Week Number Calculator
Free week number lookup: enter a date to get the ISO week and calendar week, the owning year, this week's start and end dates, and how far into the year you are.
How to Use
- Pick a date
Today is pre-filled. Click the date input to choose any date.
- Choose the week standard
ISO weeks suit international scheduling; the calendar view matches the local convention (the week containing Jan 1 is week 1).
- Read the result
The result shows the owning year, week number, the week's start and end dates, day of year and days left.
FAQ
What is the difference between ISO and calendar weeks?
ISO 8601 says weeks start on Monday and week 1 must contain at least 4 days of the year, so a date in late December can belong to week 1 of the next year. The calendar view simply makes the week containing Jan 1 week 1.
Why can 31 December show as week 1?
ISO defines week 1 as the week containing the year's first Thursday. If 31 December falls on a Thursday or later (when Jan 1 is near a weekend), it can belong to week 1 of the next year.
Does the week start on Monday or Sunday?
In China weeks start on Monday, and ISO 8601 also uses Monday as the first day. Some countries, such as the US, start the week on Sunday.
What is this tool useful for?
Project scheduling, school weeks, training programmes and content calendars all benefit from a quick way to know which week a date falls in.