ISOWEEKNUM function

Returns the ISO 8601 week number (1–53) for a date, where week 1 starts on Monday and contains at least four days in the calendar year.

=ISOWEEKNUM(date)

Generate a ISOWEEKNUM formula

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Arguments

How ISOWEEKNUM reads its arguments
daterequiredISOWEEKNUM →
ArgumentRequiredDescription
dateRequiredThe date value for which to calculate the ISO week number; can be a DATE, serial number, or text in a recognized date format. If invalid, returns #VALUE!.

Returns

Returns a number from 1 to 53 representing the ISO week number of the given date.

Availability

Excel: All · Google Sheets: Supported

Worked examples

1. Find which ISO week a property was listed

AddressBedsBathsList PriceDays on MarketList Date
123 Oak St32$450,000142025-04-07
456 Pine Ave43$625,000222025-01-15
789 Elm Rd21$275,00082025-12-29
321 Maple Dr53$850,000352025-09-03
=ISOWEEKNUM(F2)

Result: 15

April 7, 2025 falls in ISO week 15. Under ISO 8601 standards, weeks begin on Monday and week 1 must contain at least four days of the new year, making this listing date part of week 15.

2. Identify properties listed in late-year weeks

AddressBedsBathsList PriceDays on MarketList Date
123 Oak St32$450,000142025-04-07
456 Pine Ave43$625,000222025-01-15
789 Elm Rd21$275,00082025-12-29
321 Maple Dr53$850,000352025-09-03
=IF(ISOWEEKNUM(F4)>50,"Year-end listing","Earlier listing")

Result: "Year-end listing"

The property at 789 Elm Rd was listed on December 29, 2025, which falls in ISO week 52 (late-year). Since 52 > 50, the IF formula correctly identifies it as a year-end listing, useful for seasonal market analysis.

3. Group properties by ISO week for seasonal patterns

AddressBedsBathsList PriceDays on MarketList Date
123 Oak St32$450,000142025-04-07
456 Pine Ave43$625,000222025-01-15
789 Elm Rd21$275,00082025-12-29
321 Maple Dr53$850,000352025-09-03
=ISOWEEKNUM(F5)

Result: 36

September 3, 2025 falls in ISO week 36, allowing you to categorize properties by week number. This enables consistent grouping across international data and helps identify seasonal listing patterns.

Common errors

Which ISOWEEKNUM error are you seeing?
ISOWEEKNUM returned an error#VALUE!
Wrap text dates in DATEVALUE() to convert them to date values, or ensure the cell contains an actual date, not text.
#NUM!
Verify the date is within 1900–9999; if importing historical data predating 1900, use alternative date representations or text format instead.
#REF!
Update the formula to reference a valid cell containing a date, or restore the deleted cell if the reference was correct before deletion.
ErrorWhy it happensHow to fix it
#VALUE!The date argument is text in a format the spreadsheet cannot recognize as a valid date, such as "XYZ" or "13/35/2025".Wrap text dates in DATEVALUE() to convert them to date values, or ensure the cell contains an actual date, not text.
#NUM!The date serial number falls outside the supported range: before January 1, 1900 or after December 31, 9999.Verify the date is within 1900–9999; if importing historical data predating 1900, use alternative date representations or text format instead.
#REF!The cell reference in the formula points to a cell that has been deleted, moved, or is otherwise invalid.Update the formula to reference a valid cell containing a date, or restore the deleted cell if the reference was correct before deletion.

Tips and when to use something else

  • ISO weeks always start on Monday and end on Sunday, which differs from WEEKNUM()—use WEEKNUM() if you need weeks starting on Sunday.
  • ISO week 1 always contains January 4th; dates from December 29–31 of the prior year may fall into the previous year's week 52 or 53.
  • Combine ISOWEEKNUM with YEAR() for clearer reporting: =ISOWEEKNUM(date)&" of "&YEAR(date) returns results like "15 of 2025".
  • WEEKNUM() returns weeks based on locale or a specified start day; ISOWEEKNUM() always follows international ISO 8601 standards—choose the correct function for your reporting requirements.

Related functions

WEEKDAYWEEKDAY returns the day of the week (1–7) for a given date, with customizable numbering systems (Sunday=1, Monday=1, or ISO 8601).WEEKNUMWEEKNUM returns the week number (1–53) of a date within the calendar year, useful for identifying items expiring in the same week.DATEDATE returns a date serial number from year, month, and day arguments, enabling date construction from separate numeric components.TODAYTODAY returns the current date as a date value, automatically updating each time the spreadsheet opens or recalculates.DATEDIFReturns the number of time units (days, months, years) between two dates, useful for calculating durations, aging, or time-to-deadline.

Frequently asked questions

What's the difference between ISOWEEKNUM and WEEKNUM?
ISOWEEKNUM follows ISO 8601 standards (Monday–Sunday weeks, with week 1 containing January 4), while WEEKNUM uses your system locale or a specified starting day. Choose ISOWEEKNUM for international analysis and WEEKNUM for region-specific calendars.
Why does my January 1st sometimes return week 52 or 53?
Under ISO 8601, if January 1st falls on Friday, Saturday, or Sunday, it belongs to the previous year's final week (52 or 53) because ISO week 1 must contain at least four days of the new year. December 29–31 of the prior year may also belong to the current year's week 1.
Can I use ISOWEEKNUM to group real estate listings by week?
Yes; use ISOWEEKNUM in combination with COUNTIF, pivot tables, or filters to group listings by week. This is especially useful for tracking seasonal patterns, comparing weekly sales volume, or analyzing market activity across international properties.
How do I create a week label like 'Week 15, 2025' using ISOWEEKNUM?
Combine ISOWEEKNUM with YEAR and TEXT formatting: =TEXT(ISOWEEKNUM(date),"00")&", "&YEAR(date). This concatenates the zero-padded week number with the year, creating readable labels for reporting and analysis.

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Reviewed 2026-09-17