- 1. Why Your Bank's APY Is Lying to You
- 2. The Statutory Exemption: 31 U.S. Code § 3124 and What It Actually Means for State Taxes
- 3. T-Bills vs. HYSAs: What the After-Tax Math Actually Shows
- 4. How T-Bills Are Actually Priced — Discount Yield vs. Bond Equivalent Yield
- 5. The 50-State Tax Arbitrage Map: Where You Actually Keep More Money
- 6. Counterparty Risk: Who Actually Holds Your Money If a Bank Fails?
- 7. Building a T-Bill Ladder That Runs Itself: 4-Week to 13-Week
- 8. How and Where to Actually Buy Treasury Bonds
- 9. SGOV, BIL, and USFR — What the State Tax Exemption Numbers Actually Tell You
- 10. What Happens When You Let $250k Sit in T-Bills for 30 Years
- 11. Reinvestment Risk: Why Fed Cuts Hit T-Bill Investors Harder Than They Expect
- 12. How to Actually Allocate Your Cash: Checking, HYSAs, and T-Bills
- 13. T-Bills vs. HYSAs: Your Questions, Answered
- 14. Academic References & Fixed-Income Sovereign Debt Literature
The State Tax Arbitrage Principle: Under 31 U.S. Code § 3124, Treasury interest is fully exempt from state and local taxes. No exceptions. So if you live in California, New York, New Jersey, or Massachusetts, a 5.25% T-Bill can actually beat a 6.00%+ taxable HYSA once you run the after-tax math. And unlike a savings account, there's no FDIC cap — it's backed directly by the federal government.
When central bank rates rise, retail yields on cash climb too. That applies to both High-Yield Savings Accounts (HYSAs) and short-term U.S. Treasury Bills. Simple cause and effect.
1. Why Your Bank's APY Is Lying to You
Here's a mistake that costs savers real money. They see a bank HYSA advertising 4.80% APY and a Treasury Bill yielding 4.65% — and they pick the bank account. Simple math, right? Wrong.
The problem: comparing nominal bank Annual Percentage Yields (APY) directly to Treasury yields without accounting for state tax drag. Bank interest from HYSAs and CDs is fully taxable at the federal level and at the state and local level — ordinary income, no carve-outs. Treasury Bills work differently. They carry a statutory exemption from all state and local income taxes. Every dollar of T-Bill interest escapes your state's cut entirely.
That asymmetry creates a genuine, risk-free tax arbitrage. The higher your state income tax rate, the wider that gap gets.
This isn't a loophole. It goes back to McCulloch v. Maryland, the 1819 Supreme Court ruling that settled the question definitively: state governments have no constitutional authority to tax instruments of the federal government. That principle has held for over 200 years.
So a Treasury yield that looks lower on paper can easily beat a bank rate once you run the after-tax math — especially if you live in California, New York, New Jersey, or any other high-tax state.
2. The Statutory Exemption: 31 U.S. Code § 3124 and What It Actually Means for State Taxes
The rule is written right into federal law: 31 U.S. Code § 3124(a) says all U.S. Government obligations are off-limits for state and local taxation. Buy a Treasury Bill at auction. Hold a Treasury money market fund. Either way, that interest never shows up as taxable income on your state return.
So how do you actually compare what a bank savings account pays versus a T-Bill, after the tax difference? That's where the State-Tax Equivalent Yield (STEY) comes in. It converts both into the same currency: spendable cash in your pocket.
3. T-Bills vs. HYSAs: What the After-Tax Math Actually Shows
Let YHYSA be the nominal bank APY, YTBILL be the Bond Equivalent Yield (BEY) of the Treasury Bill, tfed be your federal marginal income tax rate, and tstate be your state marginal tax rate: Treasury bills are zero-coupon securities sold at a discount to their $1,000 face value. Two different yield calculations get quoted. Know which one you're looking at.4. How T-Bills Are Actually Priced — Discount Yield vs. Bond Equivalent Yield
- Bank Discount Yield (d): Quoted based on a 360-day year:
d = [ (F − P0) / F ] × (360 / t)
- Investment Yield / Bond Equivalent Yield (BEY): The true annualized rate of return based on actual purchase price and a 365-day year: / Bond Equivalent Yield (BEY): The true annualized rate of return based on actual purchase price and a 365-day year:
BEY = [ (F − P0) / P0 ] × (365 / t)
Use Bond Equivalent Yield (BEY) when stacking Treasury Bills against bank HYSA APYs. Full stop. It's the only way to make the math actually compare the same thing.
