Taxable vs Tax-Exempt Money Market Funds

A tax-exempt money market fund holds short-term municipal debt and distributes interest free from federal (and sometimes state and local) income tax. A taxable money market fund holds Treasury bills, corporate paper, and other taxable instruments. The choice between them hinges on a simple calculation: whether the higher nominal yield of a taxable fund, after taxes, exceeds the lower (but tax-free) yield of a municipal fund.

The Yield Comparison

The first step is to compare the published yields. A taxable money market fund might yield 4.5% while a tax-exempt municipal fund yields 3.0%. On the surface, taxable wins. But taxes are due on the taxable fund’s distribution each year (assuming the fund is held in a taxable account), while the municipal fund’s distribution is tax-free.

The question becomes: what is the after-tax yield of the taxable fund? If you are in the 35% federal tax bracket, your after-tax yield on the 4.5% taxable fund is approximately:

4.5% × (1 − 0.35) = 4.5% × 0.65 = 2.93%

In this case, the tax-exempt fund at 3.0% beats the after-tax taxable yield of 2.93%. You should buy the tax-exempt fund.

Conversely, if you are in the 22% federal bracket:

4.5% × (1 − 0.22) = 4.5% × 0.78 = 3.51%

The after-tax taxable yield of 3.51% beats the tax-exempt yield of 3.0%. You should buy the taxable fund.

The Break-Even Calculation

The break-even tax rate is the bracket at which both funds deliver the same after-tax yield. Rearranging the formula:

Break-even rate = (Taxable yield − Tax-exempt yield) / Taxable yield

If taxable yields 4.5% and tax-exempt yields 3.0%:

Break-even = (4.5% − 3.0%) / 4.5% = 1.5% / 4.5% = 33.3%

An investor in the 33.3% bracket is indifferent between the two. Those in higher brackets should buy tax-exempt; those in lower brackets should buy taxable.

The federal tax brackets as of 2024 are 10%, 12%, 22%, 24%, 32%, 35%, and 37%. This means the 32% bracket is close to break-even for many spreads. Most investors in the 35% and 37% top brackets strongly prefer tax-exempt. Investors in the 22% and 24% brackets should favor taxable.

State and Local Taxes Push the Scales

If the tax-exempt fund is issued by a municipality in your home state, the interest is often free from state and local income tax as well as federal tax. This dramatically improves the tax-exempt fund’s after-tax return.

A resident of California (top state rate ~13.3%) or New York (top rate ~6.85%) should include state and local taxes in the break-even calculation:

Effective rate = Federal rate + State rate (on taxable gains)

For a California resident in the 37% federal bracket plus 13.3% state bracket, the combined rate is effectively about 48% (accounting for the deductibility of state taxes). The after-tax yield of a 4.5% taxable fund becomes:

4.5% × (1 − 0.48) = 2.34%

Against a 3.0% tax-exempt municipal fund (in-state issued), the tax-exempt fund is clearly superior. The break-even bracket drops dramatically.

Conversely, a Texas resident (no state income tax) in the 37% federal bracket has an effective rate of just 37%, and the after-tax taxable yield is:

4.5% × (1 − 0.37) = 2.84%

Still below a 3.0% tax-exempt yield, but the margin is tighter.

The key: buy in-state tax-exempt funds if you live in a high-tax state and are in a high-income bracket.

Out-of-state tax-exempt municipal funds offer only federal tax exemption, losing the state and local benefit. They compete solely on the spread between taxable and tax-exempt yields; the choice is less attractive for out-of-state issuers.

Credit Risk and Safety

Both taxable and tax-exempt money market funds are considered extremely safe. They hold instruments with maturities of 1 year or less and focus on creditworthy issuers.

Taxable money market funds typically own Treasury bills (backed by the U.S. government), short-term corporate debt (commercial paper) from large corporations, and repurchase agreements. The default risk is negligible.

Tax-exempt municipal money market funds own short-term debt issued by states, cities, and special districts. Municipal default rates are historically very low—far lower than corporate defaults. Major disruptions (like the 2008 financial crisis) have caused municipal credit spreads to widen, but actual defaults on short-term municipal paper remain rare.

The SEC regulates both fund types identically under the Investment Company Act of 1940, requiring high-quality holdings and strict liquidity standards. The risk profile is essentially equivalent, so the choice should be driven by the after-tax yield comparison, not safety concerns.

Volatility and Interest Rate Risk

Money market funds are designed to maintain a stable $1 share price, unlike longer-term bond funds. Both taxable and tax-exempt versions do this by holding very short maturities (average maturity of 30–60 days) and purchasing new securities frequently.

In a rising-rate environment, both fund types experience the same pressure: yields on new purchases rise, so the distributions increase. In a falling-rate environment, yields decline. But the share price should stay stable in both cases because the fund is constantly rolling over maturing securities.

The one practical difference: in a financial crisis, taxable money market funds (holding corporate commercial paper) can face liquidity stress if companies cannot refinance short-term debt. Tax-exempt funds (holding safer municipal paper and often more government-backed securities) have proven more resilient. However, this is a rare edge case; regular market conditions offer no meaningful risk difference.

Tax-Advantaged and Tax-Deferred Accounts

An important caveat: the tax comparison is irrelevant for 401(k) plans, traditional IRAs, and other tax-deferred accounts. In these accounts, neither taxable nor tax-exempt distributions are taxed until (or unless) money is withdrawn. The after-tax yield calculation disappears.

In a tax-deferred account, you should simply buy the taxable money market fund with the higher nominal yield. The tax exemption of municipal funds provides no benefit inside a 401(k) or IRA.

Similarly, a Roth IRA allows tax-free withdrawals but offers no tax break on the contribution. Municipal money market funds do not provide a benefit here either (since the funds themselves are tax-exempt at the source, not just at withdrawal). Taxable funds are preferable for higher yield.

Practical Selection

Here’s a simplified decision tree:

• In a taxable brokerage account, top marginal bracket (35–37%), high-tax state? Buy in-state tax-exempt municipal money market funds.
• In a taxable account, middle bracket (22–24%)? Compare the after-tax yields and choose accordingly; often tilts to taxable.
• In a tax-deferred account (401(k), traditional IRA)? Buy taxable money market funds for higher yield.
• In a Roth IRA? Buy taxable money market funds.
• Out-of-state issuer or low-tax state? Taxable funds are usually preferable unless you are in the very top bracket.

See also