How ISO weeks work
ISO week: Monday start, week 1 holds the first Thursdayso a year has 52 or 53 numbered weeks
- date
- the day you enter
- week 1
- the week containing January's first Thursday
- week number
- 1 to 53 under the ISO standard
Because week 1 is fixed to the first Thursday, the first few days of January sometimes belong to week 52 or 53 of the year before, and late December can fall in week 1 of the next year.
Frequently asked questions
What week number is it?
Leave the date on today; the week-number line shows the ISO week, 1 to 53.
When does a week start?
Under ISO 8601, on Monday.
Why is early January sometimes week 52 or 53?
Because week 1 must contain the year's first Thursday, so days before it belong to the previous year's last week.
How many weeks are in a year?
Usually 52, but some years have 53 ISO weeks.