I just reviewed how the IRR actually works, and I think many investors don’t understand it well. The IRR formula is more complicated than other financial calculations, but once you master it, it becomes your best ally for comparing investments.

Let’s start with the basics: the Internal Rate of Return is that percentage metric that allows you to objectively compare two or more investment options. When you invest in fixed income, the IRR shows you the actual profitability of the bond, not just what you see in the coupon. That’s why it’s important to understand that this profitability comes from two sources: the coupons (annual, semiannual, or quarterly payments) and the price difference when the bond returns to its face value at maturity.

That’s the key point most overlook. If you buy a bond at 94.5 euros when its face value is 100, you are buying below par. That means when it matures, you will receive 100 euros plus the last coupon. That price gain adds to the coupons and is what the IRR formula captures. Conversely, if you buy it at 107.5 euros, you pay above par, and that penalizes you because at maturity you will only recover 100.

Let’s look at a concrete example. We have a bond trading at 94.5 euros, pays a 6% annual coupon, and matures in 4 years. Applying the IRR formula, we get 7.62%. That’s higher than the 6% coupon because we’re buying cheaply. Now take the same bond but trading at 107.5 euros: the IRR drops to 3.93%. The high price penalizes you even though the coupon is the same.

This explains why two bonds with different coupons can have different returns. Imagine Bond A with an 8% coupon but an IRR of 3.67%, versus Bond B with a 5% coupon but an IRR of 4.22%. If you only look at the coupon, you choose the first. But the IRR formula shows that the second is more profitable. The first probably trades very expensively in the secondary market.

Don’t confuse IRR with other rates you might see out there. The NOMINAL interest rate (TIN) is the pure nominal interest rate, without additional costs. The APR (TAE) includes commissions, insurance, and other expenses (that’s what you see with mortgages). The technical interest rate is used in savings insurance. They are different concepts but are often mixed up.

Mathematically, the IRR formula is complex because you need to solve an equation where you don’t know the exact rate. That’s why online calculators exist to do the work for you. You input the current price, the coupon, the time to maturity, and voilà: you get your IRR.

The factors that influence IRR are basically three. First, the coupon: higher coupons mean higher IRR. Second, the purchase price: below par increases IRR, above par decreases it. Third, special features of the bond: if it’s convertible, inflation-linked, or a floating rate note (FRN), other factors can modify the result.

An important warning: IRR is a powerful tool but not the only thing that matters. You need to verify the credit quality of the issuer. I remember when the 10-year Greek bond traded with an IRR above 19% during the Grexit crisis. That seemed like a bargain, but the default risk was huge. In the end, only the Eurozone bailout prevented default. The lesson: a very high IRR can be a trap if the issuer is in trouble.

So, when analyzing bonds, use the IRR formula as your main profitability metric, but always cross-check that information with credit risk analysis. That combination is what allows you to make smart decisions in fixed income.