What this calculator does
Week numbers help group dates for project plans, shift schedules, accounting and international coordination. ISO 8601 is the common European standard.
This calculator shows the week number, ISO year, week start and end, day of year and remaining days for any date.
ISO week number
Week 1 = the week containing the first Thursday of the year Weeks start on Monday. Dates near the new year can belong to the previous or next ISO year, which is why the ISO year may differ from the calendar year.
How this calculator works
- 01
Pick a date
Choose any day in the current or another year.
- 02
Read week and ISO year
The week number and ISO year are shown together.
- 03
Check the week range
Week start, week end and day of year are also displayed.
Frequently asked questions
What is week 1?
Week 1 is the week containing the first Thursday of the year. It can begin in December of the previous year.
Why does the ISO year differ?
Days around New Year may belong to the first week of the following year or the last week of the previous year.
Do all countries use ISO weeks?
No. ISO 8601 is standard in Europe, while the United States often uses a different convention. State the convention for international projects.
Data basis: 2026 · Annual constants live in structured JSON files and are updated once a year.
Results are unverified model calculations and do not replace tax, legal or financial advice.