Universal Calendar with ISO Week Numbers
Browse any month from 1900 to 2100 with ISO 8601 week numbers and today highlighted.
How the Universal Calendar works
The calendar renders each month on a Monday-first grid and labels every row with its ISO 8601 week number. ISO weeks start on Monday, and week 1 is the week that contains the year’s first Thursday.
Selecting any day shows its weekday, ISO week, day of the year and whether the year is a leap year, which makes it handy for sprint planning, payroll cycles and reporting periods.
Formulas used
- ISO week
week = ⌊(dayOfYear(Thursday of week) − 1) ÷ 7⌋ + 1- Leap year
(y mod 4 = 0 ∧ y mod 100 ≠ 0) ∨ y mod 400 = 0
How to use the Universal Calendar in 3 steps
Step 1
Pick a year between 1900 and 2100 from the year selector.
Step 2
Click a month button, or press Today to jump back to the current month.
Step 3
Select any day to see its weekday, ISO week number and day of the year.
Frequently asked questions
What week number is it today?
Open the calendar and press Today. The current week is highlighted and its ISO 8601 week number is shown in the left column of the grid.
Why does a year sometimes have 53 weeks?
An ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Years such as 2026 and 2032 have 53 ISO weeks.