Convert ISO week numbers to dates, or find the week number for any date. Works for any year.
Find answers to commonly searched week number questions:
The ISO 8601 standard defines the week number of a year. A week starts on Monday and ends on Sunday. Week 1 is the week containing the first Thursday of the year (or the week containing January 4th).
A year can have 52 or 53 weeks. Years that start on Thursday (or Wednesday in a leap year) have 53 weeks.
WeekCalc implements the ISO 8601 week date system, the international standard for numbering weeks. Here's the methodology behind every result:
Step 1: Find the Thursday. For any given date, the ISO week number is determined by finding the Thursday of that date's week. This is because Thursday is the middle of the ISO week (Mon–Sun).
Step 2: Assign the year. The ISO week-year is the year that contains that Thursday. This means dates in late December of one calendar year may belong to Week 1 of the next ISO year (and vice versa for early January).
Step 3: Count the week. Week 1 is the week containing January 4th (equivalently, the first week that has a Thursday in the new year). All subsequent weeks are numbered sequentially.
Step 4: Handle edge cases. A year can have either 52 or 53 ISO weeks. Years that start on Thursday (or Wednesday in a leap year) have 53 weeks — these are rare but common enough to matter for accurate planning.
Our algorithm handles all edge cases including cross-year weeks, leap years, and the 53-week threshold automatically. Every result has been verified against the ISO 8601 standard and cross-checked with published calendar data.
WeekCalc is a free, open-source web tool designed to give you the most accurate ISO 8601 week number results. The calculator was built from scratch using the ISO 8601 specification, not by reverse-engineering existing tools or copying data from other websites.
Why use WeekCalc?
Data Sources: ISO 8601:2004 (International Organization for Standardization). Our implementation has been verified against Wikipedia's ISO week date article and the NIST calendar reference.
What week number is today? — Use the calculator above to find the ISO week number for today's date.
What dates are in week 1? — Week 1 of each year starts on the Monday of the week containing January 4th. See our yearly calendars for exact dates.
Does every year have 52 weeks? — Most years have 52 weeks, but some have 53 weeks. A year has 53 weeks if it starts on Thursday (or Wednesday in a leap year).
How to calculate week number? — The ISO 8601 week number is determined by finding the Thursday of the week and checking which year that Thursday falls in. Week 1 is the week containing the first Thursday of the year.