Week Number Calculator
Enter any date to see its ISO 8601 week number along with the day of the year, the weekday, which quarter it falls in and how many days remain until the end of the year. Everything is computed in your browser using the international standard, so the numbers match calendars, payroll systems and spreadsheets that follow ISO weeks.
How to find the week number for a date
- Pick a date with the date picker, or start from today's date which is filled in automatically.
- Read the ISO week number and the matching weekday in the result cards.
- Check the day of the year, the quarter and the days left in the year for the same date.
Examples
First Monday of 2024
2024-01-01
ISO week 1, day 1 of 365, Monday, Q1, 365 days left
New Year that lands in the prior week 53
2021-01-01
ISO week 53, day 1, Friday, Q1
Frequently asked questions
What is an ISO week number?
ISO 8601 numbers the weeks of a year from 1 to 52 or 53. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year (equivalently, the week holding January 4th). It is the week numbering used across Europe, by many payroll and planning systems, and by spreadsheet ISOWEEKNUM functions.
Why can January 1st be week 52 or 53 instead of week 1?
Because ISO weeks always run Monday to Sunday and belong to whichever year holds their Thursday. If January 1st falls on a Friday, Saturday or Sunday, that week's Thursday was in December, so the date counts as the last week (52 or 53) of the previous year. Late December dates can likewise roll forward into week 1 of the next year.
What does day of the year mean?
It is the ordinal position of the date within its year, counting January 1st as day 1. December 31st is day 365 in a common year and day 366 in a leap year. It is handy for logs, scientific data and any system that uses Julian-style day numbers.
How is the quarter worked out?
The calendar quarter comes straight from the month: January to March is Q1, April to June is Q2, July to September is Q3, and October to December is Q4. This is the standard calendar quarter, not a company's fiscal quarter, which may start in a different month.
Does this handle leap years correctly?
Yes. Leap years are detected with the full Gregorian rule (divisible by 4, except centuries unless divisible by 400), so 2024 has 366 days while 2100 will have 365. The day of the year and the days left in the year both reflect this.
Is my date sent to a server?
No. The calculation runs entirely in your browser. The date you choose never leaves your device, so you can use it for private or sensitive scheduling without anything being uploaded.
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