Monday, December 28, 2026 to Sunday, January 3, 2027.

How do I find the week number of a date?

Pick the date above and the calculator finds the Thursday of that week, then counts Thursdays from the first Thursday of the year. Example: February 29, 2028 is a Tuesday in ISO week 9 of 2028, running Monday, February 28 to Sunday, March 5, 2028.

What is the first week of the year?

ISO week 1 is the week with the first Thursday of the year, and January 4 always falls in week 1. That is why late December dates can belong to the next year: December 29, 2025 is a Monday in ISO week 1 of 2026.