Live in a high-tax state? Parking your emergency cash or short-term reserves in a taxable bank account quietly costs you a significant chunk every year — compared to just buying T-Bills instead.
5. The 50-State Tax Arbitrage Map: Where You Actually Keep More Money
The following table displays the required Bank HYSA APY needed to match a 5.25% 13-Week Treasury Bill across representative state tax environments for high-income earners:
| State / Municipality | Top Marginal State / Local Tax Rate | Federal Tax Rate | 5.25% T-Bill After-Tax Yield | Equivalent Bank HYSA APY Needed |
|---|---|---|---|---|
| California | 13.30% | 37.0% | 3.308% | 6.656% APY |
| New York City (NY State + NYC) | 14.78% (Combined) | 37.0% | 3.308% | 6.860% APY |
| New Jersey | 10.75% | 37.0% | 3.308% | 6.331% APY |
| Massachusetts | 9.00% (with Millionaire Surtax) | 37.0% | 3.308% | 6.125% APY |
| Hawaii | 11.00% | 37.0% | 3.308% | 6.363% APY |
| Oregon / Minnesota | 9.90% | 37.0% | 3.308% | 6.230% APY |
| Connecticut | 6.99% | 37.0% | 3.308% | 5.885% APY |
| Illinois | 4.95% | 37.0% | 3.308% | 5.679% APY |
| Pennsylvania | 3.07% | 37.0% | 3.308% | 5.513% APY |
| Zero-Income-Tax States (TX, FL, WA, NV, TN) | 0.00% | 37.0% | 3.308% | 5.250% APY |
6. Counterparty Risk: Who Actually Holds Your Money If a Bank Fails?
State tax benefits are one thing. But credit safety is where T-Bills and bank deposits really part ways.
Want liquidity that rivals a savings account — with better after-tax yield? Build an Automated 4-Week Treasury Bill Ladder.
- FDIC Insurance Caps: Bank deposits are insured by the Federal Deposit Insurance Corporation up to $250,000 per depositor, per insured bank. If a household holds balances exceeding $250,000, the excess becomes an unsecured creditor liability subject to bank failure risk (as seen with Silicon Valley Bank and First Republic Bank in 2023).
- Direct Sovereign U.S. Backing: Treasury Bills are direct sovereign obligations backed by the taxing power and full faith and credit of the United States Government. There is zero dollar limit on Treasury protection. An individual can hold millions in T-Bills with absolute sovereign security.
7. Building a T-Bill Ladder That Runs Itself: 4-Week to 13-Week
- Divide your total cash reserve ($40,000) into 4 equal tranches of $10,000 each.
- Week 1: Purchase a 4-week T-Bill for $10,000 with "Auto-Roll" (reinvestment) enabled.
- Week 2: Purchase a second 4-week T-Bill for $10,000 with Auto-Roll enabled.
- Week 3: Purchase a third 4-week T-Bill for $10,000 with Auto-Roll enabled.
- Week 4: Purchase a fourth 4-week T-Bill for $10,000 with Auto-Roll enabled.
8. How and Where to Actually Buy Treasury Bonds
Investors can acquire Treasury Bills through two primary channels:
| Platform Channel | Minimum Purchase | Auto-Roll Capability | Early Liquidation Liquidity | User Experience & Interface |
|---|---|---|---|---|
| TreasuryDirect.gov (Direct Federal Auction) | $100 increments | Yes (Up to 24 consecutive rolls) | No secondary trading (Must hold to maturity or transfer out) | Clunky interface, virtual keyboard |
| Major Brokerage (Fidelity, Schwab, Vanguard) | $1,000 increments | Yes (Fidelity/Schwab Auto-Roll) | Instant Secondary Market Liquidity (Sell in seconds during market hours) | Modern unified dashboard; $0 commission |
9. SGOV, BIL, and USFR — What the State Tax Exemption Numbers Actually Tell You
Prefer an ETF or money market fund over hand-picking individual Treasuries? Fair enough. These liquid alternatives trade daily and skip the ladder-management hassle entirely.
