Week Number Calculator
Find the ISO 8601 week number and week-year of any date, the US week number and day of year, or get the dates of a given week number.
What is Week Number Calculator?
Shows which week of the year a date falls in, using both the ISO 8601 system (common in Europe and Asia) and the US system. It can also do the reverse: give a year and week number and get the dates of that week.
How to use Week Number Calculator
- In Date → week, pick a date to see its ISO week (e.g. 2026-W11), the US week number, the weekday, the day of the year and the Monday–Sunday range.
- Switch to Week → dates to enter a Year and Week number.
- Choose the Numbering system: ISO 8601 (weeks start Monday) or US (weeks start Sunday).
- Read the start and end dates of the week and the list of its seven days.
Example
2027-01-01
ISO week 2026-W53, US week 1
1 January 2027 is a Friday, so it belongs to the last ISO week of 2026.
When to use Week Number Calculator
Use ISO weeks for European business calendars, sprint planning and data reporting (YYYY-Www). Use US week numbers for calendars that start on Sunday. Some companies define their own fiscal weeks, which this tool does not cover.
How Week Number Calculator works
ISO 8601: weeks start on Monday and belong to the year that contains their Thursday, so week 1 is the week with the first Thursday of January and some days in late December or early January belong to the neighboring week-year; a year has 52 or 53 weeks. US (Excel WEEKNUM type 1): weeks start on Sunday and week 1 is the week containing 1 January, so a year can have 53 or 54 weeks. All calculations use UTC day numbers.
Common Use Cases
- Finding the current week number for a timesheet
- Converting a week number from a delivery note into dates
- Labelling weekly reports with ISO week identifiers
- Checking whether a year has 53 ISO weeks
FAQs
Why does 1 January sometimes belong to week 52 or 53?
In ISO 8601 a week belongs to the year of its Thursday. If 1 January is a Friday, Saturday or Sunday, it is part of the last week of the previous ISO year.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday and week 1 contains the first Thursday; US weeks start on Sunday and week 1 contains 1 January.
How many weeks does a year have?
52 or 53 ISO weeks. With the US system, 53, or 54 when a leap year starts on a Saturday (as in 2000).