I just read about IRR, and honestly, it's one of those concepts that completely changes how you view bond investments. The Internal Rate of Return is just a metric that allows you to truly compare which investment is more advantageous, beyond what you see at first glance.

The interesting thing is that many people only consider the coupon a bond pays, but that's a mistake. The IRR formula shows you the actual profitability, taking into account both the coupons you'll receive and the price at which you buy the bond in the secondary market. That is, if you buy a bond below its face value, it adds to your final return. If you buy it above, it penalizes you.

Let's go with an example that makes this clear. Imagine a bond trading at 94.5 euros, paying a 6% annual coupon, and maturing in 4 years. If you apply the IRR formula correctly, you get 7.62%. That's higher than the 6% coupon precisely because you bought it cheaply. Now imagine the same bond but at 107.5 euros. Here, the IRR formula gives you 3.93%, much lower. The premium you paid eats into your profitability.

The difference with other rates like the nominal interest rate (TIN) or the annual percentage rate (TAE) is important to understand. The TIN is simply the pure interest rate you agreed upon. The TAE includes additional costs, which is why you see a 2% TIN but a 3.26% TAE on a mortgage. The IRR, on the other hand, when applied to fixed income, gives you the true profitability considering the actual cash flows.

To calculate the IRR formula, you need three data points: the bond's price, the coupon it pays, and the time until maturity. The math isn't trivial, which is why online calculators exist to do the work. But what's important is to understand what that number you get actually means.

There are key factors that influence the IRR. The higher the coupon, the higher the IRR. The lower the purchase price, the higher the IRR as well. But beware, this doesn't mean you should chase the highest IRR without thinking. I remember the Greek bond case years ago, during the Grexit, it traded with an IRR above 19%. That wasn't an opportunity; it was a bomb. The credit risk of the issuer is as important as the IRR itself.

So, when you use the IRR formula to compare bonds, do it wisely. Look for the most attractive return, yes, but always verify that the issuer is solid. The IRR is your objective tool to choose between options, but it should never be your only criterion.