Week Number & Day of Year Calculator
Find the current ISO 8601 week number, day of the year, annual progress, or look up calendar metrics for any date.
Look Up Week Number for Any Date
Select any past or future calendar date to calculate its exact ISO 8601 week number and day statistics.
The Week Number and Day of Year Calculator determines the current standard ISO 8601 week number, calendar day of the year, annual progress percentage, and fiscal quarter. Widely used across Europe and international business for project planning, manufacturing schedules, sprint calendars, and logistics, the tool also includes an interactive date lookup to inspect week numbers for any historical or future date.
How to Use the Week Number & Day of Year Calculator
- Review the hero cards at the top of the page to inspect the current live ISO week number, full ISO week-date code (e.g. 2026-W38-3), and active week date span (Monday to Sunday).
- Examine the year progress bar showing percentage of the calendar year completed, current day of the year (e.g. Day 259 of 365), and days remaining.
- Check secondary metrics: active fiscal quarter (Q1–Q4) and leap year status.
- Use the "Look Up Week Number for Any Date" calendar picker to choose any specific date and view its corresponding ISO week number, start/end dates, and quarter.
Key Features & Precision
Strict ISO 8601 Compliance
Implements the international standard Thursday rule (Week 1 is the week containing the first Thursday of the year).
Full ISO Week Date Code
Displays complete international format (YYYY-Www-D) representing year, week number, and day of week.
Annual Progress Visualizer
Live progress bar illustrating the percentage of the current year elapsed with days passed and days remaining.
Historical & Future Date Lookup
Calculates the exact week number and fiscal quarter for any selected date across past and future years.
Common Use Cases
Corporate & Fiscal Planning
Align quarterly roadmaps, sprint milestones, and business reviews with standardized international week numbers.
Supply Chain & Manufacturing
Coordinate factory production batches, shipping manifests, and delivery schedules specified by week (e.g. Week 42 delivery).
Academic & School Schedules
Track term schedules, semester weeks, and exam periods structured around calendar week numbers.
Frequently Asked Questions
How is the ISO 8601 week number determined?
According to ISO 8601, weeks begin on Monday and are numbered 1 through 52 (or 53). Week 1 of a year is defined as the week containing the first Thursday of that year (which is equivalent to the week containing January 4th).
Can a date in early January belong to the previous year's week number?
Yes. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to Week 52 or 53 of the previous ISO week-year because it does not contain the first Thursday.
How many weeks are in an ISO year?
Most years have 52 ISO weeks (364 days). Years in which January 1 falls on a Thursday (or Wednesday in a leap year) have 53 ISO weeks (371 days).