What week number is it today?

The week number calculator automatically displays the current ISO week number. ISO weeks run Monday through Sunday, with Week 1 being the week that contains the year''s first Thursday.

How is the ISO week number calculated?

ISO week numbers follow ISO 8601 standard. Week 1 is the week containing January 4th (or the first Thursday of the year). Weeks run Monday to Sunday, numbered 1 to 52 or 53.

Does every year have 52 weeks?

Most years have 52 ISO weeks. Some years have 53 weeks — this occurs when January 1 falls on a Thursday, or on a Wednesday in a leap year. For example, 2026 has 53 ISO weeks.

What date does ISO week 1 start on?

ISO Week 1 always starts on a Monday and is the first week containing a Thursday. Use the week number calculator to look up the exact start and end dates for any week of any year.

How do I find which week number a specific date falls in?

Enter any date in the calculator. It instantly displays the ISO week number, the ISO year, and the Monday-to-Sunday range of that specific week.

What is the difference between ISO week numbers and US week numbers?

ISO 8601 weeks run Monday–Sunday with Week 1 defined by the first Thursday of the year. The US system often counts weeks starting Sunday–Saturday, with Week 1 simply being the week of January 1. This means a date near the start of a year may be in different week numbers depending on which system you use.

Can the ISO week year differ from the calendar year?

Yes. For example, December 31, 2025 falls in ISO Week 1 of 2026 because that week''s Thursday is January 1, 2026. The ISO year is 2026 even though the calendar date is 2025. The calculator always shows the correct ISO year alongside the week number.

Why do some years have 53 ISO weeks?

A year has 53 ISO weeks when January 1 falls on a Thursday (guaranteeing a 53rd week-Thursday at year-end), or when January 1 falls on a Wednesday in a leap year. 2026 is an example of a 53-week year.

How are ISO week numbers used in business?

ISO week numbers are widely used in manufacturing (e.g., automotive production schedules), retail planning (4-5-4 fiscal calendars), EU government procurement (tender deadlines reference week numbers), and agriculture (crop rotation and harvest scheduling).

Week Number Calculator

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Reviewed by

Priya Sharma

Calendar Systems & Cultural Time Expert

· MA Cultural Anthropology, Oxford· Research Fellow, Time & Culture Institute· Author of 'The Global Calendar' (2022)

Understanding ISO 8601 Week Numbers

ISO week numbering (ISO 8601) is an international standard for assigning a week number to each week of the year. It resolves a fundamental ambiguity: what happens to weeks that span year boundaries? For example, is December 31 in the last week of the old year, or the first week of the new one?

The ISO standard's answer is elegant: Week 1 is always the week containing the first Thursday of the year. Equivalently, it is the week containing January 4th. Weeks always run Monday through Sunday. This means the ISO calendar year can start as early as Monday, December 28 of the previous Gregorian year.

ISO Weeks vs. US Week Numbering

Property	ISO 8601	US System
Week starts on	Monday	Sunday
Week ends on	Sunday	Saturday
Week 1 definition	Week with first Thursday of year	Week containing January 1
Weeks per year	52 or 53	52 or 53
Standard	ISO 8601 (international)	No formal standard
Used in	Europe, manufacturing, EU government	United States, some software

Industries That Rely on Week Numbers

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Manufacturing & automotive — Production schedules and supplier contracts reference ISO week numbers (e.g., ''W14 delivery''). The automotive industry globally standardized on ISO weeks for supply chain coordination.
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Retail & inventory planning — Many retailers use a 4-5-4 fiscal calendar where each quarter has three periods of 4, 5, and 4 weeks. ISO week numbers anchor this system and enable year-over-year comparison.
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EU government procurement — European Union tender deadlines and public procurement timelines are formally expressed in ISO week numbers in official documents and contract notices.
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Agriculture & farming — Crop rotation schedules, planting windows, and harvest timelines are traditionally tracked by week number, giving farmers a consistent language across regions and years.
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Media & publishing — TV ratings, streaming metrics, and print circulation are measured and reported by ISO week, allowing direct comparison across different calendar configurations.

Week Number Calculator

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Understanding ISO 8601 Week Numbers

ISO Weeks vs. US Week Numbering

Industries That Rely on Week Numbers

Frequently Asked Questions