Excel Week Number: Get It from a Date

Updated 5 November 2026 · Standard formula, shown below

To get a week number from a date in Excel, use WEEKNUM, such as =WEEKNUM(A2,2) to number weeks from Monday. For ISO week numbering, use =ISOWEEKNUM(A2); the right choice depends on which day starts the week and how the first week of the year is defined.

Choose the Right Week-Numbering System

Excel offers more than one definition of a week number. A week can start on Sunday or Monday, and the first few days of January may belong to week 1—or be counted as the final week of the previous year. This means the same date can produce different week numbers under different systems.

Use WEEKNUM when your reporting rules specify a start day and a simple week 1 beginning on January 1.
Use ISOWEEKNUM when you need the ISO 8601 system, commonly used in international business and scheduling.

If your workplace, school, or reporting system already defines a calendar convention, follow that definition rather than choosing based only on which result looks familiar.

Use WEEKNUM with a Date

The basic syntax is WEEKNUM(serial_number,[return_type]). The first argument is the date, usually a cell reference; the optional second argument specifies which day starts the week and, in some cases, the numbering convention.

For example, if cell A2 contains 15 March 2025:

=WEEKNUM(A2) returns the week number using Sunday as the start of the week.
=WEEKNUM(A2,2) starts each week on Monday.
=WEEKNUM(A2,21) uses the ISO week-numbering system in Excel versions that support this return type.

You can also enter a date directly, but a cell reference is generally easier to maintain. A date typed into a formula must be constructed in a way Excel recognizes in your regional settings; for example, =WEEKNUM(DATE(2025,3,15),2) avoids ambiguity about whether a typed date means March 15 or 15 March.

Use ISOWEEKNUM for ISO Weeks

The ISO formula is =ISOWEEKNUM(A2). It returns a week number from 1 to 52 or 53, with weeks starting on Monday. ISO week 1 is the week containing the first Thursday of the calendar year. Equivalently, it is the first week with at least four days in the new year.

This rule matters around New Year. A date in early January can belong to the last ISO week of the previous ISO year, while a date in late December can belong to ISO week 1 of the next ISO year. The week number alone does not identify the year correctly for those dates.

For example, a report that groups dates by ISO week should usually include both the ISO week-year and the ISO week number. Excel’s YEAR(A2) returns the calendar year, not necessarily the ISO week-year. For a simple label, you can display the date’s ISO week with =ISOWEEKNUM(A2), but take care when combining it with the calendar year in a key or report.

WEEKNUM Return Types

The second argument to WEEKNUM controls the week start and numbering system. Common choices include:

1 or omitted: Sunday is the first day of the week; week 1 contains January 1.
2: Monday is the first day of the week; week 1 contains January 1.
11 through 17: select Monday through Sunday, respectively, as the first day; week 1 contains January 1.
21: ISO week numbering, with Monday starts and the ISO rule for week 1.

Excel also recognizes some older return types for particular Sunday-start and Monday-start conventions. If a formula is shared across different spreadsheet programs or older Excel versions, verify that the selected return type is supported and interpreted consistently. For ISO results, ISOWEEKNUM states the intention clearly.

Fill the Formula Down a Column

To number a list of dates, put the formula next to the first date and copy it down. If dates begin in A2, enter =ISOWEEKNUM(A2) or the appropriate WEEKNUM formula in B2. Use the fill handle or copy and paste to extend the formula. The reference adjusts to A3, A4, and so on.

For a reusable template, keep the formula’s date reference relative, as in A2. If the date is always in one fixed cell, use an absolute reference such as $A$2. You can then format the result as a number or combine it with text for a label. For instance, ="Week "&ISOWEEKNUM(A2) produces a label such as “Week 11.”

Common Problems and Checks

If the result is unexpected, check the date and the week convention before changing the formula. A week number is not a universal property of a date; it is the result of applying a particular calendar rule.

The cell contains text, not a date. Excel may not recognize imported or manually entered text as a date. Convert it to a real Excel date value, then recalculate.
The week starts on the wrong day. Add the correct return_type, such as 2 for Monday-start weeks, or use ISOWEEKNUM for ISO weeks.
A January date returns week 52 or 53. That can be correct under ISO rules: the date may belong to the previous ISO week-year.
A date displays as a number. Excel stores dates as serial values. Apply a date format to the source cell; the week-number result itself is normally a number.
The formula shows an error. Confirm the cell reference is valid and the date is recognized. In some regional settings, formula arguments use semicolons instead of commas.

Also distinguish a week number from a week count or a date range. A formula returning 12 tells you the assigned week number, but it does not by itself tell you the week’s Monday and Sunday or its ISO year.

Frequently Asked Questions

What is the formula for week number in Excel?
Use =WEEKNUM(A1) for the default Sunday-start convention. Use =WEEKNUM(A1,2) for Monday-start weeks, or =ISOWEEKNUM(A1) for ISO weeks.

How do I get the ISO week number in Excel?
Enter =ISOWEEKNUM(A1), replacing A1 with the cell containing your date. ISO weeks begin on Monday, and week 1 is the week containing the year’s first Thursday.

Why does Excel show week 53?
Some years have 53 weeks under a given numbering system. Whether a year has week 53 depends on the calendar and the rule used to define the first day and first week.

Can the week number be different from the year shown in the date?
Yes. Around New Year, ISO week dates can belong to the previous or next ISO week-year. The calendar year from YEAR may therefore differ from the year associated with the ISO week.

How do I show the week number with the year?
You can combine a year and a week number in a text label, but for ISO reporting do not automatically pair YEAR(A1) with ISOWEEKNUM(A1). Dates near New Year may need an ISO week-year calculation so the label stays correct.

Does WEEKNUM count weeks from January 1?
For many return types, including the common values 1 and 2, week 1 contains January 1, even if it is only part of a week. ISO numbering is different: week 1 must meet the ISO first-week rule.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.