What week number is it?
Find the official ISO 8601 week number for any date — the numbering system used in business, engineering, and project planning worldwide.
How this works
Pick any date and this calculator returns its ISO 8601 week number — the standard used by most of the world outside the US for numbering weeks (1 through 52, occasionally 53). If today's default date isn't what you want, just change it.
What "ISO week number" actually means
The ISO 8601 standard, first published by the International Organization for Standardization in 1988, defines a week as running Monday through Sunday, and defines Week 1 of any year as the week containing that year's first Thursday. Equivalently, Week 1 is the week that contains January 4th. This specific rule exists so that week numbering stays consistent year to year and doesn't get thrown off by which day of the week January 1st happens to land on.
One consequence of this rule: the very first days of January sometimes belong to the last week of the previous year, and the very last days of December sometimes belong to week 1 of the following year. For example, December 31, 2025 actually falls in ISO week 1 of 2026, not week 52 of 2025 — because the Thursday of that particular week lands in January. That's not a bug — it's how the standard is designed to work, and this calculator follows the same rule precisely.
Why some years have 53 weeks instead of 52
A standard year has 365 days, which divides into exactly 52 weeks plus one extra day. That extra day (two extra in a leap year) means roughly every 5 to 6 years, an ISO year ends up with 53 full weeks instead of 52 in order to keep the "Monday-start, contains-Thursday" rule consistent. Specifically, a year has 53 ISO weeks if January 1st falls on a Thursday, or if it's a leap year and January 1st falls on a Wednesday. This calculator handles that automatically — you don't need to know the rule to get the right answer, but it's useful context if a "week 53" result looks surprising.
Why businesses and engineers rely on week numbers
Week numbers show up constantly in contexts where "the week of August 31st" is ambiguous but "week 36" is not. Manufacturing date codes on electronics and food packaging often encode a week number directly, so a product stamped with a 4-digit code can reveal exactly when it was made. Payroll systems, especially in Europe, frequently run on week numbers rather than calendar months. Agile software teams commonly refer to sprints by ISO week ("we're shipping in week 41"), and international logistics and shipping schedules lean on week numbers specifically because they're unambiguous across languages, countries, and even which day a "week" is assumed to start on in casual conversation.
Related tools: Days between two dates · What day of the week was a date?
Frequently asked questions
Does every year have exactly 52 weeks?
Most years have 52 ISO weeks, but years where January 1st falls on a Thursday (or falls on a Wednesday in a leap year) have 53 weeks instead. This calculator accounts for that automatically — no manual adjustment needed.
Why does a date in late December sometimes show as "week 1"?
Because ISO weeks belong to whichever year contains that week's Thursday. If December 29th–31st fall in a week whose Thursday lands in January, that week counts as week 1 of the new year — even though most of the week's actual days are still in December. This is a deliberate, standardized rule, not an inconsistency.
Is this the same as the week numbers in my calendar app?
Most calendar apps that show week numbers (Outlook, most European calendar apps, Google Calendar's optional week-number display) use this same ISO 8601 standard. Some US-centric tools instead start weeks on Sunday and number them differently from day one of the year — if your numbers don't match, check which convention that specific app is using before assuming an error.
What's the difference between an ISO week and a "calendar week"?
"Calendar week" is an informal term that can mean different things depending on the country and convention. ISO week specifically refers to the ISO 8601 standard's Monday-start, Thursday-anchored definition — it's the one precise, internationally agreed-upon version among several looser regional conventions.
ISO week numbers are not US week numbers
ISO-8601 weeks start on Monday. Week 1 is the week with the year’s first Thursday. US retail or school calendars often start weeks on Sunday and may number week 1 differently in early January. If a shipment label says “week 12,” ask which standard the warehouse uses.
Late December dates can belong to week 1 of the next year under ISO rules. That looks wrong until you remember the Thursday rule.