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The Week Number Calculator finds the ISO week number for any calendar date and shows the full date range of that week. Week numbers are used extensively in project management, manufacturing, logistics, finance, and European business culture for scheduling, reporting, and planning purposes.
ISO 8601, the international standard for date representation, defines Week 1 of a year as the week containing the year's first Thursday. This means January 1 is not always in Week 1 — if January 1 falls on a Friday, Saturday, or Sunday, it is in the last week of the previous year (Week 52 or 53). Similarly, late December dates may belong to Week 1 of the following year. This nuance trips up many people and systems that naively assume Week 1 starts on January 1.
The ISO week always runs Monday through Sunday, with Monday as day 1. The year always has either 52 or 53 ISO weeks. A year has 53 weeks if January 1 falls on Thursday, or if it is a leap year and January 1 falls on Wednesday or Thursday. All other years have 52 weeks.
In the United States, many organizations use a simpler week numbering where Week 1 begins on January 1 (or the Sunday containing January 1) and weeks run Sunday to Saturday. This produces different week numbers than ISO — especially near the year boundary. This calculator supports both ISO (Monday-start) and US (Sunday-start) conventions.
Week numbers are particularly prominent in Scandinavian, German, Dutch, and Swiss business culture, where meeting invitations, project deadlines, and delivery schedules routinely reference 'Week 15' or 'KW 32' (Kalenderwoche in German). Manufacturing production schedules, retail sales reports, and financial cycles often align to week numbers for consistent 52-period annual reporting.
ISO week number is calculated per ISO 8601: find the Monday of the week containing the date, find the Monday that starts Week 1 of that year (the Monday of the week containing January 4), then compute: week_number = floor((date - year_start_monday) / 7) + 1. January 4 is used as the anchor because it is always in Week 1 by ISO definition.
If week_number shows 52 or 53 for a date in early January, the date belongs to the last week of the previous year per ISO convention. If it shows 1 for a date in late December, the date belongs to Week 1 of the following year. The week start/end dates show the Monday and Sunday bounding that week.
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March 13, 2026 is in ISO Week 11, running Monday March 9 to Sunday March 15
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December 31, 2026 falls in ISO Week 53, which runs into the first days of 2027
An ISO week number is defined by ISO 8601. Week 1 is the week containing the year's first Thursday. Weeks run Monday to Sunday and are numbered 1-52 (or 53). This system ensures weeks do not span too far across year boundaries.
ISO 8601 defines Week 1 as containing January 4 (or equivalently, the year's first Thursday). If January 1 falls on Friday, Saturday, or Sunday, it belongs to the last week of the previous year. This keeps weeks consistently Monday-Sunday aligned.
Most years have 52 ISO weeks. Years where January 1 (or December 31) falls on Thursday have 53 weeks. 2015, 2020, and 2026 have 53 ISO weeks. You can check with this calculator by looking at the 'Total Weeks in Year' output.
KW stands for Kalenderwoche (calendar week). German, Austrian, and Swiss businesses routinely use KW numbers for scheduling. 'KW 15' means ISO Week 15. This convention is standard in DACH business culture and appears on German-language calendars.
It depends on the year. If January 1 falls on Monday-Thursday, it is in Week 1 of that year. If it falls on Friday-Sunday, it is in Week 52 or 53 of the previous year. Check each specific year with this calculator.
ISO 8601 week date format: YYYY-Www-D. For example, 2026-W11-5 means Week 11 of 2026, Friday (day 5). This format is used in technical contexts, particularly in European software and logistics systems.
No. ISO week numbers (Monday-start, Week 1 = first Thursday) are standard in Europe. The US commonly uses a Sunday-start system where Week 1 begins January 1. This creates different week numbers for dates near year boundaries.
Most years end on week 52. A year has a week 53 when January 1 falls on Thursday (or leap year January 1 on Wednesday), because the Gregorian calendar is 365.25 days while 52 weeks = 364 days, leaving ~1.25 extra days that accumulate.
Project managers use week numbers for sprint planning, milestone scheduling, and Gantt charts. References like 'deliverable due Week 24' are unambiguous across team members. Weekly status reports are often labeled by week number for easy chronological sorting.
December 25 is typically in ISO Week 52. In years where December 25 falls late in a week that extends into January, it could be in Week 52 of the current year or Week 1 of the next. Use the calculator to check specific years.
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