ISO Week Number
Last reviewed: May 2026
The ISO 8601 week numbering system provides a standardized way to refer to weeks across all industries and countries.[1] It eliminates ambiguity about when weeks start and how they are counted. This calculator finds the week number for any date and can convert week numbers back to date ranges. For counting business days between dates, use the Business Days Calculator.
| Quarter | ISO Weeks | Date Range | Business Days |
|---|---|---|---|
| Q1 | W01–W13 | Dec 29, 2025 – Mar 29, 2026 | ~63 |
| Q2 | W14–W26 | Mar 30 – Jun 28, 2026 | ~64 |
| Q3 | W27–W39 | Jun 29 – Sep 27, 2026 | ~65 |
| Q4 | W40–W53 | Sep 28 – Jan 3, 2027 | ~62 |
The ISO 8601 week numbering system defines Week 1 as the week containing the first Thursday of the year — equivalently, the week containing January 4th. Weeks always start on Monday. This system produces some counterintuitive results near year boundaries: December 29, 30, and 31 may belong to Week 1 of the following year, and January 1, 2, and 3 may belong to Week 52 or 53 of the previous year. For example, January 1, 2026 is a Thursday, making it part of Week 1 of 2026. But January 1, 2023 was a Sunday, placing it in Week 52 of 2022 under ISO rules. This boundary behavior confuses many people but exists to ensure every week belongs entirely to one year — there are no partial weeks in the ISO system.
| Month | Start Week | End Week | Working Days |
|---|---|---|---|
| January | W01 | W04 | 22 |
| February | W05 | W09 | 20 |
| March | W09 | W13 | 22 |
| April | W14 | W18 | 22 |
| May | W18 | W22 | 21 |
| June | W23 | W27 | 22 |
| July | W27 | W31 | 23 |
| August | W31 | W35 | 21 |
| September | W36 | W40 | 22 |
| October | W40 | W44 | 22 |
| November | W44 | W48 | 21 |
| December | W49 | W53 | 23 |
Week numbers are the default scheduling language in many European businesses, manufacturing operations, and global supply chains. A German supplier promising delivery in "KW 23" (Kalenderwoche 23) means the week of June 1–7, 2026. Scandinavian companies routinely schedule meetings and deadlines by week number rather than date. Manufacturing operations plan production runs, shift schedules, and inventory cycles by week number because it provides a consistent 7-day unit that aligns perfectly with weekly work rhythms. Fiscal reporting by week rather than month avoids the inconsistency of months having 28–31 days — weekly reporting provides more uniform periods for comparing sales, output, and performance metrics.
Project managers use week numbers extensively for planning timelines, setting milestones, and tracking progress. A 12-week project starting in W14 ends in W25, making duration calculations trivial compared to counting days across month boundaries. Gantt charts in tools like Microsoft Project, Jira, and Asana often display week numbers as column headers. Sprint planning in Agile development frequently maps to week numbers — a two-week sprint starting W18 ends W19. When communicating across international teams, week numbers eliminate date format confusion (is 03/04 March 4th or April 3rd?) and time zone ambiguity because a "week" is the same everywhere regardless of when the local workday starts.
Most years have 52 ISO weeks, but roughly every 5–6 years, a year contains 53 weeks. A 53-week year occurs when January 1 falls on a Thursday, or when January 1 falls on a Wednesday in a leap year. Recent and upcoming 53-week years include 2004, 2009, 2015, 2020, 2026, and 2032. The 53rd week creates complications for businesses that budget, forecast, or report on a weekly basis — comparing "this year's Week 53 revenue" to last year's 52-week total introduces a one-week distortion. Many companies using 52/53-week fiscal years handle this by comparing 52-week periods regardless of calendar year boundaries, or by prorating the 53rd week when calculating year-over-year metrics.
The retail industry uses the 4-5-4 calendar (also called the NRF calendar), which divides the year into months of 4, 5, and 4 weeks respectively within each quarter. This ensures every "month" contains exactly 4 or 5 complete weeks, making same-store sales comparisons more meaningful than calendar months that vary from 28–31 days. The retail fiscal year typically begins on the Sunday closest to February 1. This calendar is distinct from ISO week numbering but shares the philosophy of using weeks as the fundamental time unit for business operations. Some companies use a 4-4-5 or 5-4-4 pattern instead. Understanding which week numbering system a business partner uses is essential for accurate scheduling and reporting coordination.
→ Clarify the system. When a colleague references "Week 15," confirm whether they mean ISO weeks (starting Monday) or a US convention (starting Sunday). The dates can differ by a day.
→ Watch year boundaries. Always verify week numbers near January 1 — the date may belong to the previous year's final week or the new year's Week 1 depending on which day of the week January 1 falls.
→ Use the ISO format. The standard notation is YYYY-Www (e.g., 2026-W19). This eliminates all ambiguity when communicating dates internationally.
See also: Business Days · Date Difference · World Clock · Time Zone Converter
Developers frequently work with week numbers in database queries, report generation, and scheduling systems. SQL databases provide functions like WEEK() in MySQL, EXTRACT(WEEK FROM date) in PostgreSQL, and DATEPART(WEEK, date) in SQL Server — but beware that these functions may not follow ISO 8601 by default. MySQL's WEEK() function has multiple modes (0–7) that determine which day starts the week and how the first week is defined. Mode 3 corresponds to ISO 8601, but the default mode 0 uses Sunday as the first day with different Week 1 rules. In JavaScript, getting the ISO week number requires a custom function since the built-in Date object does not provide it directly. Python's isocalendar() method on datetime objects returns the correct ISO year, week number, and weekday — note that the ISO year may differ from the calendar year near January 1.
Manufacturing operations worldwide use week numbers as their primary scheduling unit. Production schedules are published as "Week 22: produce 5,000 units of SKU-A, 3,000 units of SKU-B." Material requirements planning (MRP) systems calculate when raw materials must be ordered based on production week numbers minus lead times. Automotive manufacturers coordinate thousands of suppliers using week-based delivery schedules — a seat supplier might ship 500 units every Monday of Weeks 18–30 to a specific assembly plant. Just-in-time (JIT) manufacturing takes this further, scheduling deliveries by the hour within a production week. When a supplier references delivery in "CW 24" (calendar week 24), every party in the supply chain knows exactly which seven days are meant, eliminating the ambiguity that month-based scheduling introduces across different calendar systems and national conventions.
Accurate week numbering eliminates scheduling errors that cost supply chains millions of dollars annually in expedited shipping and production line downtime.
→ Weeks start on Monday. ISO 8601 standard; not Sunday.[1]
→ Week 1 contains the first Thursday. This is the defining rule for ISO week numbering.
→ Some years have 53 weeks. 2026, 2032, and 2037 all have 53 ISO weeks.
→ Use for project sprints. Two-week sprints align perfectly with even-numbered ISO weeks.
See also: Business Days · Date Difference · Day of Week · Countdown