Week Number Calculator
Find the ISO week number and day of the year for any date.
Results update as you type.
About this calculator
A week number calculator returns the ISO 8601 week number for a date — the standard used across business, logistics and Europe, where weeks run Monday to Sunday and week 1 is the week containing the first Thursday of the year. It also gives the ISO week-numbering year, the calendar quarter, the day-of-year number and how many weeks are left in the year.
Under the ISO rule a year has 52 or 53 weeks, each running Monday to Sunday, and week 1 is defined as the week that holds the year’s first Thursday (equivalently, the week containing 4 January). This is why a date in the first days of January can still belong to the last week of the previous year, and a late-December date can already sit in week 1 of the next year — the tool reports the ISO week-numbering year separately so that mismatch is clear.
For example, 1 January 2026 falls in ISO week 1 of 2026, but 1 January 2023 (a Sunday) sits in ISO week 52 of 2022. Manufacturing, shipping, publishing and European payroll systems schedule by these "CW" or "KW" week numbers, so this is the tool for answering "what week number is it", planning a delivery for "week 34", or checking which quarter a date belongs to.
Frequently asked questions
How is the ISO week number defined?
Under ISO 8601 weeks start on Monday and week 1 is the week that contains the first Thursday of the year — equivalently, the week containing 4 January. This is why early-January and late-December dates can belong to a neighbouring year.
Why does the ISO year sometimes differ from the calendar year?
Dates in the first days of January can fall in the last ISO week of the previous year, and late-December dates can fall in week 1 of the next year. The ISO week-numbering year reflects which year the week belongs to.
Which quarter is the date in?
The quarter is the calendar quarter of the ordinary date: Q1 is January–March, Q2 April–June, Q3 July–September and Q4 October–December.
How many weeks are in a year?
Under ISO 8601 a year has 52 weeks in most years and 53 in a handful (when 1 January or the leap-year 31 December falls on a Thursday). The tool tells you how many ISO weeks remain in the current year.
What is the difference between ISO and US week numbering?
ISO weeks start on Monday with week 1 holding the first Thursday. The common US convention starts weeks on Sunday and calls the week of 1 January week 1. This calculator uses the ISO standard, so numbers can differ by one from a US calendar.
What does CW or KW mean on a calendar?
CW (calendar week) and the German KW (Kalenderwoche) are just labels for the ISO week number — CW 34 is ISO week 34. Enter any date in that week to confirm which CW/KW it belongs to.
Results are estimates for general guidance only, not financial, medical or tax advice.