Week Number Calculator
Find the ISO 8601 week number for any date.
Cómo funciona
This calculator follows the ISO 8601 international standard for numbering weeks, which is widely used in business, government, and international scheduling contexts because it removes the ambiguity of "which week is this" that exists under looser conventions.
Under ISO 8601, every week starts on Monday and ends on Sunday — unlike the US convention where weeks are often considered to start on Sunday. This single difference is enough to shift which calendar week a given date falls into compared to non-ISO systems.
Week 1 of a given year is defined as the week containing that year's first Thursday, which is mathematically equivalent to saying it's the week containing January 4th. This specific rule (rather than simply "the week containing January 1st") is what keeps ISO week numbering consistent and avoids very short partial weeks at the start of the year.
Because of this rule, the last few days of December sometimes belong to week 1 of the following year, and conversely, the first few days of January sometimes belong to the last week (week 52 or 53) of the previous year. For example, December 31, 2025 (a Wednesday) falls in ISO week 1 of 2026, since that week's Thursday (January 1, 2026) is in 2026.
Most years have 52 ISO weeks, but years where January 1st falls on a Thursday (or a leap year where it falls on a Wednesday) have 53 weeks instead, since an extra partial week is needed to fully cover the year under the Monday-start, Thursday-anchored rule.
ISO week numbers are commonly used in manufacturing, logistics, payroll, and project planning because they give every week in a year a unique, unambiguous label (e.g., "2026-W15") that doesn't depend on which day of the week a month happens to start on — useful for scheduling recurring deadlines or comparing activity across weeks consistently.
This standard is officially known as ISO 8601, the same international standard that also defines the widely used YYYY-MM-DD date format — both were designed with the same goal of eliminating regional ambiguity in how dates and time periods are written and interpreted across different countries and systems.
Why does the week number sometimes belong to a different year?
The last days of December or first days of January can fall in week 1 of the next year or the last week of the previous year under ISO rules.
What day does an ISO week start on?
Monday — unlike the common US convention of weeks starting on Sunday, which can shift which week number a date falls into.
How is week 1 defined?
Week 1 is the week containing the year's first Thursday, equivalently the week containing January 4th.
Do all years have 52 weeks?
No — years where January 1st falls on a Thursday (or a leap year where it falls on a Wednesday) have 53 ISO weeks instead of 52.
Why do businesses use ISO week numbers instead of just dates?
ISO week numbers give every week a unique, unambiguous label independent of which day a month starts on, which simplifies recurring scheduling and cross-period comparisons.
Is ISO week numbering the same as the 'week of the year' shown in some calendar apps?
Not always — some calendar apps use Sunday-start weeks or a simpler 'January 1 is always week 1' rule, which can produce different week numbers than the ISO standard, especially near year boundaries.
What format is an ISO week typically written in?
The standard format is YYYY-Www, such as 2026-W15 for the 15th ISO week of 2026.
Is ISO week numbering related to the YYYY-MM-DD date format?
Yes — both are defined by the same ISO 8601 standard, which aims to eliminate regional ambiguity in how dates and time periods are written and interpreted.
Why would a project management tool use week numbers instead of dates?
Week numbers make it easy to reference and compare recurring periods (like sprint cycles or reporting periods) consistently across a year, without needing to specify exact date ranges each time.