Calendar / 02
What Week Is It?
Today's week number at a glance, with the dates this week runs from and to. Look up the week of any date or the dates of any week, and see every week of the year in one table.
What Week Is It?: The page works out the ISO 8601 week number from this device's date, along with the US-style number where weeks start on Sunday. The lookups and the year table use the same rules, including years with 53 weeks and the days around New Year that belong to the neighbouring year's weeks. Runs 100% locally in your browser with zero server file uploads.
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- Time & calendar tools
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The ISO 8601 week rule
ISO 8601 numbers the weeks of a year from Monday to Sunday. Week 1 is the week with the year's first Thursday in it, which always contains 4 January. That rule keeps every week whole: a week is never split between two years, so the few days around New Year can belong to the neighbouring year's weeks.
Because a year is 52 weeks and one or two days long, some years have a 53rd week: those that start on a Thursday, and leap years that start on a Wednesday. The ISO year of a week can therefore differ from the calendar year of a date, which is why the page shows it when they do not match.
Other week numbering systems
Software and countries do not all count alike. The common US convention starts weeks on Sunday and puts 1 January in week 1 whatever day it falls on, so its week 1 can be a single day long. Spreadsheets offer both: in Excel, ISOWEEKNUM gives the ISO number and WEEKNUM the US-style one by default.
When a colleague abroad mentions week 12, it is safer to confirm the dates than to assume which system they mean. The ISO week number calculator gives the number for a single date, and the printable calendar can print week numbers beside each row.
How to use it
- Read this week's number at the top, with its Monday and Sunday.
- Pick a date to see its ISO and US week numbers, or type a year and week to see its dates.
- Scroll the table for every week of a year, or download it as CSV.
Privacy & limitations
Everything is worked out in your browser from this device's date. Nothing is sent anywhere.
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Frequently asked questions
How are ISO week numbers counted?
Weeks run from Monday to Sunday, and week 1 is the week that contains the year's first Thursday, which is the same as the week that contains 4 January. The days before it belong to the last week of the previous year.
Why does 1 January sometimes fall in week 52 or 53?
Because week 1 is the first week with a Thursday in the new year. When 1 January is a Friday, Saturday, or Sunday, those days still belong to the last week of the old year; 1 January 2021, for example, was in week 53 of 2020.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. 2020 and 2026 have 53 ISO weeks; most years have 52. The table on this page shows how many the chosen year has.
Why is the US week number different?
In the common US convention weeks start on Sunday and week 1 is the week that contains 1 January, however short. So the two numbers are often different, usually by one, and around New Year by more. Planning tools in Europe usually mean the ISO number.
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