High-Yield Savings vs. Treasury Bills (T-Bills): State Tax Optimization & Liquidity Guide
When optimizing short-term emergency cash and liquidity reserves, many depositors default to High-Yield Savings Accounts (HYSA) without considering the tax drag of state and local income taxes. In high-tax jurisdictions (such as California, New York, or New Jersey), U.S. Treasury Bills (T-Bills) provide substantial after-tax yield advantages due to federal statutory tax exemptions.
1. State Tax Exemption Under Title 31 U.S. Code § 3124
Under federal statute 31 U.S.C. § 3124, interest and discount earned on United States Treasury obligations are 100% exempt from all state and local income taxes. By contrast, HYSA bank interest is taxed as ordinary income at both federal and state marginal rates.
# After-Tax Yield Formula for HYSA
Net HYSA Yield = Nominal APY × (1 - (Federal Tax Rate + State Tax Rate))
# After-Tax Yield Formula for T-Bills
Net T-Bill Yield = Nominal Yield × (1 - Federal Tax Rate)
# Example (California Resident - 24% Fed + 9.3% State = 33.3% Combined)
HYSA @ 4.85% APY: 4.85% × (1 - 0.333) = 3.23% Net
T-Bill @ 5.30% Yield: 5.30% × (1 - 0.24) = 4.03% Net (80 bps higher net cash return!)2. Liquidity & Duration Matching Strategy
To balance instant emergency liquidity with maximum tax-exempt yields, allocate cash reserves across a tiered structure: keep 1 month of living expenses in an HYSA for immediate debit card access, and place the remaining 3–6 month emergency fund in a rolling 4-week or 8-week T-Bill ladder.