Time & Date
Week Number Calculator
Use this week number calculator for school schedules, payroll, planning, and ISO calendar references.
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.
Related calculators
Days Between Dates Calculator
Count days and weeks between two dates with optional inclusive counting.
Business Days Calculator
Count weekdays between dates while excluding Saturdays and Sundays, with optional inclusive date counting.
Age Calculator
Calculate exact age or date difference in years, months, days, total days, weeks, hours, weekdays, and next birthday countdown.
Hours Calculator
Calculate elapsed hours between start and end clock times with break subtraction and overnight support.
Time Duration Calculator
Add or subtract durations in days, hours, minutes, and seconds with normalized time output.
Time Zone Calculator
Convert a clock time between UTC offsets and show date rollover for simple time zone planning.
Last updated: May 8, 2026