Week Number Calculator
Pick a date to see which week of the year it is, the weekday, the day of the year, the quarter and the days left in the year. The ISO week number is shown with the week's first and last day, and a second week number uses the other common rule, where the week with 1 January is week 1. You can also enter a year and a week number to get the dates of that week. Everything runs in your browser.
Find the dates of an ISO week
The first number follows ISO 8601: weeks run Monday to Sunday and week 1 is the week with the year's first Thursday, which is the week that holds 4 January. A year has 52 or 53 such weeks. The second number is the other common system, in which the week that contains 1 January is week 1, so week 1 can be short and the last days of December can be week 53. It is the default of the spreadsheet function WEEKNUM, and you can choose the day the week starts. Companies and countries number weeks in different ways, so check which one your calendar uses. Nothing you enter is uploaded.
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Student ToolsAbout the Week Number Calculator
There is more than one way to number weeks, and this tool shows the two common ones. The ISO 8601 rule says weeks run from Monday to Sunday and week 1 is the week with the year's first Thursday, which is the same as the week that holds 4 January. Each week belongs to the year that holds its Thursday, so 1 January 2027, a Friday, is in week 53 of 2026, and 4 January 2027 starts week 1 of 2027. A year has 52 or 53 ISO weeks: 53 when 1 January or 31 December is a Thursday, as in 2026, 2020 and 2015. The other rule makes the week that contains 1 January week 1, with weeks starting on a day you choose, Sunday by default. It is the default of the spreadsheet function WEEKNUM. With it, week 1 can be a short week, and the last days of December can be week 53. The two numbers often differ by one, so check which one your calendar, payroll or school uses. The reverse tool takes a year and an ISO week number and gives its Monday and Sunday, and tells you when a year does not have a week 53. Dates are whole days in the Gregorian calendar, with no time zone. Nothing you enter is uploaded.
What is an ISO week number?▾
It is the week number defined by ISO 8601. Weeks run from Monday to Sunday, and week 1 is the week that contains the year's first Thursday, which is the week with 4 January. For example, Saturday 24 October 2026 is in week 43, written 2026-W43-6.
Why is 1 January sometimes in week 52 or 53?▾
A week belongs to the year that holds its Thursday. If 1 January falls on a Friday, Saturday or Sunday, its week has its Thursday in the old year, so 1 January 2027 is in week 53 of 2026. The last days of December can likewise be in week 1 of the next year.
Which years have 53 weeks?▾
A year has 53 ISO weeks when 1 January is a Thursday, or when 31 December is a Thursday. 2015, 2020 and 2026 have 53 weeks, and the next after 2026 is 2032. All other years have 52.
Why do the two week numbers differ?▾
The ISO rule starts week 1 on the Monday of the week with the first Thursday, while the other rule makes the week containing 1 January week 1. So 1 January 2027, a Friday, is in week 53 of 2026 by the ISO rule and in week 1 by the other rule. Use the one your calendar uses.
How do I find the dates of a week number?▾
Use the second part of the tool: enter the year and the ISO week number to get the Monday and the Sunday of that week. Week 43 of 2026 runs from Monday 19 October to Sunday 25 October.