ISO week number
Look up ISO week, week year, weekday and day of year for a date.
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- Select a valid calendar date.
- Look up its ISO week and week year.
- Inspect weekday and day of year before copying the summary.
An example you can verify
Check a year boundary where ISO week year differs from calendar year.
Example input
2021-01-01
Expected result
2020-W53-5 Week year: 2020 Week: 53 ISO day: 5 Day of year: 1
Friday January 1, 2021 belongs to the final ISO week of 2020. Day of year is still 1 in the 2021 calendar.
ISO week number: uses and practical guidance
An ISO week date identifies a day through its week year, week number and weekday. Weeks begin on Monday, numbered 1, and end on Sunday, numbered 7. Week 1 contains the year's first Thursday. This rule uses complete Monday-to-Sunday weeks without artificially splitting them at a year boundary. The tool displays a label such as 2020-W53-5 and presents its components separately for inspection or transfer into another document. Keep those components together when the week-date label is being used to identify a particular day rather than simply grouping records.
Week year can differ from the year shown in the ordinary calendar date. Friday January 1, 2021 belongs to week 53 of ISO year 2020. For the same reason, some late-December dates belong to week 1 of the following ISO year. Dividing day of year by seven is insufficient, as is counting from January 1 as though it were always Monday. Inspect week year alongside week number when grouping reports: combining calendar year with an ISO week number can produce an incorrect pair. The distinction matters especially in datasets containing the final and first days of adjacent years.
Day of year is a separate measure calculated within the original calendar year. January 1 always has ordinal 1, even when it belongs to a week in the preceding ISO year. Leap years include February 29 and can reach day 366. Calculation uses UTC calendar dates without a time of day, preventing seasonal clock changes from adding or subtracting fractions of a day. Time zones are not converted and a local date is not inferred from a full timestamp. Enter the calendar date appropriate to your context before looking up its week components.
This interface uses ISO rules rather than regional conventions where weeks begin on Sunday or the first January dates always belong to week 1. Applications can display different week numbers when their conventions differ without misreading the supplied date. Confirm destination rules before combining lists. Valid calendar dates between years 0001 and 9999 are supported; nonexistent dates such as February 30 are rejected. Try an early-January date, a late-December date and a leap day to review boundary behavior. Save the complete week-date label when a day must be identified unambiguously instead of retaining only a week number without its associated year and weekday.
Settings and limits
Choose a date between years 0001 and 9999. ISO weeks start on Monday and week 1 contains the first Thursday of the year. Week year can differ from calendar year near January and December. UTC calendar dates avoid daylight-saving changes.
Frequently asked questions
Is January 1 always in week 1?
No. For example, 2021-01-01 belongs to 2020-W53-5: Friday of week 53 in ISO year 2020.