Week Number Calculator — ISO Week & Year Calendar
Find the ISO week number for any date, list all weeks in a year, and calculate the date range for any given week. Supports ISO 8601 and US/North American week numbering conventions.
ISO 8601 Week
US / North American Week
What Is the Week Number Calculator — ISO Week & Year Calendar?
This calculator finds the week number for any date under both the ISO 8601 and US/North American conventions, which can differ by one week near the start of the year. It also provides a full year calendar view, quarter information, and a week lookup tool.
- ›ISO 8601 week number — used in Europe, international business, and project management. Week 1 contains the first Thursday of the year.
- ›US week number — used in North America. Week 1 always contains January 1 and begins on Sunday.
- ›Full year calendar — see all 52–53 week numbers for any year in a compact grid, each showing the date range.
- ›Week lookup — enter a year + week number to get the corresponding date range.
- ›Quarter info — shows which fiscal/calendar quarter the date falls in and how many days remain in it.
Formula
ISO 8601 Week Number
Week 1 = the week containing the first Thursday of the year
Weeks start on Monday; year has 52 or 53 ISO weeks
US / North American Week Number
Week 1 = the week containing January 1
Weeks start on Sunday; year always has 53 US weeks
Week Start / End
ISO: Mon–Sun | US: Sun–Sat
How to Use
- 1Enter a date: Use the date picker, or leave it at today's date (the default).
- 2Read the results: The ISO week number, US week number, week start and end dates, days remaining in the week, and quarter information are shown instantly.
- 3Use Week Lookup: Enter a year and a week number (ISO or US) to find which dates that week spans.
- 4View year calendar: Click "Show Year Calendar" to see all weeks for the selected year as a scrollable grid.
- 5Check the comparison: The ISO vs US comparison panel explains exactly where the two systems diverge for the selected date with a worked example.
Example Calculation
What week number is April 21, 2026?
Date: Tuesday, April 21, 2026
ISO 8601 week: Week 17 of 2026
ISO week start: Monday, April 20, 2026
ISO week end: Sunday, April 26, 2026
US week: Week 17 of 2026
US week start: Sunday, April 19, 2026
US week end: Saturday, April 25, 2026
Quarter: Q2 (April–June) | 70 days remaining in Q2
Days left in ISO week: 5 (Wed–Sun)
When do ISO and US week numbers differ?
The ISO and US week numbers are often the same, but they can diverge at the start and end of the year. If January 1 falls on a Thursday (ISO) or Sunday (US), the numbering can be off by one for the first few days of January or the last few days of December. For example, December 31, 2018 was ISO Week 1 of 2019, while it was US Week 53 of 2018.
Understanding Week Number — ISO Week & Year Calendar
ISO 8601 Week Numbering — The International Standard
The ISO 8601 standard defines a week as running from Monday to Sunday. Week 1 of any year is the week that contains the first Thursday of that year — equivalently, it is the week containing January 4 (since January 4 is always in Week 1). This means:
- ›If January 1 is a Friday, Saturday, or Sunday, it belongs to Week 52 or 53 of the previous year
- ›If January 1 is a Monday, Tuesday, Wednesday, or Thursday, it belongs to Week 1 of the new year
- ›A year has 53 ISO weeks if it starts on Thursday, or if it is a leap year starting on Wednesday
- ›The ISO week year and the calendar year can differ in the first and last few days of January and December
US Week Numbering — The North American Convention
The US convention defines Week 1 as the week containing January 1, and weeks run from Sunday to Saturday. This is simpler but less precisely defined than ISO 8601. The US system is common in retail, payroll, sports statistics, and North American business calendars.
When Do ISO and US Week Numbers Differ?
Most of the year the two systems agree. Divergence happens only at year boundaries:
- ›The first few days of January may be in ISO Week 52/53 of the prior year but US Week 1 of the new year
- ›The last few days of December may be in ISO Week 1 of the next year but US Week 53 of the current year
- ›During the rest of the year (roughly weeks 2–51) the two systems almost always agree
Practical Uses of Week Numbers
- ›Project planning — most project management tools (Jira, MS Project) use ISO week numbers for sprint planning and milestone tracking.
- ›Retail and supply chain — many retail systems schedule deliveries, promotions, and reporting by week number rather than by date.
- ›Finance and reporting — fiscal week numbers are common in quarterly earnings reports and budget tracking.
- ›Shift scheduling — rotating shift schedules are often defined as odd/even week patterns referencing ISO week numbers.
53-week years in ISO 8601
Most years have 52 ISO weeks, but about every 5–6 years a year has 53. This happens when January 1 falls on a Thursday (any year), or on a Wednesday in a leap year. The years 2015, 2020, 2026, and 2032 are all 53-week years. Payroll systems and annual contracts that reference ISO weeks need to handle this carefully.
Frequently Asked Questions
What is ISO 8601 week numbering?
ISO 8601 weeks follow these rules:
- ›Weeks run Monday to Sunday
- ›Week 1 = the week containing the first Thursday of the year
- ›Equivalently, Week 1 contains January 4
- ›A year has 52 or 53 ISO weeks depending on which day January 1 falls on
How does the US week numbering differ from ISO 8601?
- ›ISO: Week starts Monday, Week 1 contains first Thursday of year
- ›US: Week starts Sunday, Week 1 contains January 1
- ›Divergence: only in early January and late December at year boundaries
- ›The rest of the year (weeks 2–51) they typically agree
Can a year have 53 weeks?
ISO 53-week years occur when:
- ›January 1 is a Thursday (any year)
- ›January 1 is a Wednesday in a leap year
- ›Recent examples: 2015, 2020, 2026, 2032
- ›Roughly every 5–6 years a year has 53 ISO weeks
What quarter is any given week in?
- ›Q1: January 1 – March 31
- ›Q2: April 1 – June 30
- ›Q3: July 1 – September 30
- ›Q4: October 1 – December 31
- ›The calculator shows days remaining in the current quarter for deadline tracking
How do I find what dates a specific week number covers?
Use the Week Lookup tab:
- ›Enter a 4-digit year and a week number
- ›Select ISO or US convention
- ›The calculator outputs the start date (Mon or Sun) and end date (Sun or Sat)
- ›Useful for planning around specific project sprints or payroll periods
Does the calculator save my inputs?
- ›Entered date is saved automatically
- ›All settings (ISO/US, year calendar) are preserved
- ›Data lives in your browser only — no server transmission
- ›Click Reset to clear and start fresh