Calculators & Converters
ISO Week Number Calculator
The ISO Week Number Calculator finds the official ISO 8601 week number for any date -- the standard used across Europe and by many businesses for scheduling, invoicing and reporting.
ISO weeks always start on a Monday, and week 1 is defined as the week containing the year's first Thursday, which is why the ISO week-year can occasionally differ from the calendar year near January 1st.
How to Use the ISO Week Number Calculator
- Open the ISO Week Number Calculator.
- Pick any date.
- Read the ISO week number and week-year.
Frequently Asked Questions
Why might the ISO week-year differ from the calendar year?
Dates in very late December can fall in week 1 of the following year, and dates in very early January can fall in the last week of the previous year, because ISO weeks are defined relative to the first Thursday of the year.
Is this the same as the week number shown in my calendar app?
Most calendar apps that show week numbers use the ISO 8601 standard, but some regions (notably the US) use a different week-numbering convention starting weeks on Sunday -- check your app's settings if the numbers don't match.
How many weeks are in an ISO year?
Most years have 52 ISO weeks, but years where the first day of January is a Thursday (or a leap year starting on Wednesday) have 53.