Please Note: This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: Calculating Averages by Date.

Calculating Averages by Date

by Allen Wyatt
(last updated February 6, 2016)

Suppose that you have a huge worksheet that contains all the rainfall readings for a given locale for the past hundred years or so. In cells A2:A37987 you have the dates, 1 January 1903 through 31 December 2006. In cells B2:B37987 you have the measurements for each date. Further, some of the measurements can be zero (if there is no rainfall for the day) or blank (if no reading was taken that particular day). With all this information, you want to calculate the average historic rainfall for any given day of the year.

One solution involves the use of array formulas, as detailed here:

  1. Select all the cells in column A that contain dates, and assign this range the name Dates.
  2. Select all the cells in column B that contain rainfall data, and assign this range the name Rainfall.
  3. In column D, starting in cell D2, place all the days of the year. You should end up with D2 through D366 filled with dates.
  4. In cell E2, enter the following array formula (terminate the formula by pressing Shift+Ctrl+Enter). The result of the formula is the sum of all the cells in the Rainfall range, for the date specified in cell D2.
=SUM((MONTH(Dates)=MONTH(D2))*(DAY(Dates)=DAY(D2))*Rainfall)
  • In cell F2, enter the following array formula (terminate the formula by pressing Shift+Ctrl+Enter). The result of the formula is the number of cells in the Rainfall range, for the date in cell D2, that have a value in them.
  • =SUM((MONTH(Dates)=MONTH(D2))*(DAY(Dates)=DAY(D2))*(Rainfall<>""))
    
  • In cell G2, enter the following regular formula. This is your average for the date in cell D2.
  • =IF(F2<>0,E2/F2,"")
    
  • Select the range E2:G2 and copy down for all the dates shown in column D.
  • This approach works, but it takes quite a while to calculate. This is because you effectively entered 730 array formulas, each checking over 37,000 cells. This is a lot of work, and consequently it may appear like your machine has "hung" after you complete step 7. It has not hung; it will just take it a while to complete the calculations.

    To decrease the number of calculations that must be performed, you can use a variation on the above steps. Follow steps 1 through 3, as noted, and then place the following array formula into cell E2:

    =AVERAGE(IF(ISNUMBER(Dates)*ISNUMBER(Rainfall)*(MONTH(Dates)=MONTH(D2))*(DAY(Dates)=DAY(D2)),Rainfall))
    

    You can then copy the formula down for all the dates shown in column D. The result of this formula is the actual average rainfall, the same as had been shown in column G in the previous approach.

    This formula works because of the way that Boolean arithmetic works in Excel. The ISNUMBER function returns either True or False, and the comparisons (MONTH and DAY) return either True or False. These results are all multiplied against each other, resulting in True only if all the individual tests are True. Only if they are all True will the average of the Rainfall for that particular date be calculated.

    You can reduce the calculation overhead even further by simply getting rid of all the table that calculates the averages for every day of the year. With your dates and rainfall in columns A and B, follow these steps:

    1. Select all the cells in column A that contain dates and assign this range the name Dates.
    2. Select all the cells in column B that contain rainfall data and assign this range the name Rainfall.
    3. In cell D2, place the date for which you want to check the average rainfall. (The year isn't important; only the month and day are used in the calculation.)
    4. Enter the following formula into cell E2:
    =AVERAGE(IF(ISNUMBER(Dates)*ISNUMBER(Rainfall)*(MONTH(Dates)=MONTH(D2))*(DAY(Dates)=DAY(D2)),Rainfall))
    

    That's it. Now, you can change the date in cell D2 as desired, and cell E2 will always indicate the average rainfall for that date. The formula in cell E2 is the same as the formula used in the last approach; the difference is that you aren't calculating it for all the days in a year, and thus the calculation is done much quicker.

    Another approach involves the use of Excel's filtering capabilities. Before you can use them properly, however, you must create a column that shows only the month and day for each date in your data. Use this formula in cell C2:

    =MONTH(A2) & "-" & DAY(A2)
    

    Now, turn on AutoFiltering (Data | Filter | AutoFilter) and use the drop-down list at the top of the new column to select the date for which you want an average. You then use the following formula, placed in any cell desired, to show the average rainfall for the selected date:

    =SUBTOTAL(1,B:B)
    

    ExcelTips is your source for cost-effective Microsoft Excel training. This tip (2350) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: Calculating Averages by Date.

    Author Bio

    Allen Wyatt

    With more than 50 non-fiction books and numerous magazine articles to his credit, Allen Wyatt is an internationally recognized author. He  is president of Sharon Parq Associates, a computer and publishing services company. ...

    MORE FROM ALLEN

    Applying Styles

    Styles are a powerful formatting tool for the text in your documents. Once you've created styles that describe how you want ...

    Discover More

    Changing the Office Assistant

    The Office Assistant is part of the Help system available in Excel. If you want, you can change which Office Assistant ...

    Discover More

    Bypassing the BeforeClose Event

    Hold down the Shift key as you open a workbook, and Excel bypasses any "startup macros" that may be in the workbook. If you ...

    Discover More

    Program Successfully in Excel! John Walkenbach's name is synonymous with excellence in deciphering complex technical topics. With this comprehensive guide, "Mr. Spreadsheet" shows how to maximize your Excel experience using professional spreadsheet application development tips from his own personal bookshelf. Check out Excel 2013 Power Programming with VBA today!

    MORE EXCELTIPS (MENU)

    Calculating Months of Tenure

    Need to know the number of months between two dates? It's easy to figure out if you use the DATEDIF function.

    Discover More

    Forcing Dates Forward

    Want to push a date to some pre-defined day of the month? Here's some ways to force the issue.

    Discover More

    Displaying a Number as Years and Months

    How do you display a number of years, such as 3.67 years, as a number of years and months? It's simple to do with a formula, ...

    Discover More
    Subscribe

    FREE SERVICE: Get tips like this every week in ExcelTips, a free productivity newsletter. Enter your address and click "Subscribe."

    View most recent newsletter.

    Comments for this tip:

    There are currently no comments for this tip. (Be the first to leave your comment—just use the simple form above!)

    This Site

    Got a version of Excel that uses the menu interface (Excel 97, Excel 2000, Excel 2002, or Excel 2003)? This site is for you! If you use a later version of Excel, visit our ExcelTips site focusing on the ribbon interface.

    Subscribe

    FREE SERVICE: Get tips like this every week in ExcelTips, a free productivity newsletter. Enter your address and click "Subscribe."

    (Your e-mail address is not shared with anyone, ever.)

    View the most recent newsletter.

    Links and Sharing
    Share