WorldCalcs

Week Number Calculator

Find the ISO 8601 week number and day-of-year for any date.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

ISO week

Week 24

ISO week-year

2026

Day of year

161 of 365

Wednesday

What is a week number calculator?

A week number calculator tells you which week of the year a date falls in, using the ISO 8601 standard. ISO week numbers run from 1 to 52 or 53 and are widely used in business, planning and payroll.

How it works

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. Because of this rule, early-January dates can belong to the last week of the previous year. Example: 15 June 2026 is in ISO week 25, and 1 January 2027 falls in ISO week 53 of 2026.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Pick any date and instantly see its ISO 8601 week number, the ISO week-year it belongs to, and which day of the year it is (e.g. day 158 of 365).

ISO weeks start on Monday and week 1 is always the week containing the first Thursday of the year. That's why some early-January or late-December dates fall under a different week-year than their calendar year.

Frequently asked questions

What is an ISO 8601 week?+

ISO 8601 weeks start on Monday. Week 1 of a year is the week that contains the first Thursday of the year, which is also the week containing January 4.

Why is the ISO week-year sometimes different from the calendar year?+

Dates in early January can belong to the last ISO week of the previous year, and dates in late December can belong to week 1 of the next year. The ISO week-year reflects that.

How is the day-of-year counted?+

January 1 is day 1. December 31 is day 365 in a normal year and day 366 in a leap year. February 29 only exists in leap years.

When does the week start?+

Under ISO 8601 the week starts on Monday and ends on Sunday.

Can a year have 53 weeks?+

Yes. Most years have 52 ISO weeks, but some have 53 depending on which day the year starts and whether it is a leap year.

Why is 1 January sometimes in the previous year's last week?+

ISO week 1 is the week with the year's first Thursday, so a few early-January days can still belong to week 52 or 53 of the year before.

What is ISO 8601?+

An international standard for dates and times, including the rule for numbering weeks, so the count is the same worldwide.

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