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How do I determine the inflation rate between two dates? Can I use the difference in consumer price index between the dates rather than a series of inflation rates?

Reason for the question: I am setting up a spreadsheet listing investments, the purchase price, and purchase date. I wanted to be able to calculate the real earnings if I sold today at todays price. Assuming my spread sheet can already pull the current price for a stock I could determine the real earnings if I also had the inflation from time of purchase to now.

For sanity, quarterly or monthly accuracy is all I really need.

I have found the FRED data, but I'm not sure how to turn the known inflation for a series of months into a single rate. And I do not know how to use consumer price index data.

This question seems related.

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You want percent change between the two numbers listed under whatever heading you'll be using in the CPI.

As an example, you'd probably want to use the All Items heading listed here on Page 4 of the August 2016 CPI tables as 240.853, and from August 2015 was listed as 238.316. Percent change is

(240.853/238.316)-1 = 0.0106

So 1.06% inflation from August 2015 to August 2016.

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