Week Number Lookup
Enter a calendar date or ISO week. The calculator uses UTC calendar arithmetic and reports whether the ISO week-year has 52 or 53 weeks.
A QUICK WALKTHROUGH
How to use this tool
- Choose date lookup or ISO week lookup.
- Enter an exact YYYY-MM-DD date or YYYY-Www week in years 0001–9999.
- Calculate, then read or copy the complete week result.
ISO rules and reverse lookup
Weeks run Monday through Sunday. Week 1 contains the first Thursday (equivalently January 4). An ISO week-year can differ from the calendar year. Reverse lookup validates the week-year’s actual 52/53-week count.
UTC dates and supported range
Calculations use UTC dates, avoiding time zones and daylight-saving shifts. Years 0001–9999 are supported, including 0001–0099; a result crossing that range is rejected.
Private browser calculation
Validation, calculation and copying happen locally. Nothing is uploaded or stored.
GOOD TO KNOW
Common questions
What does the week-year mean?
It is the ISO year that owns the week, which can differ from the calendar year near New Year.
Can every year have week 53?
No. A year has 53 ISO weeks if January 1 is Thursday, or Wednesday in a leap year. Other years have 52.
Does the date leave my browser?
No. All date and week calculations run locally.