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ToolPika

Week Number

See which week of the year it is today, find the week number of any date, or look up the dates of a given week.

Week number of a date

Dates of a week number

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How it works

Week number of a date. The calculator finds the Thursday of the week your date falls in. The year of that Thursday is the week’s year, and the week number is how many Thursdays have passed in that year, counting this one. It then shows the Monday and Sunday of the week, the ISO way of writing it (such as 2026-W42) and the day of the year.

Dates of a week number. Week 1 starts on the Monday of the week containing 4 January. Every following week starts seven days later, so week 42 of 2026 starts 41 weeks after Monday 29 December 2025.

Examples

Date ISO week Week runs
1 January 2026 2026-W01 29 December 2025 to 4 January 2026
1 July 2026 2026-W27 29 June to 5 July 2026
13 October 2026 2026-W42 12 to 18 October 2026
25 December 2026 2026-W52 21 to 27 December 2026
31 December 2026 2026-W53 28 December 2026 to 3 January 2027
1 January 2027 2026-W53 28 December 2026 to 3 January 2027

Where week numbers are used

  • Planning and deliveries. In Germany, the Netherlands and the Nordic countries, dates are often given as “delivery in week 42”.
  • Manufacturing. Date codes on products and components frequently use the year and week, such as 2642 for week 42 of 2026.
  • Payroll and timesheets. Weekly pay periods are numbered by ISO week.
  • School and holiday calendars. Terms and holiday rentals are commonly listed by week number.

Frequently asked questions

How are week numbers calculated?

This tool follows ISO 8601, the international standard used across Europe and in most business software. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year, which is always the week containing 4 January.

Why does 1 January sometimes fall in week 52 or 53?

Because a week belongs to the year that holds most of its days. 1 January 2027 is a Friday, so it is still part of week 53 of 2026, and week 1 of 2027 starts on Monday 4 January. The calculator tells you when a date belongs to a week of a different year.

Which years have 53 weeks?

Years in which 1 January is a Thursday, and leap years in which it is a Wednesday. 2026 has 53 weeks; the next ones are 2032 and 2037. All other years have 52.

Is the week number the same in the United States?

Not always. In the US, Canada and some other countries, calendars often start weeks on Sunday and call the week containing 1 January week 1. That count can differ from the ISO number by one, so check which system a document uses.

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