🗓️ Week Number Calculator
Pick a date to see which week of the year it falls in — under two different, commonly-confused systems.
1. Date
2. ISO-8601 week (Monday start)
The international standard. Weeks start Monday, and week 1 is the week containing the year's first Thursday — so the ISO week-year can differ from the calendar year for a few days around Dec 31 / Jan 1.
Week number
—
ISO week-year
—
3. US week (Sunday start)
Common US convention (matches Excel's WEEKNUM(date,1)). Weeks start Sunday, and week 1 begins on the Sunday on or before Jan 1 — so it does not require a full 7-day week to "count".
Week number
—
Calendar year
—
| Day of week | — |
|---|---|
| Day of year | — |