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Week Number Calculator

Find the ISO 8601 week-year, week number, and Monday-based weekday for a valid Gregorian date.

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Choose or enter a valid Gregorian date.
  2. 2.Select Calculate ISO week.
  3. 3.Use the returned YYYY-Www label only where ISO 8601 week rules are required.

About Week Number Calculator

Week Number Calculator returns the ISO 8601 week label for a Gregorian calendar date. It shows the ISO week-year, two-digit week number, and weekday number where Monday is 1 and Sunday is 7. Everything runs locally with UTC calendar arithmetic, so browser timezone and daylight-saving changes do not shift the selected date.

ISO weeks always begin on Monday. Week 1 is the week containing January 4, equivalently the week containing the year's first Thursday. Because the week-year follows Thursdays rather than calendar-year boundaries, January 1 can belong to the previous ISO year and late December can belong to the next one.

Eight independent boundary fixtures cover ordinary week 1, Sundays at New Year, dates in a previous ISO year, dates in a next ISO year, and multiple 53-week years. Examples include 2016-01-01 as 2015-W53, 2021-01-04 as 2021-W01, and 2024-12-30 as 2025-W01.

The parser accepts a strict YYYY-MM-DD value, validates the proleptic Gregorian date by round trip, moves to the Thursday of the same ISO week, derives the week-year from that Thursday, and counts seven-day blocks from January 1. Invalid dates such as 2023-02-29 fail visibly.

This tool calculates ISO week dates only. Some businesses, governments, calendars, analytics products, and locales use Sunday-start weeks, partial week 1, fiscal calendars, retail 4-4-5 calendars, or locale-specific numbering. Those systems can return a different week number for the same date.

Use the ISO result for standards-based filenames, planning, reporting, and development when all parties have agreed on ISO 8601. For payroll, tax, legal deadlines, fiscal reporting, or a vendor API, verify the exact calendar convention required by that system.

Methodology & sources

Parse and round-trip a strict Gregorian YYYY-MM-DD date in UTC, convert Sunday from 0 to ISO weekday 7, move to the Thursday of the same week, take that Thursday's year as the ISO week-year, count complete seven-day blocks from January 1, format YYYY-Www, and validate eight cross-year boundary fixtures.

Frequently asked questions

Why can January 1 belong to the previous year?
The ISO week-year is determined by the week's Thursday, and week 1 must contain January 4.
Which day starts the week?
Monday is weekday 1; Sunday is weekday 7.
Can a year have week 53?
Yes. Some ISO week-years contain 53 Mondays and therefore a W53.
Does my timezone affect the answer?
No. The calculation uses the selected calendar fields with UTC arithmetic to prevent timezone drift.

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