Municipal Bonds and Tax-Exempt Yields

A U.S. city needs to fund a new water treatment plant. It borrows by issuing a municipal bond, and the federal government, wanting to subsidize that kind of public investment without writing a direct check, makes the interest the city pays exempt from federal income tax. That single feature reshapes how these bonds are priced and compared against every other bond in a portfolio.

Comparing munis to taxable bonds

Because the tax exemption's value depends entirely on the investor's own tax bracket, munis can't be compared to corporate or Treasury bonds on yield alone. The standard conversion is the taxable-equivalent yield:

$$y_{TE} = \frac{y_{muni}}{1 - t}$$

In words: take the municipal bond's actual yield and gross it up by the investor's marginal tax rate — this tells you what a taxable bond would need to yield to leave the same amount in your pocket after tax. A muni yielding less than a Treasury in raw terms can still be the better holding once this adjustment is made, especially for investors in high tax brackets.

Worked example

A muni bond yields 3.5%. An investor in the 32% federal tax bracket compares it to a Treasury: taxable-equivalent yield is $3.5\% / (1 - 0.32) = 3.5\%/0.68 \approx 5.15\%$. If comparable-maturity Treasuries yield only 4.6%, the muni is the better after-tax holding despite its lower headline rate — the investor would need a 5.15% taxable bond to match what the muni already delivers after tax.

For an investor in a much lower, 15% bracket, the same muni's taxable-equivalent yield is only $3.5\%/0.85 \approx 4.12\%$ — below the 4.6% Treasury, meaning this investor is better off in the taxable bond instead. The identical municipal bond is the right holding for one investor and the wrong one for another, purely because of where each sits on the tax schedule.

What this means in practice

This is why muni ownership skews heavily toward high-income individual investors and why muni yields, credit spreads, and even trading patterns can diverge from taxable fixed income in ways that look odd until the tax angle is factored in. Munis also carry real credit risk — states and cities can and do default or get downgraded — so the tax exemption doesn't substitute for checking the issuer's underlying finances, revenue source (general obligation versus revenue bond), and rating.