Week Number Calculator

Find the ISO week number, day of year and day of week for any date

What is it and how does it work?

Week numbers are used in business, project management, manufacturing, and logistics to refer to a specific week of the year without naming the dates. "Deliver by W42" or "sprint W19" is more compact than "deliver by October 14–18". This calculator converts between calendar dates and ISO week numbers, answers which week number a given date falls in, and shows you all the dates in any given week number.

The ISO 8601 standard defines week numbering unambiguously: weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4th). This means some years have 53 weeks; and the last days of December or first days of January may belong to the week number of the adjacent year — a common source of confusion this tool clarifies.

Common use cases

• Project deadline: "the sprint ends in W34" — find the exact Monday–Sunday dates for week 34 of the current year to block your calendar.
• Reporting cycle: determine whether December 31st falls in week 52 or week 1 of next year for correct fiscal/reporting attribution.
• Manufacturing schedule: convert a production order date range to ISO week numbers to align with supplier weekly delivery windows.

Frequently asked questions

What is ISO week numbering and how does it differ from US week numbering?

ISO 8601 weeks start on Monday; week 1 contains the year's first Thursday. US (and some other systems) weeks start on Sunday; week 1 is the week containing January 1st. This means ISO W01 and "Week 1" in a US calendar can start on different dates. Most business and international standards use ISO.

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when January 1st falls on Thursday, or (in a leap year) when it falls on Wednesday or Thursday. This happens roughly every 5–6 years. 2015, 2020 and 2026 are examples of 53-week years.

Why does December 31st sometimes belong to week 1 of the next year?

Because ISO week 1 of the new year may start before December 31st. If December 31st is a Monday, Tuesday, or Wednesday, it falls in the last week of the next year's week 1. The week "owns" the Thursday — if the Thursday is in January, all of that week is week 1.

How do I write an ISO week date?

The ISO format is YYYY-Www-D, where W is literal, ww is the two-digit week number, and D is the day of week (1=Monday, 7=Sunday). Example: 2024-W42-3 is Wednesday of week 42 in 2024.

Time

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