ISOWEEKNUM in Google Sheets: ISO Weeks, Correct Week-Years and Grouping Keys

• Introduced: first introduction date is not established by the official history reviewed; no desktop release number is inferred.
• Availability: documented in the rolling Google Sheets service; these examples use the web worksheet workflow.
• Checked: October 14, 2026; official ISOWEEKNUM reference. Documentation and independent models, with no native Google Sheets execution or wider client certification.

=ISOWEEKNUM(date)

The required input is a date-valued cell, numeric date or date-returning expression. Google recommends DATE-generated inputs because text dates can produce errors.

The output is only the week number. It does not include the matching ISO year, the start date of the week or a business calendar. A week number recurring in another year should not be treated as the same reporting interval.

=ISOWEEKNUM(DATE(2021,1,1))
=ISOWEEKNUM(DATE(2021,1,4))

Expected results are 53 and 1. Friday January 1 is in the Monday–Sunday week beginning December 28, 2020. Monday January 4 begins the first ISO week of 2021.

January 1 is not guaranteed to have ISO week number 1. This is a consequence of the stated first-Thursday convention, not a sign that the function has returned the previous calendar year by mistake.

=ISOWEEKNUM(DATE(2024,12,30))
=ISOWEEKNUM(DATE(2025,1,5))

Both expected results are 1, and both belong to ISO year 2025. The Monday beginning that week is December 30, 2024, and its Thursday is January 2, 2025.

By contrast, December 29, 2024 belongs to week 52 of 2024. These adjacent dates are a useful check that a report’s week boundary really is Monday.

With a date-only value in A2:

=YEAR(A2-WEEKDAY(A2,2)+4)

This shifts the input to its week’s Thursday and reads that date’s year. The derived year is 2020 for January 1, 2021 and 2025 for December 30, 2024.

Combine it with the week number:

=YEAR(A2-WEEKDAY(A2,2)+4)&"-W"&TEXT(ISOWEEKNUM(A2),"00")

Expected keys for January 1, 2021, January 4, 2021 and December 30, 2024 are 2020-W53, 2021-W01 and 2025-W01. The two-digit format makes the week part consistently sortable within a year.

Using YEAR(A2) alone would attach the wrong year to the first and third examples. The key construction is an independently checked application of the documented ISO rule.

Create seven dates in A2:A8 from December 28, 2020 through January 3, 2021. Put amounts 10, 20, 30, 40, 50, 60 and 70 in B2:B8 and fill the complete key formula down C2:C8.

=SUMIF(C2:C8,"2020-W53",B2:B8)

Expected total: 280. Every row belongs to that one ISO week. Grouping by calendar year instead splits the same rows into 100 for 2020 and 180 for 2021.

Both aggregations are mathematically possible, but they answer different questions. Put the reporting convention in the header so another reader knows which grouping was intended.

=A2-WEEKDAY(A2,2)+1

For every date in the seven-row fixture, the expected Monday anchor is December 28, 2020. January 4, 2021 maps to January 4. A date-valued key can be useful when the report needs interval boundaries rather than a text label.

The anchor and complete ISO label retain the year context; ISOWEEKNUM alone does not. Do not group all week-1 values in a multiyear dataset without a corresponding year or anchor.

Google also documents WEEKNUM type 21 as ISO. Type 2 starts on Monday but uses the January-1 system: January 1, 2021 has type-2 week 1, and January 4 has type-2 week 2, while their ISO numbers are 53 and 1.

The dedicated ISOWEEKNUM page does not enumerate every invalid-input error code or historical serial boundary. These examples use ordinary Gregorian date-only values and independent calendar calculations. No native mobile, locale-conversion or complete cross-program certification is claimed.

Related helpers are DATE, WEEKDAY and YEAR.

• Google ISOWEEKNUM reference.
• Editorial testing standards.

Examples use fictional inputs, English function names and comma argument separators. Expected outputs are independently modeled from the cited rules, not native Sheets captures. Check the spreadsheet locale when reproducing punctuation-sensitive formulas.