What Week Is It? Week Number Calculator

See today's ISO week number, or look up the week number, day of the year and days remaining for any date.

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How to use it

The current week is shown automatically.
Pick any other date to look up its week number.
Read the week's Monday-to-Sunday range, the day of the year and how many days are left.

Examples

Early January

January 1, 2027 is a Friday, so it belongs to week 53 of 2026 under ISO rules. Week 1 of 2027 starts on Monday, January 4.

Mid-year

July 1, 2026 falls in ISO week 27 (June 29 to July 5) and is day 182 of the year.

ISO week numbering

ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4). A year therefore has 52 or 53 weeks, and the first or last few days of a calendar year can belong to the neighboring week-year.

This is the standard used across Europe and in most business software. Some US calendars number weeks from Sunday with week 1 starting January 1, which can differ by one.

Frequently asked questions

Why is January 1 sometimes in week 52 or 53?

If January 1 falls on a Friday, Saturday or Sunday, that week has most of its days in the previous year, so it counts as that year's last week.

How many weeks are in a year?

Most years have 52 ISO weeks; years starting on a Thursday (or leap years starting on a Wednesday) have 53.

Does the week start on Sunday or Monday?

ISO weeks start on Monday. If your workplace uses Sunday-start weeks, the number can differ for Sundays.

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More in Date & Time · Last reviewed October 2026