ISO week number

Client

ISO 8601 week-date (week starts Monday). For Unix timestamps, see timestamp converter.

ISO 8601 week-date rules

Week 1 is the week with the year’s first Thursday; weeks run Monday–Sunday. The ISO week-year can differ from the calendar year near January and December. Pair with Unix timestamp or World clock for instant and zone context.

ISO week
2026-W26
Weekday
Tue
Week start (Mon)
2026-06-22
Week end (Sun)
2026-06-28

Nearby workflows on Toolcore

Common use cases

  • Label weekly reports with ISO year-week codes (2026-W01).
  • Check which Monday–Sunday span a calendar day belongs to.
  • Align sprint planning with ISO week boundaries.

Common mistakes to avoid

  • US Sunday-start weeks

    ISO weeks start on Monday—US locale week numbers may differ.

  • UTC vs local

    This tool uses your browser local calendar date, not UTC midnight.

FAQ

What is ISO week 1?

The week containing the first Thursday of the calendar year.

Is data uploaded?

No. Calculation runs locally.

Common search terms

Phrases people search for that match this tool. See the full long-tail keyword index.

  • iso week number calculator
  • iso 8601 week from date online
  • calendar week number tool browser
  • what week of the year iso

More tools

Related utilities you can open in another tab—mostly client-side.