ISO Week Number Calculator
Find the ISO 8601 week number for any date and the start/end dates of any week. Enter values for instant results with step-by-step formulas.
Formula
ISO Week 1 = week containing Jan 4th (first Thursday of January)
ISO 8601 defines week 1 as the week containing the first Thursday of January. Weeks start on Monday and end on Sunday. The ISO week-numbering year may differ from the calendar year at the boundaries.
Worked Examples
Example 1: Finding the ISO Week for a Date
Problem: What is the ISO week number for March 15, 2025?
Solution: Date: March 15, 2025 (Saturday)\nDay of year: 31 + 28 + 15 = 74\nDay of week (ISO): Saturday = 6\nThursday of same week: March 13, 2025 (day 72)\nWeek number: ceil(72 / 7) = ceil(10.29) = 11\nISO notation: 2025-W11-6
Result: ISO Week 11, 2025 | 2025-W11-6 | Week: Mon Mar 10 - Sun Mar 16, 2025
Example 2: Year-Boundary ISO Week
Problem: What ISO week does January 1, 2023 belong to?
Solution: January 1, 2023 falls on a Sunday\nISO day of week: Sunday = 7\nThursday of same week: December 29, 2022\nThis Thursday is in 2022, so this week belongs to 2022\nISO Week 52 of 2022\nISO notation: 2022-W52-7
Result: 2022-W52-7 | January 1, 2023 belongs to ISO Week 52 of 2022 (not 2023!)
Frequently Asked Questions
What is the ISO 8601 week numbering system?
The ISO 8601 week numbering system is an international standard for representing dates using week numbers instead of traditional month-day formats. In this system, weeks always start on Monday and end on Sunday. Week 1 of any year is defined as the week that contains the first Thursday of January, which equivalently means it is the week containing January 4th. This definition ensures that week 1 always contains the majority of its days in the new year. An ISO week date is written as YYYY-Www-D, where YYYY is the ISO week-numbering year, ww is the week number (01 to 52 or 53), and D is the day number (1 for Monday through 7 for Sunday). Most years have 52 weeks, but some long years have 53 weeks.
Why do some years have 53 ISO weeks?
Most years have exactly 52 ISO weeks, but some years have 53 weeks. This occurs because a calendar year has 365 days (or 366 in leap years), which is one or two days more than 52 complete weeks (364 days). A year has 53 ISO weeks when January 1st falls on a Thursday in a common year, or when January 1st falls on a Wednesday or Thursday in a leap year. In practical terms, this happens roughly every 5 to 7 years. For example, the years 2004, 2009, 2015, 2020, and 2026 all have 53 ISO weeks. The extra week (week 53) occurs at the end of December when those final days belong to the current ISO year rather than week 1 of the next year. This pattern ensures consistent week numbering across year boundaries.
How does the ISO week year differ from the calendar year?
The ISO week-numbering year can differ from the calendar year at the boundaries of the year. Since ISO week 1 is defined as the week containing the first Thursday of January, the first few days of January might belong to the last ISO week of the previous year, and conversely, the last few days of December might belong to ISO week 1 of the following year. For example, December 31, 2020 was actually in ISO week 53 of 2020, but January 1, 2021 was in ISO week 53 of 2020 as well (since that week started on December 28, 2020). This discrepancy occurs because ISO weeks never span across week boundaries. When working with ISO week dates, always use the ISO year rather than the calendar year to avoid off-by-one errors.
Where are ISO week numbers commonly used?
ISO week numbers are widely used across industries and countries worldwide. In Europe, ISO week numbering is the standard for business planning, financial reporting, and scheduling. Many European countries display week numbers on their calendars by default. In manufacturing and supply chain management, production schedules and delivery timelines are often referenced by week number rather than specific dates. Software development teams using Agile methodology frequently plan sprints and releases using ISO week numbers. Epidemiologists and public health organizations report disease surveillance data by ISO week. Broadcasting and media companies schedule programming using week numbers. Tax authorities in several countries use ISO weeks for reporting periods. Most modern programming languages and databases support ISO week calculations natively.
How do I calculate the ISO week number manually?
To calculate the ISO week number manually, follow these steps. First, find the ordinal day of the year (January 1 = 1, February 1 = 32, etc.). Then determine the day of the week using Zeller's congruence or a reference calendar, where Monday equals 1 and Sunday equals 7. Next, calculate the Thursday of the same week by adding (4 minus the day-of-week) to the ordinal day. The ISO week number is then the Thursday's ordinal day divided by 7, rounded up. For dates near year boundaries, you must check whether the result belongs to the previous year (if it falls before week 1 starts) or the next year (if December dates belong to week 1 of the following year). This manual process is error-prone at year boundaries, so using a calculator or programming function is recommended for accuracy.
What is ISO 8601 and why should I use it for dates?
ISO 8601 formats dates as YYYY-MM-DD (e.g., 2025-03-15), eliminating ambiguity between US (MM/DD/YYYY) and European (DD/MM/YYYY) formats. It sorts chronologically as text, is internationally recognized, and is the standard for data exchange and APIs.