ISO Week Number Calculator
FreeFind the ISO 8601 week number for any date. View the full year week grid, jump to any week, and understand where weeks 1 and 53 fall. Free.
What's next
Settings guide
When a year has 53 weeks: ISO years have 53 weeks when January 1 falls on Thursday, or when it is a leap year and January 1 falls on Wednesday. 2015, 2020, 2026, and 2032 are examples of 53-week years. Week 53 contains the final days of December in those years.
Week 1 definition: Week 1 contains January 4 every year. Equivalently: it is the week with the year's first Thursday. This means Week 1 always starts on a Monday between December 29 and January 4.
US week numbering vs ISO: Some North American systems number weeks with Sunday as the first day of the week, and define Week 1 as the week containing January 1. This produces different week numbers than ISO 8601. If a system displays week numbers and does not specify ISO 8601, verify which convention it uses before relying on week numbers for cross-system planning.
Fiscal weeks: Some organizations use a fiscal year week numbering starting from their fiscal year start date rather than January 1. The ISO week calculator uses the calendar year — fiscal week calculation requires knowing the fiscal year start.
Format comparison
vs looking it up in a calendar app: Calendar apps (Google Calendar, Outlook) can show week numbers in the sidebar, but they are not always configured to display them and may use Sunday-start week numbering rather than ISO Monday-start. This calculator always uses ISO 8601 and shows the full year grid for context.
vs manual calculation: Manually determining the ISO week number for an arbitrary date requires knowing the day of the week, computing the Thursday, and applying the ISO rule. The calculation is reliably error-prone at year boundaries. The calculator handles all edge cases including the 53-week year and the December/January week boundary.
How it works
Enter any date
Click the date input or type a date. The ISO week number for that date appears immediately.
See the week's date range
The start (Monday) and end (Sunday) dates for the week are shown, confirming the exact span.
Browse the year grid
The full year displays as a 52- or 53-week grid. Click any week to see its date range. Jump between years using the navigation arrows.
About this format
The ISO week number is the week-of-year according to ISO 8601 — the international standard for date and time representation. ISO weeks start on Monday and the first week of the year (Week 1) is the week containing the first Thursday of January.
This matters because the first few days of January, or the last few days of December, can belong to the previous or next year's week numbering. January 1 may fall in Week 52 or Week 53 of the previous year if it is a Monday, Tuesday, or Wednesday. December 31 may fall in Week 1 of the next year if it is a Thursday, Friday, or Saturday.
This creates confusion for anyone doing week-based planning, reporting, or scheduling — particularly in European business contexts, manufacturing, and software development where ISO week numbers are commonly used as a shorthand for planning periods ("deliverable in W23" or "sprint starting W04").
The calculator shows the ISO week number for any date, the range of dates that week spans, the full 52-or-53-week year grid, and navigation to any specific week number.