ISOWEEKNUM Function in Apple Numbers: Syntax and Worked Examples

• Introduced: Added in Numbers 12.2 for Mac and iPad.
• Availability: Documented for Numbers in Apple’s Formulas and Functions Help 15.4; this documentation edition is not a final supported-version endpoint.
• Scope: Mac and iPad introduction versions are evidenced above; iPhone and iCloud introduction versions remain unknown.
• Checked: October 14, 2026, against the official ISOWEEKNUM reference. Documentation and independent calendar checks; no native Numbers test.

=ISOWEEKNUM(date)

The single required argument is a reference containing a date, or a function that returns a date. There is no optional week-start setting. Apple specifies ISO 8601 rather than a configurable calendar-week scheme.

ISO weeks run Monday through Sunday. Week 1 is the week containing the year’s first Thursday; equivalently, it contains January 4. A late-December date can therefore belong to the next ISO year, and an early-January date can belong to the previous one. The ISO year has either 52 or 53 numbered weeks.

Use DATE to create unambiguous practice inputs. Its numeric year, month and day arguments avoid regional date-text interpretation. The formulas here use English function names and comma separators; follow your document’s formula editor if its syntax differs.

Create a blank table with at least five columns (A:E) and nine rows. Label A1 Date, B1 ISO week, C1 ISO year, and D1 Week label, and E1 Amount. Enter only the A-column formulas first:

Enter =ISOWEEKNUM(A2) in B2, then fill down through B9. The March 1, 2022 result also matches Apple’s function example. Columns C and D remain empty until the next section.

Rows 3 and 4 belong to the same ISO week even though their calendar years differ. Rows 7 and 8 do too. Grouping on YEAR(A2) together with the ISO week would split those weeks incorrectly.

The ISO week-year differs from the calendar year only near New Year. Using the week number already calculated in B, enter this in C2, then fill through C9:

=YEAR(A2)+IF(AND(MONTH(A2)=1,B2>=52),-1,IF(AND(MONTH(A2)=12,B2=1),1,0))

The first condition shifts a January date in week 52 or 53 back one year. The second shifts a December date in week 1 forward one year. All other dates keep their calendar year. This helper is derived from the ISO calendar rule; it is not an undocumented extra argument to ISOWEEKNUM.

In D2, enter =C2&"-W"&RIGHT("0"&B2,2) and fill through D9. Expected labels in order are:

2020-W01, 2020-W53, 2020-W53, 2021-W01, 2022-W09, 2025-W01, 2025-W01, 2026-W53.

The two-digit week component makes week 9 appear as W09. Keep the numeric year and week columns for grouping and sorting. See IF for conditional results and RIGHT for taking the final characters of the padded text.

For a small grouping check, enter values 10 in E3 and 15 in E4. Those two dates share 2020-W53, with a combined amount of 25. A key made from the ordinary calendar year would separate them, even though their common Monday is December 28, 2020.

WEEKNUM is a separate function. A Monday-first weekday setting alone does not define the ISO first-week rule. If a business report follows fiscal weeks, a 4-4-5 calendar, Sunday-start weeks or a locally defined first week, establish that convention before choosing a formula.

ISOWEEKNUM does not calculate the number of elapsed weeks between two dates. For example, two dates can have the same week number in different years while being roughly a year apart. Preserve the date and ISO year rather than treating a week number as a unique identifier.

• January returns 52 or 53: inspect the ISO year before calling the value an error.
• December returns 1: the date may belong to the following ISO year.
• An imported number fails: the argument is documented as a date reference or date-returning function, not an arbitrary other application’s serial number.
• Date text changes with locale: construct the date from numeric components rather than relying on a slash-separated string.
• Unknown function in an older copy: the Mac and iPad release histories above place its introduction in 12.2. Do not infer matching introduction versions for other clients.

The short Apple entry does not enumerate blank-cell handling, malformed-text errors or every date-range boundary. The worked cases use valid modern Gregorian dates and do not assign unverified error labels to other inputs.