UnitConv

Week Number Calculator

Get the ISO week number for any date, and convert a week number back to dates.

ISO week number
2024-W01
Mon
Week number
1
ISO year
2024
Day of year
1
US week number (Sunday start)
1

That week (Mon–Sun)

Week start (Monday)
2024-01-01
Mon
Week end (Sunday)
2024-01-07
Sun

What is an ISO 8601 week number?

ISO 8601 weeks start on Monday and end on Sunday. Week 1 is the week containing the year's first Thursday — equivalently, the week that contains January 4. Because of this rule, a few days around the turn of the year can belong to the week of the neighbouring year. For example, December 31 may fall in week 1 of the next year, and January 1 may fall in the last week of the previous year. A year has either 52 or 53 weeks.

Source: ISO 8601 (International Organization for Standardization)

What is a Week Number Calculator?

A week number calculator tells you which week of the year a given date falls in. This tool shows the international ISO 8601 week number (Monday start) along with the US week number (Sunday start), the day of the year (days since January 1), the weekday, and the Monday-to-Sunday range of that week. In the 'Week number → Date' mode you can do the reverse: enter a year and week number to get the seven dates of that week. This is handy for manufacturing, logistics, and project management, where schedules are tracked by week.

How to Use

1. In 'Date → Week number' mode, pick a date to see its ISO week number, US week number, day of year, weekday, and the week's range. 2. Switch to 'Week number → Date' mode and enter an ISO year and week number to get back the Monday–Sunday dates of that week. 3. Use the 'Today' button to quickly fill in the current date.

How It Works

The ISO week number assigns each week to the year that owns its Thursday (the 'ISO year'), and week 1 is the week containing that year's first Thursday — i.e. the week containing January 4. Weeks always start on Monday. The day of year counts from January 1 (day 1). The US week number starts on Sunday and treats the week containing January 1 as week 1.

Reading the Results

'2024-W05' means the 5th week of ISO year 2024. The week range runs from that week's Monday (start) to Sunday (end). Around the turn of the year the ISO year can differ from the calendar year — for example, January 1, 2021 belongs to week 53 of 2020 under ISO. Because the US week number starts on Sunday, it can differ from the ISO week number by one for the same date.

Frequently Asked Questions

What is an ISO week number? ▾

It is the week number defined by ISO 8601. Weeks start on Monday, and week 1 is the week containing the year's first Thursday (the week that contains January 4). It is the most widely used international week-numbering standard.

How does it differ from the US week number? ▾

The US system starts the week on Sunday and treats the week containing January 1 as week 1. ISO starts on Monday and uses the week with the first Thursday as week 1. As a result the two week numbers can differ by one for the same date.

Why is January 1, 2021 in week 53 of 2020? ▾

An ISO week is assigned to the year that owns its Thursday. The week containing Friday, January 1, 2021 has its Thursday on December 31, 2020, so under ISO that week belongs to 2020 and is its last week — week 53.

Which years have 53 weeks? ▾

A year has 53 weeks if January 1 is a Thursday, or if it is a leap year and January 1 is a Wednesday. All other years have 52 weeks. Most years have 52, but a 53-week year appears roughly every 5–6 years.

What are week numbers used for? ▾

They are widely used in business accounting and shipping schedules, manufacturing lot tracking, project planning, and European business calendars. Sharing periods by week (e.g. 'delivery in week 12') is a key benefit.

This tool computes results using ISO 8601. Some organizations or countries use their own week-numbering conventions. For critical work, confirm the definition used by your organization.