Find the ISO week number and full date range for any date
ISO Week Number
26
of 53 weeks in 2026
Week range
22 Jun 2026
Monday (start)
28 Jun 2026
Sunday (end)
Day of week
Thursday
ISO notation
2026-W26
Progress through 2026
Days in this week
Need to know what week number a date falls on? This free ISO week number calculator follows the ISO 8601 standard — weeks start on Monday, and Week 1 is the week containing the first Thursday of the year. Enter any date to instantly see its week number, the full Monday-to-Sunday range, ISO notation (e.g. 2025-W23), and year progress.
ISO 8601 defines a standard week numbering system where Week 1 is the week containing the first Thursday of the year. Weeks always run Monday through Sunday, and a year has either 52 or 53 ISO weeks.
Because ISO Week 1 must contain a Thursday, it can begin as early as December 29. Conversely, the last few days of December sometimes fall in Week 1 of the following year.
It is the standard way to write an ISO week: the 4-digit week-year, the letter W, and the 2-digit week number. This format is used in project planning, finance, and logistics to reference a specific week unambiguously.
Yes. A year has 53 ISO weeks if January 1st or December 31st falls on a Thursday (in a leap year, if January 1st falls on a Wednesday or Thursday). This happens roughly every 5–6 years.