Week Number Calculator
Find the ISO 8601 week number for any date. See the day of the week and a visual calendar view of the entire week.
25
ISO Week
2026
Year
Mon
Day of Week
Week 25
Mon
Jun 15
Tue
Jun 16
Wed
Jun 17
Thu
Jun 18
Fri
Jun 19
Sat
Jun 20
Sun
Jun 21
How to Use the Week Number Calculator
- Enter the required date, time, or duration values.
- Select any additional options such as time zone or format.
- View the computed result instantly.
- Copy the result or adjust inputs as needed.
Quick Reference
| From | To |
|---|---|
| 1 minute | 60 seconds |
| 1 hour | 3,600 seconds |
| 1 day | 86,400 seconds |
| 1 week | 604,800 seconds |
| 1 year | 31,536,000 seconds |
| 1 year | 525,600 minutes |
Use Cases
- •Tracking elapsed time for productivity or time management.
- •Setting precise intervals for cooking, exercise, or training.
- •Planning schedules and events across different time zones.
- •Calculating durations and deadlines for project management.
Formula
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. The week number is calculated by finding the nearest Thursday and counting weeks from January 1.
Frequently Asked Questions
What is the ISO 8601 week number?
ISO 8601 defines a standard way to number weeks. Week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4). Weeks always start on Monday and end on Sunday.
How many weeks are in a year?
Most years have 52 weeks, but some years (called 'long years') have 53 weeks. A year has 53 ISO weeks if January 1 falls on a Thursday, or if December 31 falls on a Thursday.
Why do ISO weeks start on Monday?
The ISO 8601 standard chose Monday as the first day of the week to align with business practices in most countries. This differs from some locales (like the US) where Sunday is traditionally considered the first day.