Week Number Calculator
Find an ISO week number from a date, or see the Monday-to-Sunday dates for any ISO week.
Find any date's ISO week.
ISO weeks run Monday to Sunday. Today is shown in Sydney time.
Choose a date from 1900 to 2199.
Sydney today: 2026-09-21
Results update as you type.
Your result
2026-W39ISO week 2026-W39
Week 39
- Week starts
- Monday
- Weeks in 2026
- 53
How ISO week numbers work
ISO weeks start on Monday and end on Sunday. Week 1 is the week containing 4 January, so dates near New Year can belong to the previous or following ISO week-year. Compare annual weeks, working days and pay periods in our guide to weeks in a year.
The calculator follows the ISO 8601 week-date method.
Frequently asked questions
What is an ISO week number?
ISO week numbering divides a year into Monday-to-Sunday weeks. Week 1 is the week containing 4 January, and a week-year has either 52 or 53 weeks.
Why can a January date belong to the previous week-year?
A week belongs to the calendar year of its Thursday. This means the first few days of January can still be in the final ISO week of the previous year, while late December can be in week 1 of the next year.
When does a year have 53 ISO weeks?
A week-year has 53 ISO weeks when 1 January is a Thursday, or when it is a Wednesday in a leap year. The calculator checks the week-year you enter.
Related tools
Picked for this page.
Weeks in a year
Compare weeks, working days and pay periods across calendar and financial years.
Date calculator
Count the days between dates, or add and subtract days, weeks and months.
Working days calculator
Count business days between dates, excluding your state's public holidays.
Leave planner
Find the best dates to book annual leave around public holidays.