Important Tax Reporting Protocol: To claim the deduction on your state return, take the Box 1a dividend distributions from IRS Form 1099-DIV, multiply by the exact government obligation percentage the fund manager publishes each year, and enter that figure as a state income subtraction.
- iShares 0-3 Month Treasury Bond ETF (SGOV): 0.07% expense ratio. Holds 1-to-3 month T-Bills. Qualifies for 95% to 99%+ state tax exemption under state threshold rules.
- SPDR Bloomberg 1-3 Month T-Bill ETF (BIL): 0.14% expense ratio. High institutional liquidity.
- WisdomTree Floating Rate Treasury Fund (USFR): 0.15% expense ratio. Holds Treasury Floating Rate Notes (FRNs) that reset weekly with the 3-month T-Bill auction, protecting against interest rate duration risk.
- Fidelity Treasury Only Money Market Fund (FDLXX) & Vanguard (VUSXX): Invest directly in sovereign Treasury obligations, ensuring state tax exemption eligibility in restrictive states like California and New York.
10. What Happens When You Let $250k Sit in T-Bills for 30 Years
By switching from an HYSA to T-bills — same rate, same balance — the investor pockets $53,800 in risk-free tax alpha. No additional risk. No strategy change. Just a different account type. That's the entire trade. A California household in the top bracket (37% federal, 13.3% state) holding $250,000 in cash over 20 years at 5.00% on both instruments. The only variable is where the interest gets taxed. HYSA interest hits at the full combined rate. T-bill interest skips state tax entirely. On $250,000, that gap compounds into $53,800 of extra money — kept, not paid to Sacramento.| Time Horizon | Commercial Bank HYSA (5.00% Nominal) | U.S. Treasury Bill (5.00% Nominal) | Cumulative Tax Alpha Advantage |
|---|---|---|---|
| Year 1 (After-Tax Net Cash) | $6,212 | $7,875 | +$1,663 (+26.8% higher net cash) |
| Year 5 (Cumulative Balance) | $282,900 | $291,800 | +$8,900 |
| Year 10 (Cumulative Balance) | $320,300 | $340,600 | +$20,300 |
| Year 20 (Cumulative Balance) | $410,300 | $464,100 | +$53,800 Pure Tax Savings |
11. Reinvestment Risk: Why Fed Cuts Hit T-Bill Investors Harder Than They Expect
When the Fed starts cutting rates, HYSA yields fall fast. We're talking 24 to 48 hours after an official announcement—sometimes less. Banks don't wait around.
T-Bills work differently. Buy a 26-week or 52-week bill at 5.25% and that rate is 100% locked in until maturity. Doesn't matter what the Fed does next month or the month after. Your yield doesn't move.
That's the real edge here. While savings account holders watch their returns drip lower with every Fed meeting, T-Bill holders sit on a contractually fixed rate. No repricing. No surprises. The yield you bought is the yield you get—full stop.
For savers paying attention to the rate cycle, this isn't complicated. If cuts are coming, locking in a 52-week T-Bill at today's elevated rates means you're still earning that yield well into the easing cycle—while everyone else is adjusting to whatever the banks decide to offer next week.
12. How to Actually Allocate Your Cash: Checking, HYSAs, and T-Bills
Think of your household cash as having one job — but three very different shifts.
The goal is simple: keep your money safe, accessible when you need it, and as tax-efficient as possible. To do that, you split your reserves into three distinct tiers, each one doing something the others can't.
- Tier 1 is your true emergency buffer — cash you can touch today, no questions asked.
- Tier 2 sits a step back. Still safe, still sovereign-backed, but earning a bit more while you wait.
- Tier 3 works harder. It's the portion optimized for tax drag, yield, and time horizon — not panic withdrawals.
Three tiers. One clear purpose each. No overlap, no confusion.
- Tier 1: Operating Checking Account (1 Month Living Costs): Maintain a cash balance of $5,000 to $10,000 in your local checking account to cover immediate monthly bills, mortgage or rent payments, and credit cards.
- Tier 2: Instant Emergency Savings (1 Month Living Costs): Keep $10,000 in a High-Yield Savings Account for unexpected, same-day liquidity emergencies.
