30 September 2026
A Wednesday in week 40 of 2026.
Week Number Calculator
Find the ISO week number of any date, or the dates of any week of the year.
Week number of a date
Week
Pick a date.
Dates of a week
Dates
Week 1 of 2026
29 December 2025 – 4 January 2026
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How the Week Number Calculator works
Many businesses and calendars number weeks with ISO 8601: weeks start on Monday, and week 1 is the week that contains the year's first Thursday. Pick a date to see its week number, or enter a year and week to see its Monday-to-Sunday dates.
Formula
week = the week containing that date's Thursday, counted from the week holding 4 January
Examples
Weeks that cross the new year
1 January 2021 is in week 53 of 2020, and 30 December 2024 is in week 1 of 2025.
Frequently asked questions
Why do some years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday (or a Wednesday in a leap year) — 2020 and 2026 do.
Is this the US week number?
US calendars often start weeks on Sunday and count 1 January as week 1, so numbers can differ by one.
How is it written?
As 2026-W40, the ISO format, which the result copies.
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