I have cash inflows and cash outflows for a 7 year period and my MIRR and inflation rate. That's all the information I have.
How do I calculation the IRR taking the inflation rate into account in Excel ?
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OK, you need to create a price series based on your "inflation" rate. Deflate all your cashflows by this, to give you a series of real cashflows. And then you can do a normal IRR calculation, based on these real cashflows.
Even if your inflation rate is constant rather than time-varying, this can make a difference. Imagine a series of cashflows that goes -100, +100, -100, +100 ad infinitum, versus the opposite that does +100, -100, +100, -100 ad infinitum. IRR'ing these will give you the same nominal 0% IRR. Subtracting any constant inflation rate will thus give you the same real returns. Except if inflation is running at, say, 100% a year, the real returns will be VERY different ;-)
So this would in reality become:
-100, +50, -25, +12.5, -6.25...
+100, -50, +25, -12.5, +6.25...
Not the same thing at all. Obviously, an extreme example. But hopefully the point is obvious. Just discount all the cashflows by your inflation price series, to give you a real-terms IRR.