Find the ISO week number for any date.
How It Works
How Week Number Calculator Works
Using the ISO 8601 standard, weeks run Monday through Sunday, and week 1 of each year is defined as the week containing that year's first Thursday — which is the same as the week containing January 4th. Dates near the very start or end of a calendar year can end up belonging to a week number from the adjacent year under this rule.
FAQ
Frequently Asked Questions
Why might a date in late December show as week 1?
Because ISO week 1 is defined by which week contains January 4th, the last few days of December sometimes fall into what's technically week 1 of the following year rather than the final week of the current one — this is expected, standard behavior, not an error.
Is this the same "week number" my spreadsheet shows?
Not always — some spreadsheet functions use a simpler Sunday-start numbering instead of the ISO Monday-start, Thursday-anchored standard, which can shift the week number by one near year boundaries.
