Week number calculator

Pick a date to find its ISO week number, day of the year and quarter. Or enter a year and a week number to see the dates from Monday to Sunday.

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  • Runs in your browser
week-number-calculator

Fill in the values above to see the result.

100% private — the calculation is done in your browser and nothing is sent to any server.

How it works

Pick a date or a week

Find the week of a date, or the dates of a week number.

Read the ISO week

The week number as used in ISO 8601, with the Monday-to-Sunday range.

Check the extras

Day of the year, quarter, days left and the US-style week number.

Which week is this?

Week numbers are used to plan projects, schedule production, book holidays, name delivery windows and label timetables and reports. Everyone says "week 38" and expects the same thing, but there are actually several ways of numbering weeks. This calculator uses the international ISO 8601 standard, which is what most of Europe, and most planning and business tools, use.

How ISO weeks work

In ISO 8601 a week always starts on Monday, and week 1 is the week that contains the first Thursday of the year, which is the same as saying that it is the week with at least four days in the new year. A consequence is that the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. A year has 52 or 53 weeks. That is why the tool shows the ISO year alongside the week: 30 December can be in week 1 of the following year.

The US system is different

In the United States and some other countries a week traditionally starts on Sunday, and week 1 is the one that contains 1 January. The numbers can differ by one from ISO for many dates. Since two people can both say "week 38" and mean different weeks, the tool shows the US-style number as well, and the range of the ISO week from Monday to Sunday, so that you can check that you are on the same page.

From week to dates

The second mode does the reverse: give a year and a week number and get the Monday and Sunday of that week, together with the full dates. It is handy for planning a project that runs "from week 12 to week 20", to check when a shipment promised for "week 47" is due or to fill in a form that asks for dates when you only have the week.

Other useful figures

For any date the tool also gives the weekday, the day of the year, useful for ordinal dates, the quarter and how many days remain in the year.

What it does not do

It does not follow other numbering conventions used in some organisations, such as fiscal weeks starting on other days or a week 1 that begins on 1 January regardless. If you rely on a company calendar, check its rules. To add a period of weeks to a date, use the add days to a date tool.

Frequently asked questions

How do I find the week number of a date?
Pick the date. The ISO week number appears, with the Monday-to-Sunday range of that week.
Which week is week 1?
In ISO 8601, the week containing the first Thursday of the year, that is, the week with at least four days in the new year.
Can 1 January be in week 52 or 53?
Yes. If it falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year. The tool shows the ISO year too.
How many weeks does a year have?
52 or 53 ISO weeks. Years with 53 weeks start on a Thursday, or on a Wednesday in a leap year.
How do I find the dates of week 30?
Choose "The dates of a week number", type the year and 30. The Monday and Sunday of that week are shown.
Is my data sent anywhere?
No. The calculation is done in your browser.

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