Date & Time
ISO Week Number Calculator
Find the ISO 8601 week number for any date, or convert an ISO year and week back into the Monday-to-Sunday date range. Handles 52- and 53-week years and cross-year edge cases like Dec 31, 2024 belonging to ISO week 2025-W01.
ISO 8601 week lookup
Any date in YYYY-MM-DD format.
ISO 8601 week
2026-W25-5
Gregorian date 2026-06-19 falls on Friday of ISO week 25 in ISO year 2,026.
Gregorian date
2026-06-19
ISO year
2,026
ISO week
25
ISO weekday
5 (Friday)
Week starts (Mon)
2026-06-15
Week ends (Sun)
2026-06-21
Weeks in ISO year
53
Cross-year week?
No
Frequently Asked Questions about the ISO Week Number Calculator
How does ISO 8601 define a week number?
ISO 8601 weeks always start on Monday and end on Sunday. Week 1 of any ISO year is the week that contains that year's first Thursday, which is the same as saying the week that contains January 4, or the first week with at least four of its seven days in the new year. Every other week is numbered sequentially from there, ending at week 52 or 53.
Why can a date in early or late year shift its ISO year?
Because the ISO year is anchored to Thursdays, not to January 1. December 29, 30, and 31 can fall into ISO week 1 of the next year if they share a week with a Thursday in January. The other way around, January 1, 2, and 3 can land in week 52 or 53 of the previous ISO year. For example, December 31, 2024 is a Tuesday, so it sits in ISO 2025-W01-2, even though the calendar year is still 2024.
When does an ISO year have 53 weeks instead of 52?
An ISO year is a long year (53 weeks) when January 1 falls on a Thursday, or when January 1 falls on a Wednesday in a leap year. Both conditions force an extra Thursday into the year, which means one extra ISO week. Examples: 2015, 2020, and 2026 are 53-week ISO years, while most years have 52 weeks. The calculator computes this automatically and rejects week 53 in any year that only has 52.
Where are ISO week numbers actually used?
ISO weeks are the default planning unit across most of Europe, in international shipping, in payroll and supply chain software, and in many financial reporting calendars. You will see week numbers printed on European pay stubs, production schedules, ERP systems, project Gantt charts, and academic calendars. They are also the standard week format in databases (PostgreSQL, MySQL) and in date libraries like ISO_WEEK, Luxon, and date-fns.
How is the ISO week different from the US week convention?
US calendars usually treat Sunday as the start of the week and have no formal definition of week 1, so different vendors disagree. Most US payroll systems just count week 1 as the week containing January 1. That means an early January date can be in US week 1 but ISO week 52 or 53 of the previous year, and the two systems can be off by one for several weeks. If your software, partner, or country uses Monday-start weeks, you almost certainly want the ISO 8601 number this calculator returns.