Syntax
WEEKNUM(serial_number, [return_type])
| Parameter | Description |
|---|---|
| serial_number | Parameter of the WEEKNUM function. |
| [return_type] | (Optional.) Parameter of the WEEKNUM function. |
Examples
Week number (Sunday start)
Formula
=WEEKNUM("2026-04-09")
Returns 15 — April 9, 2026 falls in the 15th week of the year using the default Sunday-start system.
Week number (Monday start)
Formula
=WEEKNUM("2026-04-09", 2)
Returns 15 using Monday as the first day of the week. The result may differ from the Sunday-start version for dates near week boundaries.
Group by week
Formula
="Week " & WEEKNUM(A1, 2) & " - " & YEAR(A1)
Creates labels like "Week 15 - 2026" for grouping transactions or events by week in pivot-style reports.
Common Errors
#VALUE!
Occurs when the date is not valid or return_type is not a recognized option.
#NUM!
Occurs for date serial numbers outside the valid range.
Tips
Not ISO week numbers
WEEKNUM uses a US convention where week 1 always contains Jan 1. For ISO 8601 week numbering (used in Europe), use ISOWEEKNUM instead.
Return types
1 (default) = Sunday start, 2 = Monday start. Types 11-17 allow any day as week start. Most business reporting uses type 2 (Monday start).
Year boundary quirk
Dec 31 is always in week 52 or 53 of the current year with WEEKNUM. ISO week numbering may place it in week 1 of the next year.
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