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Week Number Calculator

Find the ISO week number, week-year, and weekday for a selected date. Keep the week-year with the number when dates fall near New Year.

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How Week Number Calculator works

The week’s Thursday determines its ISO week-year and Monday-based number.

The selected date is shifted to the Thursday of its ISO week to determine the week-year. The week number is then counted within that year, and Monday is represented as ISO weekday one.

A date near New Year can belong to the previous or next ISO week-year.

Conceptual ISO calendar week crossing a year boundary with its week-year shown separately.
An ISO week-year can differ from the calendar year near New Year.

How to use Week Number Calculator

Select a calendar date and read its ISO week number together with the ISO week year.

Choose the date to classify

Select the calendar date and check its year, month, and day. This tool uses the ISO 8601 Monday-based week convention. It does not offer a Sunday-first or institution-specific week-number mode.

Choose the date to classify
Calendar date
Week Number Calculator native input panel with the actual Calendar date settings used for this example.
Configure Calendar date; select Calculate.

Calculate the ISO week number

Select Calculate and read the ISO week number. The calculation uses the Thursday of the selected week to identify its week-year. A date near New Year can therefore belong to a different week-year from its calendar year.

Calculate
Week 1
Week Number Calculator actual result panel showing Week 1.
Submitted result: Week 1. ISO week-year: 2021.

Keep week-year and weekday together

Review ISO week-year, Weekday, and ISO weekday alongside the main week number. Monday has ISO weekday one. Retain the week-year in a weekly report so a boundary date is not accidentally placed in the wrong annual sequence.

Keep week-year and weekday together
ISO week-year
Week Number Calculator actual iso week-year region and result note after submitting the pictured settings.
Review ISO week-year, Weekday, ISO weekday in this Week Number Calculator example. Read the result note.

Use ISO week numbers in reports and calendar comparisons

Keep the calendar convention explicit when comparing weekly records.

Label an ISO based weekly report

Use the week number and ISO week year as a pair in a report that follows this convention. The page does not generate report files or group records automatically. It provides the calendar values needed for a label you prepare elsewhere.

Try these inputs
Date and time inputs represented by calendar and clock shapes.
Enter dates and time settings. Concept illustration.

Check the ISO week-year at New Year

January first 2021 belongs to ISO week fifty three of 2020. This calendar example shows why the selected year and displayed week year can differ. Include both values when checking a boundary date rather than assuming the page has changed the year incorrectly.

Compare a scenario
Two date or time scenarios represented for comparison.
Compare another date or time. Concept illustration.

Compare two calendar conventions

If another calendar gives a different week number, check its first weekday and definition of the first week. This calculator uses the ISO convention. A locally configured calendar can follow a different convention without either system using an invalid date.

Use the tool
Date or time information used in a planning workflow.
Plan with the result. Concept illustration.

The ISO week year is part of the result

Keep ISO week-year and calendar year distinct around year boundaries.

Week one is the week containing January fourth, and weeks start on Monday. A date at the beginning or end of a calendar year can therefore belong to an adjacent ISO week year.

The weekday details identify the chosen calendar date; the ISO week year identifies its week grouping. These are distinct labels. The calculator does not display a whole week grid or convert a local time between zones.

Check the outputTry another input
Date or time output connected to its original settings.
Read dates with their settings. Concept illustration.

Check the convention before reconciling numbers

Check the date and week convention before labelling a weekly record.

The form requires a valid calendar date. For a disagreement near New Year, inspect the ISO week year first. Adding a calendar year to a bare ISO week number can create a label that describes the wrong week.

Use Day of Year Calculator for an ordinal day and Business Days Calculator for a weekday count across a range. Neither result substitutes for an ISO week identifier, even when all three tools use the same selected date.

Check your inputsReturn to the tool
Calendar boundaries and time settings represented in a review illustration.
Check boundaries and settings. Concept illustration.

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Use Week Number Calculator

Find the ISO week number, week-year, and weekday for a selected date.

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Week Number Calculator: common questions

Answers about using Week Number Calculator and understanding its results.

Why can January 1 be in last year's week?

ISO week 1 must contain January 4, so some early-January dates remain in the prior week-year.

Which day starts the week?

Monday.

Can December dates belong to the next week-year?

Yes. A late-December date can fall in ISO week one of the following week-year. Read the separate week-year output rather than assuming the calendar year.

Can I choose a Sunday-first week system?

No. This page implements ISO 8601 weeks, which begin on Monday. It does not offer an alternate week convention.