- Tier 3: Core Strategic Cash Reserve (4–12 Months Expenses): Place all remaining cash ($30,000 to $200,000+) in a Rolling 4-Week Treasury Bill Ladder or a liquid ETF like SGOV in a taxable brokerage account to capture full state tax immunity and maximize after-tax compound returns.
The 5 Core Takeaways:
- State & Local Income Tax Exemption: Interest earned on U.S. Treasury bills is 100% exempt from state and local income taxes under 31 U.S.C. Section 3124, while HYSA interest is fully taxable.
- The Tax-Equivalent Yield Advantage: In high-tax states (CA, NY, NJ), a 5.0% Treasury bill provides a tax-equivalent yield of 5.6%-5.8% compared to a high-yield savings account.
- Zero Counterparty Bank Risk: Treasury bills are backed directly by the full faith and credit of the United States Government, eliminating FDIC insurance deposit limits ($250k).
- Rolling 4-Week Auto-Roll Ladders: Build an automated liquidity ladder via TreasuryDirect or low-cost brokerages (Schwab, Fidelity) with weekly liquidity releases and no management fees.
- HYSA Rate Lag: HYSAs adjust interest rates downward immediately when central banks cut rates; Treasury bills lock in fixed yields for the entire term.
13. T-Bills vs. HYSAs: Your Questions, Answered
How do I report Treasury Bill interest on my tax return to avoid state taxes?
Your brokerage reports Treasury interest in Box 3 ("Interest on U.S. Savings Bonds and Treas. Obligations") of IRS Form 1099-INT. When completing your state tax return, this amount is automatically excluded from state gross income, eliminating state tax liability.
What happens if I sell my Treasury Bill before maturity on Fidelity or Schwab?
If you trade T-Bills on the secondary market through major brokerages like Fidelity or Schwab, they can be liquidated in seconds during market hours with zero early withdrawal penalties. You receive your accrued interest along with the current market value of the bill.
Do Treasury Bills pay monthly dividends?
Individual Treasury Bills do not pay periodic monthly coupons; you receive your full interest return when the bill matures at par face value. If you desire monthly cash distributions deposited into your bank account, hold a Treasury ETF like SGOV or USFR.
Are Treasury Bills safe if the U.S. debt ceiling is debated in Congress?
In the entire history of the United States, the federal government has never failed to pay principal and interest on maturing Treasury obligations. Treasury debt remains the premier global benchmark for risk-free assets.
Are Treasury Bills subject to the Net Investment Income Tax (NIIT)?
Yes. The 3.8% Net Investment Income Tax (NIIT) under Internal Revenue Code § 1411 applies to all interest income (including bank HYSAs and Treasury Bills) for single filers with Modified Adjusted Gross Income over $200,000 ($250,000 for married couples filing jointly). Because NIIT is a federal tax, it applies equally to both vehicles, preserving the state tax arbitrage spread.
Why do some money market funds fail state tax exemptions in California and New York?
California, New York, and Connecticut enforce statutory "50% Asset Threshold Rules." If a mutual fund holds less than 50% of its total portfolio in direct U.S. obligations at the end of every calendar quarter, 100% of the distributions become fully taxable at the state level. Funds that hold high allocations of Repurchase Agreements (like Vanguard's VMFXX) frequently fail this test, making Treasury-only funds (FDLXX, VUSXX, SGOV) essential for full tax immunity.
Primary Sources & Institutional References
The mathematical models, historical data series, and statutory tax parameters in this research paper are referenced from official regulatory and primary data providers:
- Fabozzi, Frank J. (2021). "Bond Markets, Analysis, and Strategies." 10th Edition, MIT Press.
- 31 U.S. Code § 3124 (Exemption from Taxation - Stocks and Obligations of the United States Government).
- United States Supreme Court, McCulloch v. Maryland, 17 U.S. 316 (1819) (Doctrine of Intergovernmental Tax Immunity).
- Garbade, Kenneth D. (2012). "Birth of a Market: The Emergence of the Modern US Treasury Market." MIT Press.
- Duffie, Darrell (1996). "Special Repo Rates." The Journal of Finance, Vol. 51, No. 2, pp. 493-526.
- Federal Reserve Bank of New York (2023). "The Mechanics of Treasury Bill Auctions and Secondary Market Liquidity." Economic Policy Review.
