Week Number Calculator
Find the ISO 8601 or US week number for any date, and see which days fall in that week under each standard.
Example: January 1, 2024 was a Monday, so ISO week 1 of 2024 spans the first 7 days of the year. But December 31, 2023 (a Sunday) falls in ISO week 52 of 2023 — even though it's the last day before the new year — because ISO weeks always belong to the year containing most of their days.
ISO 8601 defines week 1 as the week containing the year's first Thursday; the US system starts week 1 on January 1st regardless of weekday.
How week numbers are calculated
ISO 8601 and US week numbering use different rules for where a year's weeks begin. ISO weeks start on Monday, and week 1 is anchored to the week containing the year's first Thursday (equivalently, the week containing 4 January) — this is what allows a handful of December days to belong to week 1 of the following ISO year, and why some years have 53 ISO weeks instead of 52. US weeks are simpler but less internationally standard: they start on Sunday, and week 1 always begins on 1 January regardless of which weekday that falls on.
The reverse lookup runs the same logic backwards — given a year and ISO week number, it returns that week's Monday-to-Sunday date range, useful for translating a "week 23" reference back into actual calendar dates. Week numbers are one of several ways to describe a position within the year; for a fiscal (non-calendar-year) week reference instead, see the Fiscal Year Calculator, and to find the exact number of days or weeks between two specific dates, see the Date Difference Calculator.
Frequently asked questions
- What is the difference between ISO week and US week numbering?
- ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4th). US weeks start on Sunday, and week 1 begins on January 1st regardless of weekday.
- Why does a date in late December sometimes show a week number in the next year?
- Under ISO 8601, the last few days of December can belong to week 1 of the following year (if Jan 1-3 fall on a Mon-Wed) — the isoYear field reflects this and may differ from the calendar year.
- What does the reverse lookup return?
- Given a year and an ISO week number, it returns that week's Monday-to-Sunday date range.
- What is the ISO 8601 week number?
- The ISO 8601 week number is the internationally standardised way of numbering weeks within a year — weeks run Monday to Sunday, and week 1 is defined as the week containing the year's first Thursday. It's widely used in Europe and in business/manufacturing contexts that need an unambiguous, globally consistent week reference.
- What is the first week of the year under ISO 8601?
- ISO week 1 is the week that contains 4 January, equivalently the week containing the year's first Thursday. Depending on which weekday 1 January falls on, this means the last few days of December can actually belong to week 1 of the following ISO year.
- Why can a year have 53 ISO weeks?
- Most years have 52 ISO weeks, but a year has 53 whenever 1 January falls on a Thursday, or in a leap year where 1 January falls on a Wednesday — this extra week absorbs the days that don't evenly divide into 52 seven-day weeks.
- How is week numbering used in business and manufacturing?
- Many businesses — manufacturing, retail, payroll, and project management in particular — use week numbers instead of calendar dates for scheduling and reporting, since it gives a consistent, unambiguous unit of time that's shorter than a month but longer than a day, and avoids confusion between different date format conventions internationally.