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Week Number Calculator

Find the ISO 8601 week number and ISO week-year for a calendar date.

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Calculator guide

How to use the Week Number Calculator

Find the ISO 8601 week number and ISO week-year for a calendar date. Enter the requested values and select Calculate. The result and supporting values are produced locally in your browser.

Formula

ISO week 1 is the week containing January 4, with weeks starting Monday

Worked example

January 1 can belong to the final ISO week of the previous ISO week-year, depending on the weekday.

What to keep in mind

  • This calculator follows ISO 8601: weeks begin Monday and week 1 contains January 4.

Methodology

This calculator uses deterministic arithmetic and performs the calculation locally in your browser. Verify assumptions, units, and source values for your specific use case.

Review standard: Level A · Standard formula. How calculator reviews work.

Frequently asked questions

What does this calculator calculate?

Find the ISO 8601 week number and ISO week-year for a calendar date.

How is the result calculated?

ISO week 1 is the week containing January 4, with weeks starting Monday.

Can I use the result for an important decision?

Use the result as a calculation aid and verify the inputs, assumptions, and applicable professional or regulatory requirements before relying on it for an important decision.

Does CalculateMeasure store the values I enter?

The calculator performs its arithmetic in your browser. The calculator itself does not need to send your entered values to a server to produce the result.