Week Number Calculator

Find the current week number or the week number for any date. Supports ISO week numbers, US week numbering, and custom week starts.

Week number for any date

Selected date

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ISO week number

Today

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ISO: —

US: —

Custom: —

Show full year week calendar
Week Start date End date

How the week number calculator works

This tool calculates week numbers for any date using three different systems:

  • ISO week number (ISO-8601, widely used in Europe and in software)
  • US week number (weeks starting on Sunday, week 1 contains January 1)
  • Custom week number (you choose the first day of the week and how week 1 is defined)

ISO week number rules (ISO-8601)

ISO-8601 defines weeks like this:

  • Weeks start on Monday.
  • Week 1 is the week that contains January 4 (or equivalently, the first Thursday of the year).
  • Week numbers run from 1 to 52 or 1 to 53.

ISO week number algorithm (conceptual)

  1. Take the date and move it to the Thursday of the same week.
  2. Compute the number of weeks between that Thursday and the Thursday of week 1.
  3. Add 1 to get the week number.

The calculator uses a robust implementation of this algorithm that correctly handles years with 52 or 53 weeks.

US week numbers

Many US calendars and business systems use a simpler rule:

  • Weeks start on Sunday.
  • Week 1 is the week that contains January 1.

This can cause the US week number to differ from the ISO week number, especially near the start and end of the year.

Custom week start and week 1 definition

In the Custom tab you can:

  • Choose any day (Monday–Sunday) as the first day of the week.
  • Choose how week 1 is defined:
    • Week containing January 1 – common in business calendars.
    • First full week – the first week that lies entirely in the new year.
    • ISO-like (4+ days in year) – a week is week 1 if it has at least 4 days in the new year.

Examples

  • 2025-01-01 (1 January 2025)
    • ISO week: likely week 1 (depending on weekday)
    • US week: week containing January 1 is week 1
  • Last days of December
    • Under ISO, they may belong to week 1 of the next year.
    • Under US rules, they are always in the last week of the same year.

FAQ

What week number is it today?

When you open this page, the calculator automatically loads today’s date and shows the current week number in the Selected date and Today panels.

Why do some years have 53 weeks?

A year has 365 days (or 366 in a leap year). That’s 52 full weeks plus 1 or 2 extra days. Depending on how week 1 is defined, those extra days can create a week 53. ISO-8601 has specific rules that sometimes produce week 53.

Can I export or print the week calendar?

You can expand the full year week calendar section and print the page or copy the table into a spreadsheet for further formatting.

Which system should I use?

  • Use ISO weeks if you work with international teams, software systems, or European partners.
  • Use US weeks if your organization follows a Sunday–Saturday business calendar.
  • Use Custom if your company or project has its own definition of weeks.