Week Number Calculator
Find the ISO week number and week of the year for any date.
Last updated · View algorithm, constants and default inputs (JSON)
Results
About this calculator
ISO week numbering, used across Europe and in most business software, defines week one as the week containing the first Thursday of the year. That means early January can belong to the last week of the previous year.
Worked example
calculated with the default values below
Date 2026-10-31 ISO week number 44 · ISO week year 2,026 · Day of the year 304 days · ISO weeks in the year 53 · Calendar quarter 4
General background references
Disclaimer
These calculators give general date results. Public holidays, working-day conventions and time-zone rules are set locally and change, so confirm against the official calendar for anything legally or contractually binding.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
W = 1 + round((T_d − T_1) ÷ 7)
- T_d
- Thursday of the date's week
- T_1
- Thursday of week 1 (contains Jan 4)
- W
- ISO 8601 week number
Date is shifted to its Thursday (Monday starts the week); ISO year is that Thursday's year. 53 weeks when Jan 1 is Thursday, or Wednesday in a leap year; day-of-year and quarter also output.
Frequently asked questions
Why can the ISO week year differ from the calendar year?
ISO week 1 is the week containing the first Thursday. Dates near New Year can therefore belong to week 52 or 53 of the previous ISO year; the calculator reports both the week number and ISO week year.