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Week Number (ISO) Calculator

What week number is it?

How it is calculated

The tool reports the ISO 8601 week number of the date, the day of the year and the days left in the year. ISO weeks start on Monday, and week 1 is the week that contains the first Thursday of the year, which equals the week that contains 4 January. So the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. Day of year counts 1 January as day 1. Days left is 365 − day of year, or 366 in a leap year. Some countries, such as the United States, number weeks differently, so printed calendars may disagree.

Worked example

Date 2025-03-10. Day of year: 31 days of January + 28 days of February + 10 = 69. The year 2025 is not a leap year, so days left: 365 − 69 = 296. The date is a Monday, and the first Thursday of 2025 was 2 January, which sits in week 1 (starting Monday 30 December 2024). Counting weeks forward, 10 March starts ISO week 11.

Try this example →

Common mistakes

  • Assuming week 1 always starts on 1 January. In ISO numbering it can start in the last days of the previous December.
  • Comparing with a calendar that starts weeks on Sunday, such as many US calendars, and expecting the same number.
  • Treating the day of year as the date index in leap years without allowing for 29 February: after February, the day number is one higher than in an ordinary year.
  • Typing the date in a local format like 10/03/2025, which can be read as 3 October instead of 10 March.

About this calculator

This tool finds a date's ISO week number — the standardized numbering (ISO 8601) where week 1 is the week containing the year's first Thursday, so week numbering stays consistent even though January 1st can fall on any weekday — along with the day-of-year count and how many days remain until year-end.

It is used in business and project management contexts where planning is organized by week number rather than calendar date (common in manufacturing, payroll and European corporate scheduling), and by anyone who needs to reference "week 33" on a report or invoice and wants to confirm exactly which dates that corresponds to.

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Comments (1)

  • Editorial team

    The tool does not show the ISO year, and that can mislead near New Year. 30 December 2025 is in week 1, but of 2026, while 1 January 2027 falls in week 53 of 2026. When you write a week reference for a schedule or invoice, pair the week with its ISO year, not the calendar year of the date. Which system does your workplace use, ISO weeks or a different one?

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