How this calculator works
Every date maps to one of seven weekdays because the calendar repeats on a seven-day cycle. The day of the year counts from January 1, so January 1 is day 1 and December 31 is day 365 in a common year or day 366 in a leap year. The ISO week number follows the international standard for business calendars: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. That is why early January can belong to the last week of the previous year, and late December can belong to week 1 of the next.
Formula
The weekday comes from the proleptic Gregorian calendar, with day 0 equal to Sunday. The day of the year is the number of calendar days since January 1 plus one. The ISO week number is calculated by moving to the Thursday of the current ISO week, finding the Thursday of week 1 in that year, and dividing the difference by seven. A year is a leap year when it is divisible by 4, except for century years, which must also be divisible by 400.
Worked example
November 1, 2026 is a Sunday. It is day 305 of the year and falls in ISO week 44 of 2026. February 29, 2024 was a Thursday, day 60 of a leap year, in ISO week 9.
Advertisement
Frequently asked questions
Why does the ISO week number sometimes disagree with my calendar app?
Some apps show the week number of the local week system, where weeks may start on Sunday and week 1 may be different. The ISO number here always follows Monday starts and the first-Thursday rule.
Does the calculator account for the calendar change in 1582?
No. It uses the proleptic Gregorian calendar for every year, so dates before 1582 are converted as if the Gregorian rules had always applied. That is useful for astronomy and genealogy but not for comparing historical documents.
How do I find the weekday for a date far in the future?
Enter the full ISO date, including the year. The Gregorian calendar repeats every 400 years, so the calculation remains exact for any year the date input accepts.
Is the weekend always Saturday and Sunday?
This page labels Saturday and Sunday as the weekend, which is the convention in Canada and the United States. Many countries use a Friday and Saturday weekend, and some use a single rest day.