CD vs Treasury Bill After-Tax Yield Comparison

Comparing a certificate of deposit (CD) to a Treasury bill often looks like comparing apples to apples: both are short-term, safe, income-bearing instruments issued by trusted entities. But a critical tax difference separates them. Treasury bill interest is exempt from state and local income taxes, while CD interest is not. This tax differential can swing the after-tax return in favor of T-bills, especially for investors in high-tax states and high tax brackets. Understanding how to calculate and compare after-tax yields is essential.

The Tax Difference: Why It Matters

The tax treatment of Treasury bills is unique. Interest earned on T-bills is subject to federal income tax but exempt from state and local income tax. This is a constitutional feature: interest on federal debt instruments cannot be taxed by states.

A certificate of deposit is issued by a bank, not the federal government. CD interest is fully subject to federal, state, and local income taxes—the full weight of the tax code.

This difference is not trivial. A high-income investor in California (state rate 13.3%), New York (8.82%), or Illinois (4.95% plus some local levies) can owe 20–23% combined state and local tax on CD interest. Meanwhile, T-bill interest faces only federal tax (24% to 37% depending on bracket). The state tax exemption is worth 5–15 percentage points of return, depending on where you live.

Calculating After-Tax Yield

To compare, you must compute the after-tax yield for each instrument.

For a Treasury Bill:

After-tax yield = Gross yield × (1 − federal tax rate)

Example: You buy a 6-month T-bill yielding 4.8% gross. Your federal tax bracket is 24%.

After-tax yield = 4.8% × (1 − 0.24) = 4.8% × 0.76 = 3.65%

For a Certificate of Deposit:

After-tax yield = Gross yield × (1 − (federal rate + state rate + local rate))

Example: You buy a 6-month CD yielding 5.0% gross. Your federal rate is 24%, state rate is 10%, and local rate is 1%.

Combined tax rate = 24% + 10% + 1% = 35%

After-tax yield = 5.0% × (1 − 0.35) = 5.0% × 0.65 = 3.25%

Comparison:

T-bill after-tax yield: 3.65% CD after-tax yield: 3.25% T-bill advantage: 0.40%

Even though the CD’s gross yield was 20 basis points higher (5.0% vs. 4.8%), the T-bill wins on an after-tax basis because of the state tax exemption.

Varying Scenarios by Tax Bracket and State

The advantage of T-bills widens as tax rates climb and as state taxes increase.

Low Tax Bracket Scenario (12% federal, 0% state, 0% local)

T-bill gross yield: 4.8% T-bill after-tax: 4.8% × (1 − 0.12) = 4.22%

CD gross yield: 5.0% CD after-tax: 5.0% × (1 − 0.12) = 4.40%

Winner: CD by 18 basis points. State tax exemption does not matter if there is no state tax.

High Tax Bracket Scenario (35% federal, 10% state, 1% local)

T-bill gross yield: 4.8% T-bill after-tax: 4.8% × (1 − 0.35) = 3.12%

CD gross yield: 5.0% CD after-tax: 5.0% × (1 − 0.46) = 2.70%

Winner: T-bill by 42 basis points. The state and local tax exemption is now worth a lot.

Middle-Income, High-Tax-State Scenario (24% federal, 8% state, 0% local)

T-bill gross yield: 4.8% T-bill after-tax: 4.8% × (1 − 0.24) = 3.65%

CD gross yield: 5.0% CD after-tax: 5.0% × (1 − 0.32) = 3.40%

Winner: T-bill by 25 basis points. T-bills pull ahead once you add meaningful state tax.

The Tax-Equivalent Yield Approach

A common shorthand is the “tax-equivalent yield”—what pre-tax yield a CD would need to match the after-tax yield of a T-bill.

Formula:

Tax-equivalent CD yield = T-bill yield ÷ (1 − combined tax rate)

Example: A T-bill yields 4.8%. Your combined federal, state, and local rate is 32%.

Tax-equivalent CD yield = 4.8% ÷ (1 − 0.32) = 4.8% ÷ 0.68 = 7.06%

A CD would need to yield 7.06% to match the T-bill’s after-tax return. If CDs are offering 5.0%, T-bills are the better deal.

Duration, Liquidity, and Other Factors

The after-tax comparison is necessary but not sufficient. Other factors matter:

Duration mismatch

T-bills are issued in terms up to 52 weeks. CDs come in various terms (3 months to 5 years, sometimes longer). If you need 2-year safety and yield, a 2-year CD might be your only option; a T-bill ladder would require buying and rolling over multiple bills. The convenience of a single CD can be worth a small yield penalty.

Liquidity and penalties

Treasury bills have a deep, liquid secondary market. You can sell a T-bill before maturity and recover nearly full value. CDs often carry early withdrawal penalties—typically a loss of 6 to 12 months of interest. If you might need the money early, a T-bill’s liquidity is valuable.

Conversely, if you are confident you will hold to maturity, the CD penalty does not apply.

FDIC insurance

CDs are bank deposits backed by FDIC insurance up to $250,000. Treasury bills are backed by the full faith and credit of the U.S. government (arguably safer, but you cannot claim FDIC insurance proceeds as a bank failure loss in tax terms). For very large sums beyond FDIC limits, T-bills might be preferred for safety.

Reinvestment risk

Both T-bills and CDs mature and must be reinvested. If rates fall, your next T-bill or CD will earn less. A longer-term CD locks in a rate for years; a T-bill must be rolled over. Neither is obviously safer.

A Practical Comparison Framework

To decide between CDs and T-bills:

1. Calculate your marginal tax rate. Include federal, state, and local income tax.
2. Look up current T-bill and CD yields for the same maturity.
3. Compute after-tax yields using the formulas above.
4. Adjust for liquidity and convenience. If you value the simplicity of a 2-year CD and the FDIC insurance, you might accept a 20–30 basis point yield penalty vs. T-bills.
5. Consider the environment. If rates are expected to fall, lock in a longer CD. If you expect rates to rise, keep maturity short and consider a T-bill ladder.

The Bottom Line for High-Income Investors

For investors in the 32%+ combined tax bracket (especially those in high-tax states), T-bills often win on an after-tax basis, even if CDs advertise a slightly higher gross yield. The state tax exemption is a real economic benefit, compounded over multiple years.

For lower-income investors in low-tax states, the tax difference narrows, and CDs may be competitive or even superior if they offer better liquidity or simplicity.

See also