Week Number Calculator
Find the ISO 8601 week number for any date — instantly. Also shows the ISO week-year and day of the week.
Sunday, June 21, 2026
ISO Week
W25
Week number
25
ISO week-year
2026
Day of week
Sunday
ISO week code
2026-W25
What is an ISO week number?
ISO 8601 divides the year into numbered weeks (W01–W52 or W53), with each week starting on Monday and ending on Sunday. Week 1 is always the week that contains the first Thursday of the year — equivalently, the week that contains January 4. This can mean a few days at the end of December belong to Week 1 of the following year, or that January 1–3 belong to the last week of the previous year. When this happens, the ISO week-year differs from the calendar year, and this tool highlights it.
ISO week code format
The standard notation is YYYY-Www, for example 2025-W03. Adding a day suffix gives the full ISO 8601 date in week format: 2025-W03-1 is the Monday of that week.
Privacy — runs in your browser
All calculations run in your browser using the JavaScript Date API. No data is sent to any server. No signup required.
Frequently Asked Questions
Why can week 1 start in December?
ISO 8601 defines Week 1 as the week containing the year's first Thursday. If January 1 is a Friday, Saturday, or Sunday, the previous week (which contains the first Thursday) belongs to the new year's W01 — so December 29–31 can be in Week 1 of the following year.
How many weeks are in a year?
Most years have 52 ISO weeks. About 71 out of every 400 years have 53 weeks. A "long year" occurs when January 1 or December 31 is a Thursday.
Do all countries use ISO week numbers?
ISO 8601 week numbers are used in many countries and widely in business contexts, manufacturing, and logistics. Some systems (notably the US) start the week on Sunday and use different week numbering schemes.