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Week Number Calculator
Find the ISO week number of any date, or see the start and end dates of any week.
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Week Number Calculator Online
Find the ISO week number of any date, or see the start and end dates of any week.
Week Number Calculator: Find the ISO Calendar Week
Many schedules, invoices and shipping notices refer to a calendar week instead of a date. Enter any date to see its ISO week number, or enter a week number to see its first and last days. The tool follows the ISO 8601 standard used across Europe and in many businesses.
How to use the calculator
1. Pick a date to find its week number.
2. Or enter a year and week number to see the date range.
3. Read the week number, start date (Monday) and end date (Sunday).
4. Copy the result, for example 2026-W17.
The ISO 8601 week rules
- Weeks start on Monday.
- Week 1 is the week that contains the first Thursday of the year.
- A year has either 52 or 53 weeks.
Why week 1 can start in December
Because of the Thursday rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. The tool shows the ISO week-year which may differ from the calendar year.
Where week numbers are used
Production and delivery schedules, school and university timetables, payroll and invoicing, project plans and sprints, and shipping and logistics in many countries.
ISO weeks versus the US week system
The ISO system starts weeks on Monday and defines week 1 by the first Thursday. The common US system starts on Sunday and puts 1 January in week 1. The two disagree for the first days of January and the last days of December, which is why the same date can show as week 1 in one calendar and week 52 or 53 in another.
Frequently asked questions
What is an ISO week number?
It is the week of the year as defined by ISO 8601: weeks start on Monday, and week 1 is the week containing the first Thursday.
How do I find the week number of a date?
Pick the date in the calculator and read the ISO week number it returns.
What are the start and end dates of a given week?
Enter the year and week number. The tool returns the Monday start and Sunday end dates.
How many weeks are in a year?
Every year has 52 or 53 ISO weeks. 53-week years happen roughly every five to six years.
Why is 1 January sometimes in week 52 or 53?
If 1 January falls on Friday, Saturday or Sunday, it belongs to the last week of the previous year under ISO rules.
Why does my calendar show a different week number?
Your calendar may use the US system (Sunday start, week 1 contains 1 January). Check its settings for ISO week numbering.
Can I see the current week number?
Yes. Enter today date or use the current date to see the current ISO week.
About this tool
Many schedules, invoices and shipping notices refer to a calendar week instead of a date. Enter any date to see its ISO week number, or enter a week number to see its first and last days. The tool follows the ISO 8601 standard.
How to use: Pick a date or enter year and week number to get results.
- ISO week number and ISO week-year
- Start and end dates of any week (Monday to Sunday)
- Day of week and day of year
- Current week support