# Computing the Number of Friday the 13ths in a Year Using Excel

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Figure 1: Number of Friday the 13ths By Year.

I have been tutoring math at the local library and using Excel as a vehicle for encouraging people to explore everyday math. While at the library, I heard a young man ask "How many Friday the 13ths are in a year?" Since I am always looking for computational examples, I showed him how to use Excel to find the answer for himself. This post shows how I taught him to solve the problem. The solution turned out to be a good example of using Excel's date and array capabilities.

Figure 1 shows a simple table with the number of Friday the 13ths by year. I deliberately made the table 28 years per column to show that calendars repeat with a period of 28 years.  I use this characteristic of calendars to buy old calendars that I can reuse; I love old Christmas and tool company calendars.

Figure 2 shows the Excel array formula I used to compute the number of Friday the 13ths. My workbook is here.

Figure 2: Formula Breakdown for Calculating the Number of Friday the 13ths in a Year.

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### 3 Responses to Computing the Number of Friday the 13ths in a Year Using Excel

1. RONAN A MANDRA says:

Nice way of finding the number of Friday the 13ths in a year. Please share your workbook as my formula in a test workbook is not working. Thanks

• mathscinotes says:

Hi Ronan,

Sorry about forgetting the workbook. I just put it out there. This was a quick post before I started to get ready before Thanksgiving and was a bit rushed!

mark

• Ronan Mandra says:

Thanks