Week Number Calculator

Find the ISO week number for any date, or look up what dates a given week number covers. Full year calendar view.

Date → Week Number

Select a date to see its week number

Week Number → Dates

Enter a week number and year to see the date range

Full Year Calendar with Week Numbers

What is an ISO Week Number?

An ISO week number is a numbering system defined by the ISO 8601 international standard. Each week starts on Monday and ends on Sunday. Week 1 of a year is the week that contains the first Thursday of that year, which is equivalent to the week containing January 4th.

This system is widely used in business, government, and IT systems across Europe and many other regions. It eliminates ambiguity when referring to specific weeks across international boundaries.

Week Numbers for 2026 — Full Calendar

Week Monday (Start) Sunday (End)
12025-12-292026-01-04
22026-01-052026-01-11
32026-01-122026-01-18
42026-01-192026-01-25
52026-01-262026-02-01
62026-02-022026-02-08
72026-02-092026-02-15
82026-02-162026-02-22
92026-02-232026-03-01
102026-03-022026-03-08
112026-03-092026-03-15
122026-03-162026-03-22
132026-03-232026-03-29
142026-03-302026-04-05
152026-04-062026-04-12
162026-04-132026-04-19
172026-04-202026-04-26
182026-04-272026-05-03
192026-05-042026-05-10
202026-05-112026-05-17
212026-05-182026-05-24
222026-05-252026-05-31
232026-06-012026-06-07
242026-06-082026-06-14
252026-06-152026-06-21
262026-06-222026-06-28
272026-06-292026-07-05
282026-07-062026-07-12
292026-07-132026-07-19
302026-07-202026-07-26
312026-07-272026-08-02
322026-08-032026-08-09
332026-08-102026-08-16
342026-08-172026-08-23
352026-08-242026-08-30
362026-08-312026-09-06
372026-09-072026-09-13
382026-09-142026-09-20
392026-09-212026-09-27
402026-09-282026-10-04
412026-10-052026-10-11
422026-10-122026-10-18
432026-10-192026-10-25
442026-10-262026-11-01
452026-11-022026-11-08
462026-11-092026-11-15
472026-11-162026-11-22
482026-11-232026-11-29
492026-11-302026-12-06
502026-12-072026-12-13
512026-12-142026-12-20
522026-12-212026-12-27
532026-12-282027-01-03

Frequently Asked Questions

What is ISO week numbering?
ISO week numbering is defined by the ISO 8601 standard. Weeks start on Monday. Week 1 is the week containing the first Thursday of the year (or equivalently, the week containing January 4th). Most years have 52 weeks, but some have 53.
What week number is today?
Use the calculator above to find today's week number. The tool defaults to the current date and immediately shows the ISO week number, day of the week, and the Monday–Sunday range for that week.
Does Week 1 always start on January 1?
No. In the ISO system, Week 1 is the week containing the first Thursday of the year. This means Week 1 can start as early as December 29 of the previous year or as late as January 4. January 1 may fall in Week 52 or 53 of the previous year.
What is the difference between ISO and US week numbering?
The main differences are: ISO weeks start on Monday, while US weeks start on Sunday. ISO Week 1 contains the first Thursday of the year, while US Week 1 typically contains January 1. This can cause the week number to differ by one between the two systems.
How many weeks are in 2026?
The year 2026 has 53 ISO weeks. This is because January 1, 2026 is a Thursday, making it part of Week 1, and December 31, 2026 is also a Thursday, which creates a 53rd week.