How is the ISO week number defined?

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. This means the first few days of January can fall in the last week of the previous year, and late December can fall in week 1 of the next year.

Why does the ISO year sometimes differ from the calendar year?

Because a week belongs to the year that contains its Thursday. For example, 1 January 2027 is a Friday, so it belongs to ISO week 53 of 2026, and its ISO year is 2026 even though the calendar year is 2027.

How many weeks are in an ISO year?

An ISO year has either 52 or 53 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. All other years have 52.

Does the ISO week start on Monday or Sunday?

ISO 8601 weeks always start on Monday and end on Sunday. This differs from the US convention where calendar weeks often begin on Sunday, which is why ISO and US week numbers can disagree.

What week number is Christmas?

It depends on the year, but 25 December usually falls in ISO week 51 or 52. Use the date picker to get the exact ISO week and the Monday-to-Sunday range for any specific year.

Is my date sent anywhere?

No. The calculation runs entirely in your browser and nothing is uploaded.

ISO Week Number Calculator

ISO 8601 week number for any date

Enter a date and get its ISO 8601 week number in the standard year-Wnn format, along with the weekday and the Monday-to-Sunday span of that week. ISO week numbers are widely used in manufacturing, payroll, project planning and European business, where “week 14” is an unambiguous way to refer to a period.

How it works

ISO weeks always start on Monday, and week 1 is the week containing the year’s first Thursday (equivalently, the week containing 4 January). To find a date’s week number, the calculator takes the Thursday of that date’s week and counts how many weeks it is from the first Thursday of that ISO year. Because a week belongs to whichever year holds its Thursday, the ISO year can differ from the calendar year at the boundaries of January and December.

Example

29 May 2026 falls in 2026-W22, running Monday 25 May to Sunday 31 May.
1 January 2027 is a Friday, so its week’s Thursday (31 Dec 2026) belongs to 2026 — it is 2026-W53, with an ISO year of 2026 even though the calendar year reads 2027.

Everything is computed locally in your browser; nothing is uploaded.