Week Number Calculator: ISO and US Week of the Year

How to use the week number calculator

1. Choose Date to week number to find the week of the year, or Week number to date to reverse the lookup.
2. For date to week, type a date in YYYY-MM-DD format. For week to date, enter the year and week number.
3. Select ISO 8601 for the international standard or US for the Sunday-start system.
4. The result updates instantly showing the week number, week year and day of the year, or the starting date of that week.

How ISO 8601 week numbering works

The ISO 8601 standard defines week 1 as the week containing the first Thursday of the calendar year. Equivalently, it is the week containing January 4. Weeks always start on Monday, and the last week of the year can be W52 or W53.

To compute this, find the Thursday of the given date's week. The ISO week year is the calendar year of that Thursday. Then count how many Thursdays have occurred from the first Thursday of that year up to and including the target Thursday. That count is the ISO week number.

For example, 2026-01-01 is a Thursday. It falls in the week starting Monday 2025-12-29. Because January 1 is itself a Thursday, that week is ISO week 1 of 2026. The previous Monday, 2025-12-29, belongs to ISO week 1 of 2026 as well, not to 2025.

US week numbering explained

The US week system starts weeks on Sunday and assigns week 1 to the period beginning January 1, regardless of what day of the week January 1 falls on. If January 1 is a Wednesday, then Sunday through Tuesday belong to week 1, and the following Sunday begins week 2.

This means partial weeks at the start and end of the year are common. Most years have 53 US week numbers, because a 365 or 366 day year never fits neatly into whole Sunday-to-Saturday weeks. A 54th week number only appears in a leap year that starts on a Saturday (1916 and 2000 are examples, and 2028 will be the next): week 1 is a single day, and December 30 and 31 spill into week 54.

This system is widely used in North American business scheduling, payroll and retail planning where Sunday is treated as the traditional start of the week.

Common ISO week dates reference

The table below shows selected dates in 2026 with their ISO week number, ISO week year and day of the year. Use it as a quick reference alongside the week number calculator above.

When the ISO week year differs from the calendar year

A frequent source of confusion is that the ISO week year does not always match the calendar year. Late December dates can belong to week 1 of the next year, and early January dates can belong to the last week of the previous year.

For instance, 2025-12-29 is a Monday. Under ISO 8601, the Thursday of that week is 2026-01-01, so the entire week (Monday 2025-12-29 through Sunday 2026-01-04) is ISO week 1 of 2026. Conversely, 2027-01-01 is a Friday whose Thursday is 2026-12-31, placing it in ISO week 53 of 2026. In the same way, 2021-01-03 is a Sunday that belongs to ISO week 53 of 2020, and 2024-12-30 belongs to ISO week 1 of 2025.

This is why the tool reports both the ISO week number and the ISO week year separately. When storing or transmitting week-based dates, always use the ISO week year rather than the calendar year to avoid off-by-one errors in reporting periods.

Practical uses for week of the year calculations

Knowing the week of the year is essential in project management, manufacturing schedules and financial reporting. Many organisations plan sprints, production runs and budget reviews by ISO week rather than by calendar month because weeks provide uniform seven-day periods.

In logistics, shipping carriers often quote transit times in business weeks. Converting a ship date to a week number makes it straightforward to calculate expected delivery windows. Similarly, epidemiologists and public health agencies track disease incidence by ISO week to maintain consistent reporting intervals across years.

If you need to figure out what day of the week a historical event fell on, the day of the week calculator handles that directly. For adding or subtracting specific day counts, see the add days to date tool, and for measuring spans between two dates, use the days between dates calculator.

Worked examples