ISO Week Number Calculator
Read the ISO week number, the ISO week-year, and the Monday–Sunday span of any calendar date.
Processed entirely in your browser — nothing you enter here is ever uploaded.
Result
About this tool
The field is a date input. The page reads it as year-month-day and builds a UTC date. A value that is not a real calendar day, such as a month number the control cannot hold, is rejected. There is no time and no time zone in the arithmetic. The week number is the ISO-8601 week, not a week that starts on Sunday and not a week numbered from January 1 as week 1 regardless of the weekday.
The algorithm moves the date to the Thursday of its week, because the ISO week-year is the calendar year of that Thursday. Monday is day 1 and Sunday is day 7. From the original date it subtracts the weekday and adds 1 to land on Monday, then adds 6 days to land on Sunday. The week number is the count of seven-day steps from January 1 of the week-year up to that Thursday, rounded up. 1 January 2026 is a Thursday, so it is already the Thursday of its week. That Thursday is in 2026, the week number is 1, and the span is Monday 29 December 2025 through Sunday 4 January 2026.
Dates near New Year are the cases that surprise people. 31 December 2024 is a Tuesday. The Thursday of that week is 2 January 2025, so the ISO week-year is 2025 and the week number is 1. The span is Monday 30 December 2024 through Sunday 5 January 2025. The other direction happens too: 1 January 2021 is a Friday, its Thursday is 31 December 2020, and the page reports week 53 of 2020, from Monday 28 December 2020 through Sunday 3 January 2021. The week-year on the result line is that ISO year, which is not always the calendar year of the day you typed.
The result line is the word Week, the number, and the week-year. The detail line is the Monday date, a dash, and the Sunday date, formatted with the page language's month names but still the UTC calendar days. The page does not list every week of a year, does not convert a week number back into a date, and does not apply a company calendar that starts its week on Sunday. Fiscal weeks that are not ISO weeks will not match this number. A second check of the Thursday rule: 1 January 2026 is itself a Thursday, so its week-year stays 2026 and the number is 1, even though the Monday of that week, 29 December 2025, belongs to the previous calendar year. The weekday of the day you typed is not printed here. The day-of-week page is the one that names Thursday and counts the day of the year.
Example. 1 January 2026 is a Thursday and reads Week 1, 2026, spanning Monday 29 December 2025 through Sunday 4 January 2026. 1 January 2021 reads Week 53, 2020.
Limit. The week is the ISO week, Monday through Sunday, with the week-year taken from that week's Thursday. A Sunday-start business calendar is not a mode on this page.
Frequently asked questions
What week number is 1 January 2026?
It is a Thursday, so it is the Thursday that sets the ISO week. The page reads Week 1, 2026. The Monday of that week is 29 December 2025 and the Sunday is 4 January 2026. The week-year stays 2026 because the Thursday falls in 2026.
Why is 31 December sometimes week 1 of next year?
ISO weeks belong to the year of their Thursday. 31 December 2024 is a Tuesday. That week's Thursday is 2 January 2025, so the result is Week 1, 2025, from 30 December 2024 through 5 January 2025.
Can 1 January fall in week 53 of the previous year?
Yes. 1 January 2021 is a Friday. Its Thursday is 31 December 2020, so the page reads Week 53, 2020, spanning 28 December 2020 through 3 January 2021. The number on the result is the ISO week-year, not always the year printed in the date you picked.
Does the week start on Sunday?
No. Monday is day 1 and Sunday is day 7. The detail line always runs from that Monday through that Sunday. A payroll week that starts on Sunday will not use the same number.