Free time and date tool
ISO Week Number Calculator
Apply ISO 8601 rules, where weeks start Monday and week 1 contains the first Thursday.
✓ Your time and date calculations are not stored.
Worked examples you can reproduce
These fixed inputs are calculated by the same engine as the tool above. Compare the result and the boundary rule, then load an example to change one variable. Business-day examples do not subtract holidays.
Example 1
January 1 can belong to the previous ISO week-year. Use the ISO year together with the week number, not the calendar year alone.
Inputs
- Date
- 2021-01-01
Calculated result
- Date
- January 1, 2021
- ISO week
- 53
- ISO year
- 2020
- Start date
- December 28, 2020
- End date
- January 3, 2021
Example 2
The last days of December can belong to week 1 of the next ISO year. Group reports by the full ISO year/week pair.
Inputs
- Date
- 2024-12-30
Calculated result
- Date
- December 30, 2024
- ISO week
- 1
- ISO year
- 2025
- Start date
- December 30, 2024
- End date
- January 5, 2025
Example 3
ISO weeks run Monday through Sunday. The displayed boundaries cover the full week, not only your workweek.
Inputs
- Date
- 2026-10-01
Calculated result
- Date
- October 1, 2026
- ISO week
- 40
- ISO year
- 2026
- Start date
- September 28, 2026
- End date
- October 4, 2026
Detailed reading
Continue with the English editorial guides for worked reasoning, checklists, and source references.
Primary references
How it works
Find the ISO week number, week-year, start and end for any date.
- Select any calendar date.
- Read the ISO week number and ISO week-year.
- Use the displayed Monday start and Sunday end to plan the full week.
Example
January 1 can belong to the final ISO week of the previous week-year when the first Thursday falls later.
How accuracy is handled
ISO 8601 weeks start on Monday, and week 1 is the week containing the year’s first Thursday.
Privacy: Inputs are validated and calculations use calendar-aware IANA time-zone rules.
Frequently asked questions
Can the ISO year differ from the calendar year?
Yes. Dates close to New Year can belong to the adjacent ISO week-year.
What day starts an ISO week?
Monday.