The break-even tax rate formula answers one question: at what marginal tax rate does a tax-exempt bond and a taxable bond leave you with the same after-tax return? The formula is Break-Even Tax Rate = 1 − (Tax-Exempt Yield ÷ Taxable Yield). Run the numbers, compare the result to your own combined marginal rate, and you have your answer. If your rate is higher than the break-even, the tax-exempt bond wins. If it’s lower, the taxable bond wins. Everything else in this article is about getting the inputs right so the answer is worth trusting.
Running the Formula
The math has two steps. Divide the tax-exempt yield by the taxable yield. Subtract that number from 1. What’s left is the marginal tax rate at which the two investments produce identical after-tax income.
Say a municipal bond pays 3.50% and a comparable corporate bond pays 5.00%. Divide 3.50 by 5.00 and you get 0.70. The muni already delivers 70% of the corporate bond’s gross return without any tax. Subtract 0.70 from 1.00 and you get 0.30, or 30%. Anyone whose combined marginal rate is above 30% keeps more money in the muni. Anyone below 30% keeps more in the corporate bond after paying tax.
The formula works because municipal bond interest is excluded from federal gross income by statute.1Office of the Law Revision Counsel. 26 US Code 103 – Interest on State and Local Bonds That exclusion is what creates the spread between taxable and tax-exempt yields, and the formula identifies the tax rate that closes the spread.
Finding Your Combined Marginal Rate
Your marginal rate is the rate applied to your last dollar of taxable income, not your average rate across all income. Federal income tax uses a bracketed structure where each additional slice of income is taxed at a progressively higher rate.2Office of the Law Revision Counsel. 26 US Code 1 – Tax Imposed For 2026, single-filer brackets range from 10% at the bottom to 37% on income above $640,600; the joint-filer 37% bracket begins above $768,700.3Internal Revenue Service. Rev Proc 2025-32 Look at your most recent return, find where your taxable income falls, and you have your federal marginal rate.
State income tax belongs in the same number. Top marginal state rates run from about 2.5% to over 13%, and eight states impose no individual income tax. Add your state’s top marginal rate to your federal bracket for a workable combined rate. A single filer in the 24% federal bracket who lives in a state with a 5% top rate has a combined marginal rate of roughly 29%.
State income tax is deductible on your federal return if you itemize, and the federal deduction for state and local taxes is capped at $40,000 for most filers in 2026. If your state taxes exceed that cap, the excess isn’t deductible and your effective combined rate is slightly higher than a plain sum of the two brackets. For a bond comparison, the plain sum is usually close enough.
The 3.8% Net Investment Income Tax
Higher-income taxpayers owe an additional 3.8% on net investment income when modified adjusted gross income exceeds $200,000 (single) or $250,000 (joint).4Office of the Law Revision Counsel. 26 US Code 1411 – Imposition of Tax Interest from taxable bonds is net investment income; interest from municipal bonds is not.5Internal Revenue Service. Net Investment Income Tax If you’re above the threshold, add 3.8 percentage points to your federal marginal rate before running the formula. A single filer in the 35% bracket who owes the surcharge is really looking at 38.8% federal on taxable bond interest.
A Worked Comparison
Back to the 3.50% muni and 5.00% corporate bond. The break-even is 30%.
A single filer with $80,000 in taxable income sits in the 22% federal bracket. Add a 5% state rate and the combined marginal rate is 27%, below the 30% break-even. Paying 27% tax on the 5.00% corporate yield leaves 3.65%, which beats the muni’s 3.50%. The taxable bond wins.
Push that same investor into the 32% federal bracket, above roughly $202,000 in taxable income. The combined rate climbs to 37%. Paying 37% on the 5.00% corporate yield leaves 3.15%, well short of the muni’s 3.50%. The muni wins, and the gap widens as the bracket climbs.
