# ISO Week Number Calculator

> Find the ISO 8601 week number, quarter and day of year for any date.

To find an ISO week number, enter a date. ISO 8601 weeks begin on Monday and week 1 is the week containing the first Thursday of the year, which means the first days of January can belong to week 52 or 53 of the previous year.

**URL:** https://convertto.tech/t/week-number-calculator
**Category:** Date & Time Tools (https://convertto.tech/c/date-time-tools)
**Privacy:** Runs entirely in the browser; no upload
**Cost:** Free, no sign-up
**Last updated:** 2026-08-01

## Key facts

- **Privacy:** Runs entirely in your browser — nothing is uploaded
- **Cost:** Free, unlimited, no sign-up

## How to use

1. Enter or paste your date.
2. The result appears immediately — copy or download it.

## FAQ

### Why is 1 January sometimes in week 53 of the previous year?

Under ISO 8601 a week belongs to whichever year contains its Thursday. If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday was in December — so those days count as the last week of the old year.

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