The ISO 8601 week number is the standard way to say "which week of the year" a date falls in. Weeks start on Monday, and week 1 is the week containing the year's first Thursday (the same as the week with January 4). Numbers run 1 to 52 or 53. This calculator returns the ISO week, the day of the year, how many days remain, and the weekday for any date you pick.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the ISO 8601 week-date algorithm, recomputed in code.
How ISO weeks work
Anchoring week 1 to the first Thursday keeps most of January in week 1 while letting a stray Friday–Sunday at the very start belong to the previous year's final week. That's why a date like 1 January can occasionally read as week 52 or 53, and a late-December date can read as week 1. The "ISO week-year" tracks the week, so it may differ from the calendar year by a day or two at the boundaries.
Worked example — 27 June 2026
Scenario: look up 27 June 2026.
27 June 2026 sits in ISO week 26, exactly halfway through the year — it's day 178 of 365 with 187 days remaining, and falls on a Saturday. Because 2026 is a 53-week ISO year, its 31 December lands in week 53; in a typical 52-week year that same date would be week 52 or even week 1 of the next year.
Frequently Asked Questions
ISO 8601's weekly numbering: weeks start Monday, week 1 holds the first Thursday (Jan 4). 27 Jun 2026 → week 26.
52 in most years, 53 when the year starts on Thursday (or a leap year starting Wednesday). 2026 has 53.
Week 1 must contain the first Thursday. A Fri/Sat/Sun Jan 1 belongs to the prior year's last week.
Monday, under ISO 8601 (ends Sunday). The US convention often starts the week on Sunday instead.
The date's position from Jan 1 = day 1. 27 Jun 2026 = day 178. The tool also shows days remaining.