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Week Number

The current ISO week number, plus the week number for any date you pick.

Defaults to today. Change it to look up any date. Calculated in your browser, nothing is uploaded.

Pick a date to see its ISO week number.

How to find the current week number

  1. Open the tool

    The current ISO week number appears as soon as the page loads, along with today's date and the week's Monday to Sunday range.

  2. Look up any date

    Change the date field to see the ISO week number, week-year, and date range for any day you choose.

  3. Copy the week code

    Use the copy button to grab the standard year and week code, like 2026-W27, for calendars, spreadsheets, and file names.

Why use this tool

Current week on load

The page shows today's ISO week number the moment it opens, with the full Monday to Sunday date range for the week.

Any date lookup

Enter any calendar date to see the ISO week that contains it, updating live as you change the field.

Correct week-year handling

Dates in late December or early January show the ISO week-year, which can differ from the calendar year, with a plain note when it does.

52 or 53 week fact

See whether the year you are viewing has 52 or 53 ISO weeks, so long-year planning is never a surprise.

Copy the ISO code

Copy the standard year and week string in the 2026-W27 format for schedules, sheets, and file names.

Nothing leaves your browser

The date math runs on your device. Nothing you enter is uploaded or stored.

About this tool

A week number is a compact way to name a week without spelling out dates. Instead of saying the week of 29 June, a factory line, a retail buyer, or a project manager can just say week 27 and everyone lands on the same seven days. Manufacturing, logistics, retail planning, and delivery schedules lean on week numbers heavily, which is why so many calendars in Europe print them down the side.

This tool uses the ISO 8601 standard, the version most business software agrees on. Under ISO 8601 every week starts on Monday and ends on Sunday, and week 1 is the week that holds the first Thursday of the year. The first-Thursday rule is a tidy way of saying week 1 is the week with most of its days in the new year, so a stray day or two at the turn of the year does not get its own tiny week.

That rule creates one edge case worth knowing. Because a week belongs to the year that owns its Thursday, a date can sit in a week that carries a different year from the one on the calendar. 1 January 2023, a Sunday, actually falls in week 52 of 2022, so its ISO code is 2022-W52. The tool shows this ISO week-year and adds a plain note whenever it differs from the calendar year, so the New Year never trips you up.

Need more than a week number? Measure the gap between two dates with the date difference calculator, add or subtract time with the duration calculator, or work out an age with the age calculator. Everything here runs in your browser.

Frequently asked questions

How is the ISO week number defined?
ISO 8601 counts weeks from Monday to Sunday. Week 1 is the week that contains the year's first Thursday, which is the same as the week containing 4 January. That keeps every week whole and tied to a single year.
Why can the week-year differ from the calendar year?
A week belongs to the year that owns its Thursday, so a few days at either end of the year can fall in a neighbouring year's week. For example, 1 January 2023 sits in week 52 of 2022 and is written 2022-W52. The tool flags this whenever it happens.
Why do some years have 53 weeks?
Most years have 52 ISO weeks, but a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. Roughly one year in six is a 53-week year. The tool shows the count for whichever year you are viewing.
Why do ISO weeks start on Monday?
The ISO 8601 standard fixes Monday as the first day of the week and Sunday as the last. Most of the businesses and schedules that rely on week numbers, especially in Europe, follow the same convention.
Is any of this sent to a server?
No. The date and week math all happen in your browser, and nothing you type is uploaded or stored.

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