ISO Week Number Calculator

Formula

ISO 8601 week algorithm

Worked example

2025-01-06 — 2

Quick answer

What does the ISO Week Number Calculator do?

Find the ISO 8601 week number for a calendar date.

What is the ISO Week Number Calculator?

Find the ISO 8601 week number for a calendar date. Use it when you have the required inputs and want a deterministic, repeatable result with the calculation method visible.

How to use this calculator

Enter the values requested by the tool, keep units and time periods consistent, resolve any validation message, calculate, then review the primary answer together with the displayed formula and supporting values.

Formula and method

This calculator uses the deterministic formula ISO 8601 week algorithm. Enter the required values, then review the displayed substitution and result breakdown to verify the calculation.

Worked example

2025-01-06 2

Common mistakes to avoid

Check units, decimal points, percentages, signs, date/time order, selector choices, and rounding. For important decisions, verify the result against an independent source or qualified professional when appropriate.