Week Number Calculator
Select any date to see its ISO week number, day of the week, and which week of the year it falls in.
Reviewed by the ToolNestr Editorial Team — July 2026
Pick a date to see its week information.
Week number distribution across the year
How many days fall in each ISO week number during a typical year.
Project Manager
Uses ISO week numbers for sprint planning, milestone tracking and reporting in agile software development workflows.
Payroll Admin
Determines which pay period a date falls into, especially for weekly or bi-weekly payroll cycles that follow ISO weeks.
Teacher
Plans lessons by week number, aligns curriculum with school term weeks and schedules exams in specific weeks of the year.
Event Planner
Coordinates events by week number to simplify scheduling across different organizations and industries.
| Date | ISO Week | Week Year | Notes |
|---|---|---|---|
| Jan 1 (normal year) | 1 | Same year | Jan 1 is in week 1 if it is Monday–Wednesday; week 52/53 if Thursday–Sunday |
| Jan 1 (leap year) | 1 | Same year | Same rule applies regardless of leap year |
| Dec 31 | 52 or 53 | Same or next | Can be week 1 of the next year if Dec 31 is Monday |
| First Thursday | 1 | Same year | Week 1 always contains the first Thursday |
| Feb 29 (leap) | 9 | Same year | Always in week 9 |
| Jun 30 | 26 or 27 | Same year | Mid-year boundary, varies by year |
| Sep 30 | 39 or 40 | Same year | End of Q3 |
| Dec 25 | 51 or 52 | Same year | Christmas always near end of year |
| 53-week year example | 53 | Same year | 2026 has 53 ISO weeks (Jan 1 is Thursday) |
How to use the Week Number Calculator
Pick a date
Use the date picker to select any date you want to analyze.
Review ISO week data
See the ISO week number, week year, day of week, day of year and quarter all at once.
Check week boundaries
View the Monday start and Sunday end dates of the week, plus which occurrence of that weekday within the month.
Tips for working with week numbers
ISO vs US week numbering
ISO weeks start on Monday and week 1 is defined by the first Thursday. In the US system, weeks start on Sunday and week 1 always starts on January 1. Make sure you know which system your organization uses.
Week year matters for reporting
When tracking metrics by week, use the week year (not the calendar year) to keep weeks together. January 1 may belong to the previous year's last week, and reporting it under that year's data is more consistent.
53-week years are rare
Only about 4–5 years in a 28-year cycle have 53 weeks. 2026 is one of them. If your payroll or planning system needs to handle 53-week years, make sure it accounts for the extra period.
What is ISO week number
ISO 8601 defines a week date system where weeks start on Monday. Week 1 of a year is the week that contains the first Thursday of that year. This means the first few days of January may belong to week 52 or 53 of the previous year, and the last days of December may belong to week 1 of the next year.
How week numbering works
The ISO week calculation uses the first Thursday rule: find the Thursday of the same week as the target date, then compute (ordinal day of that Thursday - 1) / 7 + 1. The week year of the target date is the year of that Thursday. This ensures every week has exactly 7 days and belongs to exactly one year.
Worked example
Week year vs calendar year
The week year (also called ISO year) can differ from the Gregorian calendar year. For example, January 1, 2027 is a Friday, which means it falls in week 53 of ISO year 2026. The week year is 2026, not 2027. This is important for any system that groups data by week for reporting or analysis.
Frequently asked questions
What is an ISO week number?
The ISO week date system (ISO 8601) assigns a week number to each week of the year. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
How is the week year different from the calendar year?
The week year can differ from the calendar year. For example, January 1 may belong to week 52 or 53 of the previous year, making its week year the previous year.
When does week 1 of the year start?
Week 1 is the week that contains the first Thursday of the Gregorian year. It always starts on the Monday of that week.
How many weeks are in a year?
Most years have 52 weeks. A year has 53 weeks if the year starts on a Thursday (common year) or on a Wednesday or Thursday (leap year).
What is the day of year?
Day of year is a number from 1 to 365 (or 366 in a leap year) representing the sequential day count from January 1.
What quarter is a date in?
Q1 is January–March, Q2 is April–June, Q3 is July–September, Q4 is October–December.
What week of the month is it?
Week of the month counts which occurrence of that weekday within the month. For example, the second Monday of the month.
Can I see the date range of a week?
Yes. The calculator shows the Monday and Sunday dates of the week the selected date falls in.
Does it work for dates far in the past or future?
The ISO week algorithm works for any date that JavaScript can represent, covering roughly year 1 to year 9999.
Why do some years have 53 weeks?
A year has 53 weeks if it starts on a Thursday (common year) or on a Wednesday if a leap year. This ensures the first Thursday rule is satisfied.