What is ISO Week Number?

ISO Week Number finder tells you the ISO 8601 week number for any date. ISO weeks start on Monday and the first week of the year contains the first Thursday. Useful for fiscal reporting, project timelines, and any workflow built on week-based calendars.

Under ISO 8601 the year is split into 52 or 53 numbered weeks. Each week starts Monday and ends Sunday, and the first week of the ISO year is the one containing the first Thursday. That is why early January or late December can belong to the neighbouring ISO year. Common in European payroll, manufacturing, and freight scheduling.

How to use

1. Step 1 — Select or type a date using the date picker. Today's date is pre-filled.
2. Step 2 — View the ISO week number, day of week, day of year, and which quarter the date falls in.
3. Step 3 — Use the week calendar view to see all dates in the same ISO week, and navigate between weeks to plan ahead.

When to use

• Filing weekly reports labelled W12 or 2025-W42 for European payroll and finance teams.
• Aligning Jira sprints or production runs that always start Monday with ISO calendar weeks.
• Checking which calendar dates fall inside the same ISO week before booking shifts or freight slots.

Result

You need to file a report for 'Week 12, 2025.' Enter March 20, 2025 and confirm it falls in ISO Week 12 (March 17–23), which is in Q1. The tool shows this is day 79 of the year, a Thursday.

FAQ

Why does January 1st sometimes show as week 52 or 53?

If the first Thursday of January is on the 4th or later, the days before it belong to the last week of the previous ISO year. So January 1, 2023 (a Sunday) is W52 of 2022. The ISO year and the calendar year do not always agree.

How is ISO week numbering different from the US week numbering?

ISO weeks start Monday and number from the week containing the first Thursday. US conventions usually start the week on Sunday and call the week containing January 1 Week 1. The two systems can disagree by a full week at the year boundary.

Can an ISO year have 53 weeks?

Yes. Roughly every five to six years the ISO year contains 53 weeks, which is why payroll templates that hard-code 52 weeks sometimes break in years like 2015, 2020, and 2026. The tool shows the correct count for each date you enter.

What does the ISO year column tell me?

It is the year the ISO week belongs to, not necessarily the calendar year of the date. December 31, 2024 falls in ISO week 1 of 2025, because that week contains the Thursday January 2, 2025.

Is week 1 the same as the first week of January?

Not always. Week 1 is the week that contains the first Thursday of January. If January 1 falls on a Friday, Saturday or Sunday, those days are in the previous ISO year and the first Monday after them starts the new ISO week 1.

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