Date Difference
Find the time between any two calendar dates.
Date & Time
Week Number Calculator
Find the ISO week, week dates, and day of year.
Your details
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ISO week number
43
- ISO week year
- 2,026
- Week start (Monday)
- 2026-10-19
- Week end (Sunday)
- 2026-10-25
- Day of year
- 292
- ISO weeks remaining
- 10
How to use this tool
- Select the calendar date you want to inspect.
- Calculate its ISO week number and ISO week year.
- Read the Monday-to-Sunday week range and day-of-year value alongside the number.
Formula & method
ISO 8601 weeks start on Monday. Week 1 is the week containing 4 January, so dates near New Year can belong to the previous or next ISO week year. Weeks remaining refers to that ISO year.
A practical example
1 January 2021 falls in ISO week 53 of 2020. The week runs from 28 December 2020 through 3 January 2021.
When this tool is useful
Label a weekly report, identify a sprint week, or match a date to an ISO-based planning calendar. Include the ISO year when sharing a week number around New Year to prevent ambiguity.
Common mistakes and limitations
- The ISO week year can differ from the ordinary calendar year near January 1.
- Some calendar applications use Sunday-start or other numbering systems; compare the convention before comparing results.
Frequently asked questions
Why is January 1 sometimes in week 52 or 53?
ISO week 1 is the week containing January 4. Early January dates can therefore belong to the final ISO week of the previous year.
What is the first day of an ISO week?
Monday. This tool does not switch the week-number convention when you change display language.