Time & Date
Week Number Calculator
Use this week number calculator for school schedules, payroll, planning, and ISO calendar references.
Try an example
Result
ISO week
—
- Week number
- —
- ISO week-year
- —
Student quick launch
Grade planning, algebra checks, and formulas students reach for most.
Study path
Use this calculator with
Follow these when you want the formula behind the answer, a short lesson, or nearby tools in the same topic.
What the week number calculator solves
Use this week number calculator for school schedules, payroll, planning, and ISO calendar references.
Formula
How to use it
- Enter a date.
- Read the ISO week number.
- Check the ISO year for dates near New Year's.
How to read the answer
Some early-January dates belong to the final ISO week of the previous year, and some late-December dates belong to week 1 of the next ISO year.
Common mistakes and edge cases
- US week numbering and ISO week numbering can differ.
- The ISO week starts on Monday.
- The ISO week-year can differ from the calendar year.
Worked examples
Current default
ISO week
2026-W19
Near New Year
ISO week
2026-W53
Frequently asked questions
What is ISO week 1?+
It is the week containing the first Thursday of the ISO week-year.
Why can the ISO year differ?+
Week-based years keep full Monday-through-Sunday weeks together.
About this calculator
- Written by
- mathcheck editorial team
- Last reviewed
- May 8, 2026
Method
- Uses the values entered by the user and stable formulas documented on the page.
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Last updated: May 8, 2026