Date & Time
Age, duration & timezone calculators
Week Number of a Date (ISO-8601)
Find the ISO-8601 week number for any specific date, along with the ISO year and the week’s Monday-to-Sunday date range. Weeks start on Monday and week 1 is the week containing the year’s first Thursday, so the same date can belong to a different ISO year than its calendar year near the boundaries. For example 29 December 2025, a Monday, is ISO week 1 of ISO year 2026 — the classic year-boundary edge — which is why the tool always shows the ISO year with the week. Enter a date and read the exact week number, matching Excel and Google Sheets ISOWEEKNUM() rather than the older Sunday-start WEEKNUM default. All the math runs on UTC midnights and everything is computed in your browser.
ISO week = Thursday-anchored week number for the entered date
- 29 Dec 2025 (Monday) = ISO week 1 of ISO year 2026 (boundary edge)
- Also shown: the ISO year and the week’s Monday–Sunday date range
Frequently asked questions
How do I find the ISO week number of a date?
Enter the date and the calculator shows its ISO-8601 week number, the ISO year, and the week’s Monday-to-Sunday range. Weeks start on Monday and week 1 is the week containing the year’s first Thursday.
Can a date’s ISO year differ from its calendar year?
Yes. Late-December dates can fall in week 1 of the next ISO year and early-January dates in week 52/53 of the previous one. For example 29 December 2025 is ISO week 1 of ISO year 2026, so the tool shows the ISO year alongside the week.