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universal Week Number

universal

Free Week Number Calculator

Find the week number of any date, the dates of any week or the weeks in a year — free, instant, in your browser.

✓ ISO 8601 week numbers✓ No account needed✓ Dates never uploaded✓ Any device
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    Main result: ISO 8601 – weeks start on Monday, week 1 contains the first Thursday. Dates use the Gregorian calendar, years 1–9999, also before 1582 – historical dates may differ.

    The dates you enter never leave your browser. Nothing is sent to a server and nothing is stored.

    How it works

    Three steps, no account

    Pick what to look up

    Choose the week number of a date, the dates of a week number, or the number of weeks in a year. The page opens on today's date, so the current week number is already on screen.

    Enter a date or a week

    Tap the date field and choose a date, or type a year and a week number. Press Today or This week to jump to the present. The answer updates as you type.

    Read and copy the result

    The week number is shown large with a sentence that names the Monday and the Sunday of that week. Below it you see the week-year, the US week number, the day of the year and more. Copy the result with one tap.

    Why it matters

    A week number is not the same everywhere

    The same date can be in week 1 in one system and in week 53 in another. Plans, invoices and shipping dates that quote a week number only work if both sides use the same rule.

    This calculator uses the ISO 8601 rule that most of Europe and many companies use: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year, which is the week with 4 January in it. That is why 29 December can be in week 1 of the next year and 1 January can be in week 52 or 53 of the old one. For comparison it also shows the US-style number, where weeks start on Sunday and week 1 contains 1 January, and the Monday-start version of the same idea.

    Use cases

    What people use a week number calculator for

    Work and project planning

    Find the calendar week of a deadline, a sprint or a meeting so that everyone talks about the same days.

    Shipping and delivery

    Turn a delivery in week 47 into real dates, or see in which week a parcel is due.

    School and university terms

    Check which week of the year a term starts and ends and how many weeks it lasts.

    Payroll and shift schedules

    Match a shift plan, a holiday request or a timesheet to the right week of the year.

    Pregnancy and events

    Place a due date, a festival or a trip in its calendar week and see the Monday to Sunday range.

    Spreadsheets and software

    Check what Excel, Google Sheets or your own code should return for ISOWEEKNUM and WEEKNUM.

    Under the hood

    How the week number is calculated

    Every date is turned into a whole number of days since 1 January 1970 using UTC, so time zones and daylight saving time never change a result. The ISO week of a date is found from its Thursday: the Thursday of the same Monday-to-Sunday week decides which year the week belongs to, and the week number counts the weeks from 1 January of that year. A year has 53 weeks if 1 January is a Thursday, or if it is a leap year and 1 January is a Wednesday. The US number counts weeks that start on Sunday, with week 1 being the week that contains 1 January. Dates use the Gregorian calendar for the years 1 to 9999. Everything runs locally in your browser.

    Questions

    Frequently asked questions

    Basics
    Is this week number calculator really free?
    Yes. The calculator is completely free, with no limit on how many dates you look up. There is no paid tier and no feature locked behind a payment.
    Do I need an account to use it?
    No. There is nothing to sign up for, no email address to give and no login. Open the page and the current week number is already shown.
    Are my dates stored or sent anywhere?
    No. All calculations run in your browser. The dates you enter are not uploaded, logged or stored by this site.
    What is an ISO week number?
    It is the calendar week number defined by the international standard ISO 8601. Weeks run from Monday to Sunday, and week 1 of a year is the week that contains the first Thursday, which is the same as the week that contains 4 January. A year has 52 or 53 such weeks.
    Week numbers and rules
    How do I find the current week number?
    Open the first tab. The date field starts on today's date, so the large number is the current ISO week number, and the sentence below names the Monday and Sunday of this week. Pick any other date to look up its week.
    Which day does a week start on?
    In the ISO system, which this calculator uses for its main result, a week starts on Monday and ends on Sunday. In the United States, Canada and some other countries the week traditionally starts on Sunday, and in some Middle Eastern countries on Saturday. The extra rows show the Sunday-start and Monday-start counts for comparison.
    Why can week 1 start in December?
    Because week 1 is the week that contains the first Thursday of January, and a week always runs from Monday to Sunday. If 1 January falls on a Friday, Saturday or Sunday, that week still belongs to the old year and week 1 starts on the Monday before 4 January, which can be as early as 29 December.
    Can a year have 53 weeks?
    Yes. Years in which 1 January is a Thursday, and leap years in which 1 January is a Wednesday, have 53 ISO weeks. Examples are 2015, 2020 and 2026. About 71 of every 400 years have 53 weeks. The third tab shows the number for any year.
    Why is the US week number different from the ISO one?
    The US system starts weeks on Sunday and makes the week that contains 1 January week 1, even if it has only one day in the new year. The ISO system starts on Monday and needs at least four days in the new year. Near the turn of the year the two numbers can differ by one or more.
    How do I find the dates of a week number?
    Open the second tab, enter the year and the week number, and the calculator shows the Monday to Sunday dates of that ISO week, one line per day. It tells you if the year has no such week, for example week 53 in a year with only 52.
    Is this the same as WEEKNUM in Excel and Google Sheets?
    The ISO result matches ISOWEEKNUM in Excel and Google Sheets. The plain WEEKNUM function uses the US rule by default, with weeks starting on Sunday and week 1 containing 1 January, which matches the US row shown here.
    Technical
    Which dates can I enter?
    Any valid date from 1 January of the year 1 to 31 December 9999 in the Gregorian calendar. Invalid dates such as 31 February are rejected. Years before 1582 use the Gregorian rules too, so they can differ from historical records.
    Do time zones or daylight saving time change the result?
    No. The calculator works with calendar dates, not clock times, so your time zone and daylight saving time have no effect. A date has the same week number everywhere in the world.
    Does it work on a phone?
    Yes. The date field uses your phone's own date picker, and the layout adapts to small screens. Once the page has loaded, the calculations also work without a connection.