ISO Week Number
Find the ISO week number and week-numbering year for a date.
About ISO Week Number
The ISO week number uses a week containing the year's first Thursday as week one, so early January can belong to the previous year's final week. ISO 8601 weeks are the standard for business planning, reporting and manufacturing schedules.
Formula
ISO Week Number
ISO week from the Thursday rule: week 1 contains January 4
- week year
- The year the week belongs to, which can differ from the calendar year
Worked example
With the default values below, ISO Week Number returns ISO Week Number of 1. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Year
- 2,025
- Monthaccepts 1–12
- 1
- Dayaccepts 1–31
- 4
Results
- ISO Week Number
- 1
- Week Year
- 2,025
- ISO Label
- 2025-W1
Step by step
- 1Values used
- 2ISO Week Number
- 3ISO Week Number= 1
- 4Week Year= 2,025
- 5ISO Label= 2025-W1
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 Number calculated?
ISO week from the Thursday rule: week 1 contains January 4 The ISO week number uses a week containing the year's first Thursday as week one, so early January can belong to the previous year's final week.
Why does ISO Week Number matter?
ISO 8601 weeks are the standard for business planning, reporting and manufacturing schedules.
What values do I need to enter?
This calculator takes 3 inputs: Year, Month, 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.