Date
Find the ISO week number for one local calendar date.
Week Number Calculator
Find the ISO week number for a date, with ISO week-year, weekday, week start, week end, and calendar year shown clearly.
How the ISO week number is calculated
This calculator uses the ISO week-date rules. Weeks run from Monday through Sunday. Week 1 is the week that contains January 4, which is also the week with the year's first Thursday. The calculation moves the selected date to its Thursday, finds that Thursday's week-year, then counts complete seven-day periods from week 1.
For example, January 1, 2021 was a Friday. Its week's Thursday was December 31, 2020, so the result is week 53 of ISO week-year 2020. January 4, 2021 was a Monday and started week 1 of ISO week-year 2021. The input is a local calendar date. Time of day and timezone offsets do not enter the calculation.
Sources: ISO 8601 date and time format.
How this week number calculator works
Choose a calendar date and the result shows the ISO week number. ISO weeks start on Monday, and the ISO week-year is shown because it can differ from the calendar year near New Year.
Supporting rows show the selected date's weekday, the Monday start date for that ISO week, the Sunday end date, and the calendar year of the selected date.
When week numbers are useful
Week numbers are useful for project planning, work schedules, classroom calendar examples, reporting periods, and personal planning. This page is a calendar lookup helper and does not apply payroll, tax, HR, government, or official reporting rules.
Related date calculators
To identify a weekday without week numbering, use the weekday calculator. To add or subtract from a date, use the date calculator. To count days between dates, use the date duration calculator. For weekday ranges, use the business days calculator.
Week number FAQ
What is an ISO week number?
ISO weeks start on Monday, and week 1 is the week that contains the first Thursday of the ISO week-year.
Can the ISO week-year differ from the calendar year?
Yes. Dates near January 1 can belong to the final ISO week of the previous year or the first ISO week of the next year.
Is this for payroll, tax, or government reporting?
No. It is a planning and calendar lookup tool, not a payroll, tax, HR, government, or official calendar system.