Everyday & Conversions
Week Number Calculator (ISO 8601)
Convert a date into its ISO 8601 week number and see the Monday-to-Sunday range for that week.
Your inputs
Change any value to update the result instantly.
ISO week number
1 of 2026
Week starts (Monday)
Monday, 29 December 2025
Week ends (Sunday)
Sunday, 4 January 2026
Weekday
Thursday
Day of the year
1
ISO week-numbering years can differ from the calendar year: 1 January can belong to week 52 or 53 of the previous year.
What this calculator does
Converts a date into its ISO 8601 week number.
What your result means
The ISO week number, the ISO week-numbering year and the Monday-to-Sunday range.
Formula and working
ISO week: weeks start on Monday and week 1 is the week containing the first Thursday of the year
- 1
Collect the inputs
Date = 2026-01-01
- 2
Apply the formula
ISO week: weeks start on Monday and week 1 is the week containing the first Thursday of the year
- 3
Result
ISO week number = 1 of 2026
Frequently asked questions
Symbols and assumptions
- W
- ISO week number
- Y
- ISO week-numbering year
- • Proleptic Gregorian calendar with real month lengths (28, 29, 30 or 31 days).
- • Leap years follow the rule: divisible by 400, or by 4 but not by 100.
- • Dates are handled in UTC so a device time zone cannot shift a calendar day.
- • ISO weeks start on Monday; week 1 contains the first Thursday of the year.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Dates before 15 October 1582 used the Julian calendar in most of Europe and are outside the intended range.
- • Clock-time results ignore daylight-saving transitions unless the tool asks you to set them.
- • Some countries number weeks from Sunday or from 1 January; those conventions differ from ISO 8601.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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