What is an ISO week number?

An ISO week number is a way of identifying weeks within a year according to the ISO 8601 international standard. Under this system, weeks always start on Monday and end on Sunday. Week 1 is defined as the week that contains the first Thursday of the year, which means it always includes January 4th. This convention is widely used in business, project management, accounting, and international communication because it removes ambiguity about when a week begins and which year it belongs to.

Because ISO weeks are anchored to Thursday, a date near the start or end of a calendar year may fall into a different ISO week-numbering year. For example, December 31 can sometimes belong to week 1 of the following year, and January 1 can belong to week 52 or 53 of the previous year. Most years have 52 ISO weeks, but years where January 1 or December 31 falls on a Thursday contain 53 weeks.

Tool description

This tool calculates the ISO 8601 week number for any date you enter. It instantly returns the week number along with detailed date context including the ISO week-numbering year, week start and end dates, day of the week, day of the year, quarter, total weeks in the year, and whether the year is a leap year. The current date is pre-filled by default for quick reference.

Examples

Input date ISO week Notation Week range
2026-01-01 W01 2026-W01 2025-12-29 to 2026-01-04

Features

  • Calculates ISO 8601 week number and week-numbering year for any date
  • Shows the full Monday-to-Sunday date range for the selected week
  • Displays day of year, quarter, and total weeks in the year at a glance
  • Indicates whether the selected year is a leap year
  • Pre-fills today's date with a one-click "Today" button for quick lookup

How it works

The ISO 8601 week date system assigns each date to a week number (1–53) within a week-numbering year. The algorithm finds the nearest Thursday to the input date, since Thursday determines which week and year a date belongs to. From there it calculates the week number by counting the days elapsed since January 1 of that Thursday's year and dividing by seven. The week start (Monday) and end (Sunday) are derived by offsetting the input date based on its day of the week.

Use cases

  • Project planning: Reference ISO week numbers in schedules, sprint planning, and status reports so all team members share the same timeline regardless of locale
  • Payroll and accounting: Determine which fiscal week a date falls into for timesheet processing and financial reporting
  • International coordination: Communicate deadlines and delivery windows using a standard week format that avoids confusion across countries with different week-start conventions