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Kordu Tools
Time & Date Runs in browser Updated 08 Apr 2026

Week Number Calculator

Find the ISO week number for any date. See day of year, quarter, and days remaining.

Current ISO week

17

Thursday, 23 April 2026

Check any date

ISO week numberWeek 17
Day of weekThursday
Day of year113 of 365
Days remaining in year252
QuarterQ2
Leap yearNo
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How to use Week Number Calculator

  1. See the current week number

    The current ISO week number is displayed at the top of the page automatically — no input needed.

  2. Check a specific date

    Use the date picker to select any past or future date to see its ISO week number and other calendar details.

  3. Read the details

    See the day of week, day of year, days remaining in the year, quarter, and whether it is a leap year.

Week Number Calculator FAQ

What is an ISO week number?
ISO week numbers (ISO 8601) number the weeks of the year from 1 to 52 or 53. Week 1 is the week containing the first Thursday of the year. Weeks start on Monday. This is the standard used in European business and most international contexts.

Why might the first days of January be in week 52?
If January 1 falls on a Friday, Saturday, or Sunday, those first days belong to week 52 or 53 of the previous year, because the week they are in does not contain a Thursday in January.

What is the difference between ISO weeks and US week numbers?
ISO weeks start on Monday and week 1 contains January 4. US calendars often start weeks on Sunday and define week 1 as the week containing January 1. This can cause differences of 1 week, especially at the start and end of the year.

How many weeks are in a year?
Most years have 52 ISO weeks. About 71% of years have exactly 52 weeks; the remaining 29% (called long years) have 53 weeks. 2026 is a 53-week year.

Is my data sent anywhere?
No. All calculations happen in your browser. Nothing is uploaded or stored.

Background

The Kordu Week Number Calculator shows the current ISO week number at the top of the page — the week number changes every Monday at midnight. Select any date to find its ISO week number, the day of the week, the day of year (1–365/366), days remaining in the year, the quarter (Q1–Q4), and whether the year is a leap year.

ISO week numbering (ISO 8601) defines weeks as starting on Monday. Week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4th). This means the first few days of January may belong to week 52 or 53 of the previous year, and the last few days of December may belong to week 1 of the following year.

Common uses: project planning, payroll periods, weekly reporting, financial quarters, academic calendars, and checking what week a specific date falls in.

All calculations run client-side in your browser. Nothing is uploaded or stored.

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