Comprehensive Calculator Guide
Week numbers count the weeks of a year from 1 to 52 or 53 and are used in reports, production and shipping plans. This calculator gives the ISO 8601 week number of any date, the week's year and how many weeks that year has. 19 October 2026 falls in week 43, and 2026 has 53 weeks.
The ISO 8601 rule
Weeks start on Monday, and week 1 is the week holding the year's first Thursday. So early January days can belong to the previous year's last week, and late December days to the next year's week 1: 29 December 2025 is week 1 of 2026, and 1 January 2027 is week 53 of 2026.
Calendars that start weeks on Sunday, or count the week of 1 January as week 1, can differ by one.
How to Use
- 01Pick a date.
- 02Read the week number and its year.
- 03See how many weeks that year has and the weekday.
Formula Used
Weeks start Monday; week 1 contains the year's first Thursday (ISO 8601)Practical Examples
Example 1
19 October 2026: week 43.
Example 2
1 January 2027: week 53 of 2026.
Example 3
29 December 2025: week 1 of 2026.
Important Tips
- Quote the week with its year, such as week 1 of 2026.
Common Mistakes to Avoid
- Assuming 1 January is always in week 1.
Frequently Asked Questions
- What week number is it?
- Pick today's date; 19 October 2026 is week 43.
- How many weeks are in a year?
- 52, or 53 in some years such as 2026.
- Why does my calendar show a different number?
- It may start weeks on Sunday or use another week-1 rule.