Week Number Calculator
Free, locally-processed week number calculator for fast and secure results.
Runs locally in your browser
Your inputs are processed locally in your browser and are not uploaded or stored by us.
Calculate the exact ISO 8601 week number and week-year for any calendar date.
How to use the Week Number Calculator
- Enter any calendar date.
- Select Calculate to see its ISO week number.
Calculation methodology (ISO 8601)
Week 1 = The week containing January 4
The calculator conforms strictly to the ISO 8601 standard. Weeks begin on Monday. The first week of the year is mathematically defined as the week containing the first Thursday of the year.
Worked example
DateJanuary 1, 2021
January 1, 2021 fell on a Friday. Under ISO 8601 rules, this date belongs to the final week of the previous calendar year.
Result: Week 53 of 2020
Notes
- Why is the week-year different from the calendar year? Around New Year, a few days can belong to the previous year's final week (like Jan 1, 2021) or the next year's first week (like Dec 30, 2019).