Week Number Calculator
NewFind the ISO 8601 week number for any date, plus its ISO week year, day-of-year, and the exact Monday–Sunday range of that week.
—
ISO 8601 week number
—
Simple week number (US-style)
Frequently Asked Questions
What exactly is the ISO 8601 week number rule?
Week 1 is defined as the week containing the year's first Thursday (equivalently, the week containing January 4th), with weeks always running Monday through Sunday — this is the standard used across most of Europe and by most software date/time libraries.
Why does the "ISO week year" sometimes differ from the calendar year?
Because ISO weeks always span Monday–Sunday, the very first or last few days of a calendar year can actually belong to a week officially assigned to the adjacent year — for example, December 31st can fall in "week 1" of the following ISO week year if it lands on a Monday.
What's the difference between the ISO week number and the "simple" week number shown alongside it?
The simple count is a common US-style alternative that just divides the day-of-year by 7 starting from January 1st, regardless of what weekday the year begins on — it's simpler to compute but doesn't follow any official standard and can drift from the ISO number, especially near the start/end of the year.
Does this account for leap years correctly?
Yes — the calculation is based on actual `Date` object day-of-year offsets, which correctly account for February 29th in leap years without any special-casing needed.
Can I use this to find which week a project deadline falls in?
Yes — many project management and manufacturing/logistics systems reference "week numbers" (e.g. "ship by week 42"), and this tool lets you quickly translate an actual calendar date into that ISO week number, or vice versa by scanning nearby dates.