What is ISO 8601 week numbering?

ISO 8601 defines week 1 as the week containing the first Thursday of the year (equivalently, the week containing January 4). Weeks run Monday through Sunday, and the year can have 52 or 53 weeks.

How does US week numbering differ from ISO?

The US convention starts weeks on Sunday, and week 1 is the week containing January 1. This can differ from the ISO week by one or two weeks around the new year.

What does the ISO calendar date string (e.g. 2025-W03-2) mean?

It follows the ISO 8601 format: YYYY-Www-D where YYYY is the ISO year, ww is the zero-padded week number, and D is the day of the week (1 = Monday, 7 = Sunday).

Can a year have 53 ISO weeks?

Yes. A year has 53 ISO weeks if January 1 is a Thursday, or if it is a leap year and January 1 is a Wednesday or Thursday. This occurs roughly every 5 to 6 years.

What is the day of year, and how is it calculated?

The day of year (ordinal date) is the sequential day count starting from 1 on January 1. For example, February 1 is day 32. Leap years have 366 days; all others have 365.

Week Number Calculator

Name: Week Number Calculator
Availability: InStock
Author: Nham Vu

Pick a date to see its ISO 8601 week number, US week number, day of year, and day of week.

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Summary

Pick a date to see its ISO 8601 week number, US week number, day of year, and day of week.

How it works

The date picker defaults to today — results appear immediately.
Click "Today" to jump back to the current date at any time.
Use the "- 1 Week" and "+ 1 Week" buttons to step through weeks one at a time.
Use the left and right year arrows to jump an entire year back or forward.
Read the ISO week, US week, day of year, quarter, and weekday from the result panel.

Use cases

Find the ISO 8601 week number for a project deadline or sprint.
Determine which US fiscal or payroll week a date falls in.
Calculate the day of year for logging, analytics, or astronomy.
Quickly see which quarter a date belongs to for financial reporting.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu