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ISO Week Number Finder

Find the ISO-8601 week number for any date.

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What Is a Week Number?

Week numbers are a standardized way of identifying weeks within a year. According to ISO 8601, the international standard, weeks start on Monday, and Week 1 is the week containing the first Thursday of the year (or equivalently, the week containing January 4). This system is widely used in Europe, manufacturing, logistics, and project management.

Week numbers allow teams to communicate about time periods without referring to specific calendar dates — a project might be scheduled for "Week 14" rather than "the week of April 1–7." Our Week Number Finder shows you the ISO week number for any date instantly, along with the start and end dates of that week.

ISO 8601 Week Numbering Rules

The ISO 8601 standard defines the following rules for week numbering:

  • Each week starts on Monday and ends on Sunday.
  • Week 1 is the week containing the year's first Thursday. This means Week 1 always contains January 4.
  • A year has either 52 or 53 weeks. Years with 53 weeks occur when January 1 falls on Thursday (or Wednesday in a leap year).
  • The days of late December that fall in the next year's Week 1 are assigned to week 1 of the following year — not week 53 or 52 of the current year. Similarly, the days of early January that fall in the previous year's last week are assigned to week 52 or 53 of the previous year.
  • The ISO week year can differ from the calendar year for the first and last few days of January and December.

How to Use the Week Number Finder

  1. Enter any date using the date picker above — past, present, or future.
  2. Click Calculate to see the ISO 8601 week number for that date, along with the Monday and Sunday that bound that week.
  3. Use the Copy result button to copy the output and paste it into your project plan, spreadsheet, or communication.

Key Use Cases for Week Numbers

  • Project management — Agile sprints and project timelines are often expressed in week numbers to avoid confusion when dates span months.
  • Logistics and supply chain — Delivery schedules, production runs, and inventory restocking are commonly planned by week number in manufacturing.
  • Manufacturing — Production planning in automotive, electronics, and other industries uses ISO week numbers for scheduling factory work orders.
  • Payroll and HR — Weekly payroll cycles, timesheet submissions, and reporting periods are often tracked by week number.
  • Academic calendars — Universities and schools in many countries publish their term calendars using ISO week numbers.

Frequently Asked Questions

  • ISO 8601 is an international standard published by the International Organization for Standardization (ISO) that covers the exchange of date and time-related data. It defines a consistent format for expressing dates (YYYY-MM-DD), times, durations, and week numbers. The week numbering system defined in ISO 8601 is the most widely adopted standard for week numbers in business, government, and technology contexts worldwide.
  • Not necessarily. Because Week 1 is defined as the week containing the first Thursday of the year, it always starts between December 29 and January 4. This means Week 1 can begin in the last few days of December of the previous calendar year. For example, if January 1 falls on a Saturday, the preceding Monday (December 27) is the start of Week 1.
  • Most years have 52 ISO weeks, ending with Week 52. However, some years have 53 weeks. This happens when January 1 falls on a Thursday (or Wednesday in a leap year), because the last week of the year then has enough days to form a full week before the next year's Week 1 begins. Years with 53 weeks include 2015, 2020, and 2026.
  • No. While ISO 8601 week numbers are standard throughout Europe and widely adopted in manufacturing and logistics globally, some countries — most notably the United States — use a different convention. In the US, weeks are often numbered starting from the week containing January 1, and weeks typically begin on Sunday rather than Monday. If you need US-style week numbers, note that results may differ from ISO 8601 by one week, especially early and late in the year.