Week Number Calculator

Determine the week number of a given date. Enter a date to see its ISO week number, the day of the week and the week's start and end dates.

Enter a date. The calculator returns its ISO-8601 week number, the day of the week, and the Monday and Sunday that bound that week.

Date

ISO week number —

Day of week —

Week starts (Mon) —

Week ends (Sun) —

A week number calculator finds the ISO-8601 week number of any date — the standard used across Europe for scheduling, where weeks start on Monday and week 1 contains the year's first Thursday.

ISO-8601 weeks

Weeks run Monday to Sunday
Week 1 is the week with the year's first Thursday (and 4 January)
Some years have 52 weeks, some have 53

Examples

8 July 2026 falls in week 28
That week runs Monday 6 July to Sunday 12 July
Late-December dates can land in week 1 of the next year

Frequently asked questions

Why is week 1 sometimes in late December?

Under ISO-8601, week 1 is the one containing the first Thursday, so dates at the very end of December can belong to week 1 of the following year, and early-January dates can belong to week 52 or 53.

Is this the same week numbering used everywhere?

ISO-8601 (Monday-start) is standard in Europe. The US often uses Sunday-start weeks with week 1 containing 1 January, so the numbers can differ by a week.

ISO-8601 weeks

Examples

Frequently asked questions

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