Why WEEKNUM Returns Week 2 Instead of Week 1 in Excel
Question details
The user wants to know why the WEEKNUM function returns week 2 for an early January date (like January 3, 2021) instead of week 1, and how to correct the week numbering.
- Product
- Excel
- Device & OS
- not provided
- Scenario
- Calculating week numbers for dates at the beginning of the year using the WEEKNUM function.
- Observed behavior
- The formula =WEEKNUM(DATE(2021,1,3),1) unexpectedly returns week 2, even though the user expects it to be week 1, while the previous week is numbered 53.
Determine whether your organization or region follows standard calendar week numbering (where Jan 1st dictates Week 1) or the ISO 8601 week date system (where Week 1 is the first week containing a Thursday).
Use ISOWEEKNUM or Return Type 21 for ISO Standards
By default, WEEKNUM assumes the week containing January 1st is Week 1. If you use European/ISO standard numbering, you should use the ISOWEEKNUM function instead to get accurate results.
In the ISO 8601 week-numbering system, weeks always begin on Monday, and the first week of the year is defined as the week containing the first Thursday of the year. January 3, 2021, falls on a Sunday, which makes it the last day of ISO Week 53 of 2020.
Select the cell where you want the week number to appear. Replace your existing formula with =ISOWEEKNUM(A1), assuming A1 contains your target date.
Alternatively, you can keep the WEEKNUM function but change its return type. Enter the formula =WEEKNUM(A1, 21). The '21' argument instructs Excel to apply ISO week numbering rules.
Change the Start Day of the Week in WEEKNUM
If you do not use ISO standards but want to define a different day as the start of your week (e.g., Monday instead of Sunday), adjust the 'return_type' argument.
Effortlessly Calculate Dates & Week Numbers with WPS Spreadsheet
WPS Office provides a highly compatible spreadsheet application that seamlessly handles WEEKNUM, ISOWEEKNUM, and hundreds of other advanced functions to streamline your data processing.
- 1. Open WPS Spreadsheet: Launch WPS Office and open your workbook containing the calendar dates.
- 2. Enter the Function: Click on the target cell and type =ISOWEEKNUM( to begin the formula.
- 3. Select the Date Cell: Click the cell with your date (e.g., A1), or type it directly, then close the parenthesis.
- 4. Apply the Formula: Press Enter. Drag the fill handle down to apply the precise week numbering to your entire dataset.
Frequently Asked Questions
What is the exact difference between WEEKNUM and ISOWEEKNUM?
WEEKNUM defaults to System 1, where the week containing January 1st is always defined as Week 1, regardless of how many days of that week fall into the new year. ISOWEEKNUM strictly follows the ISO 8601 standard, where weeks begin on Monday and Week 1 is the week containing the first Thursday of the year.
Why does my calendar year have 53 weeks?
A standard year has 365 days (52 weeks and 1 day), and a leap year has 366 days (52 weeks and 2 days). Depending on the day of the week January 1st falls on and your chosen week numbering system, those extra 1-2 days can easily push the final days of the calendar year into a 53rd week.
How do I make weeks start on Monday in WEEKNUM?
You can force the week to start on Monday by changing the return_type argument to 2. The modified formula will be written as =WEEKNUM(A1, 2).
