---
title: "ISO Week Number Calculator | Numequo"
description: "Find the ISO week number and week-based year for any date. Get a clear result with the formula, examples and useful explanations."
canonical: "https://www.numequo.com/time/dates-calendar/iso-week-number-calculator/"
locale: "en"
---
[Numequo](https://www.numequo.com/) › [Time](https://www.numequo.com/time/) › [Dates and calendar](https://www.numequo.com/time/dates-calendar/)

# ISO Week Number Calculator

Enter a calendar date to identify its ISO 8601 week.

## Input data

- **Date** - type: date; default: 2026-08-12

## How it works

ISO weeks begin on Monday. Week 1 is the week containing the year’s first Thursday.

## Quick examples

- **12 August 2026** - Date: 2026-08-12
- **1 January 2021** - Date: 2021-01-01
- **31 December 2024** - Date: 2024-12-31

## Understanding your result

Dates near New Year can belong to the previous or following ISO week-based year.

Last verified: 29 September 2026 (2026-09-29)

## Frequently asked questions

### When does an ISO week start?

On Monday.

### Can week 1 begin in December?

Yes. The first ISO week can begin in the previous calendar year.

### Can early January belong to the previous ISO year?

Yes, when those dates fall before the first ISO week.

## Related calculators

- [Day of the Week Calculator](https://www.numequo.com/time/dates-calendar/day-of-week-calculator/) - Find which weekday corresponds to any valid calendar date.
- [Leap Year Calculator](https://www.numequo.com/time/dates-calendar/leap-year-calculator/) - Check whether a Gregorian calendar year contains 365 or 366 days.
- [Date Difference Calculator](https://www.numequo.com/time/dates-calendar/date-difference-calculator/) - Count days and weeks between any two dates.
- [Age Calculator](https://www.numequo.com/time/dates-calendar/age-calculator/) - Calculate an exact age between two dates.
- [Date Add or Subtract Calculator](https://www.numequo.com/time/dates-calendar/date-add-subtract-calculator/) - Add or subtract days, weeks, months or years from any calendar date.
- [Business Days Calculator](https://www.numequo.com/time/dates-calendar/business-days-calculator/) - Count business days between two dates with configurable weekends and holidays.

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