Tax-Equivalent Yield: The Same Question Flipped
The same comparison can be run from the other direction. Instead of solving for the tax rate that makes two bonds equal, solve for the taxable yield that would match a given muni after tax:
Tax-Equivalent Yield = Tax-Exempt Yield ÷ (1 − Marginal Tax Rate)6Fidelity. Fixed Income: Taxable-Equivalent Yields for Individual Bonds, CDs, and SPDAs
A 3.50% muni for someone in a combined 37% bracket has a tax-equivalent yield of 3.50 ÷ (1 − 0.37) = 5.56%. A taxable bond would have to pay at least 5.56% to beat it. Both formulas answer the same underlying question. Pick whichever framing is easier to hold in your head.
Adjustments That Change the Inputs
In-State Munis and Treasury Interest
A municipal bond issued in your home state is usually exempt from your state’s income tax on top of the federal exclusion. When you’re comparing an in-state muni against a fully taxable corporate bond, use your full combined federal-plus-state marginal rate. Treasuries are different: Treasury interest is exempt from state and local tax by federal statute,7Office of the Law Revision Counsel. 31 US Code 3124 – Exemption From Taxation so when the taxable side of your comparison is a Treasury, use your federal marginal rate alone (plus NIIT if it applies), not the combined rate. Using the wrong rate in either direction will tilt the answer.
Fund Expense Ratios
If you hold bonds through a mutual fund or ETF, the expense ratio comes out of the yield before you see it. A muni fund with a 3.50% gross yield and a 0.20% expense ratio nets 3.30%. A taxable bond fund with a 5.00% gross yield and a 0.40% expense ratio nets 4.60%. Run the formula on net yields, not gross, or the higher-cost fund will look better than it is.
AMT and Private Activity Bonds
Not every muni is fully tax-exempt in practice. Interest on certain private activity bonds, which fund projects like airports, housing, and stadiums rather than general government operations, is a tax preference item for the Alternative Minimum Tax.8Office of the Law Revision Counsel. 26 US Code 57 – Items of Tax Preference For 2026, the AMT exemption is $90,100 for single filers and $140,200 for joint filers, with a phase-out at higher incomes.3Internal Revenue Service. Rev Proc 2025-32 If you’re subject to the AMT, private activity bond interest is no longer fully exempt, and the break-even formula will overstate the muni’s advantage unless you treat that interest as partially taxable. General obligation and essential-service revenue bonds sidestep the problem.
Reading the Answer
Line the break-even rate up against your combined marginal rate:
- Your marginal rate is higher than the break-even. The tax-exempt bond nets more. The wider the gap, the stronger the advantage.
- Your marginal rate is lower than the break-even. The taxable bond nets more, and paying the tax still leaves you ahead.
- The two rates are equal. After-tax cash flow is the same either way. Credit quality, liquidity, and call features drive the decision from there.
The answer isn’t permanent. Your marginal rate changes with income, retirement, and new tax legislation. The break-even rate itself moves whenever the yield spread between muni and taxable bonds moves. Rerun the numbers whenever you’re making a new purchase or rebalancing.
What the Formula Doesn’t Tell You
The break-even formula isolates tax efficiency and ignores everything else. Useful for what it does, silent on the rest.
Credit risk isn’t in the math. Munis have historically defaulted at lower rates than similarly rated corporates, but that gap narrows in downturns. A muni that clears the break-even by a thin margin isn’t worth it if the issuer’s finances are shaky.
Liquidity isn’t in the math either. The municipal market is thinner than the Treasury or investment-grade corporate market, and wider bid-ask spreads can eat into your return if you sell before maturity. That cost never shows up in the yield.
Capital gains treatment sits outside the formula too. A discounted bond that appreciates may generate long-term capital gains taxed at a lower rate than ordinary income. Bond funds that distribute a mix of interest and capital gains add another layer the formula wasn’t built to handle.
One boundary worth flagging. A separate concept called the break-even tax rate for Roth IRA conversions asks a different question: at what future tax rate does converting a traditional IRA to a Roth leave you with the same after-tax value as leaving it alone? The inputs and logic aren’t the same as the muni-versus-taxable calculation, and the formula above won’t answer it.