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ISO Week Number Calculator

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INPUT

Auto Process

OUTPUT

Client Side

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Guide

ISO Week Number Calculator

The ISO Week Number Calculator converts any date into its ISO 8601 week number, week-year, and Monday-to-Sunday range. It also works in reverse, turning a week-year and week number back into exact dates, and shows a full-year calendar of every ISO week. Because ISO 8601 numbering has subtle edge cases around week 53 and the year boundary, this tool gives you instant, verifiable answers for scheduling, payroll, sprint planning, and reporting.

How to Use

Pick any date to instantly see its ISO week number, ISO week-year, and the Monday and Sunday that bound that week. Use the Previous week, This week, and Next week buttons to step through the calendar, or click any row in the full-year week list to jump straight to it. To go the other way, enter a week-year and a week number in the reverse section to get the exact start and end dates for that ISO week.

Features

Date to ISO week – Get the week number, ISO week-year, day of week, and day of year for any date.
Week range – See the Monday start and Sunday end dates for the selected week.
Reverse lookup – Convert a week-year plus week number back into an exact date range, with validation for week 53.
Quick navigation – Jump to the previous week, next week, or today with a single click.
Full-year calendar – Browse every ISO week of the year with the current week highlighted.
Edge-case accurate – Correctly handles 53-week years and dates that fall into the previous or next ISO week-year.

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 FAQ

What is an ISO 8601 week number?

ISO 8601 defines a week as starting on Monday and ending on Sunday. Weeks are numbered 1 to 52 or 53 within an ISO week-year. Week 1 is the week containing the first Thursday of the year, which is equivalent to the week containing January 4th.
Why can a date have a different ISO week-year than its calendar year?

Because ISO weeks never split across the Monday-Sunday boundary, the days at the very start or end of a calendar year can belong to a week owned by the neighbouring year. For example, December 31st can fall in week 1 of the next year, and January 1st can fall in week 52 or 53 of the previous year.
When does a year have 53 ISO weeks?

An ISO week-year has 53 weeks when January 1st is a Thursday, or when it is a leap year and January 1st is a Wednesday. All other years have 52 weeks. Equivalently, a year is long if December 28th falls in week 53.
How does ISO week numbering differ from US week numbering?

The common US convention starts weeks on Sunday and treats the week containing January 1st as week 1, so week boundaries and numbers often differ from ISO. ISO 8601 always starts weeks on Monday and anchors week 1 to the first Thursday, which makes it unambiguous across locales.