Week Number Calculator
Find the ISO week number for any date, plus the day of the year, days left in the year and the quarter.
How ISO week numbers work
The ISO 8601 standard numbers the weeks of a year from 1 to 52 or 53. Weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4). That means early January can belong to the last week of the previous year, and late December to week 1 of the next.
Tuesday, October 20, 2026 is in week 43. Friday, January 1, 2027 still belongs to week 53 of 2026, because that week's Thursday falls in 2026.
Where week numbers are used
Week numbers are standard in business planning, manufacturing, retail calendars and payroll, especially in Europe. In the US many calendars count weeks starting on Sunday with week 1 containing January 1, so US and ISO numbers can differ by one near the start of the year.
Years with 53 weeks
A year has 53 ISO weeks if it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 is one of them.
Frequently asked questions
What week number is it today?
Open the calculator; it shows the ISO week number for today by default.
Does the week start on Sunday or Monday?
ISO weeks start on Monday. Many US calendars start on Sunday, which can shift the number.
How many weeks are in a year?
52 or 53 ISO weeks.
Last reviewed October 2026. Results are estimates; check important decisions with a qualified professional.