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ISO Week Date Calculator

Calculate the ISO 8601 week number and week-based year for a date.

About ISO Week Date Calculator

ISO weeks always start on Monday and belong to whichever year contains their Thursday. That is why 1 January can fall in week 52 or 53 of the previous week-based year. Financial and manufacturing reporting uses ISO weeks precisely because every week has exactly seven days, unlike calendar months.

Formula

ISO Week Date

The ISO week is the week containing the Thursday, which determines the week-based year

ISO weekday
Monday is 1 through Sunday is 7
week-based year
May differ from the calendar year in late December or early January

Worked example

With the default values below, ISO Week Date Calculator returns ISO Week Number of 1. Change any input above and every figure here is recalculated in your browser.

Inputs used

Date yearaccepts 1,970–2,200
2,026
Date monthaccepts 1–12
1
Date dayaccepts 1–31
1

Results

ISO Week Number
1
ISO Week-Based Year
2,026
ISO Weekday Number
4
ISO Week Date Notation
2026-W01-4
Day of the Week
Thursday

Step by step

  1. 1Values used
  2. 2ISO Week Date
  3. 3ISO Week Number= 1
  4. 4ISO Week-Based Year= 2,026
  5. 5ISO Weekday Number= 4
  6. 6ISO Week Date Notation= 2026-W01-4
  7. 7Day of the Week= Thursday

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is ISO Week Date calculated?

The ISO week is the week containing the Thursday, which determines the week-based year ISO weeks always start on Monday and belong to whichever year contains their Thursday. That is why 1 January can fall in week 52 or 53 of the previous week-based year.

Why does ISO Week Date matter?

Financial and manufacturing reporting uses ISO weeks precisely because every week has exactly seven days, unlike calendar months.

What values do I need to enter?

This calculator takes 3 inputs: Date year, Date month, Date day. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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ISO Week Date Calculator – Calculate ISO Week Number