Week Number Calculator
Find the ISO 8601 week number for any date.
ISO week numberWeek 40, 2026
Uses the ISO 8601 standard, where week 1 is the week containing the year’s first Thursday.
Ad space — always below the tool, never inside it
How to use week number calculator
1Pick a dateChoose any date.
2Read the week numberSee its ISO 8601 week number and year.
Questions people ask
ISO 8601 defines week 1 as the week containing the year's first Thursday, so late-December dates can belong to week 1 of the following year, and vice versa.
Not always — Excel's default week numbering differs from ISO 8601 unless you specifically choose the ISO option.