Two accounts, two headline rates, and a decision that looks simple on the surface: a high-yield savings account (HYSA) advertising 4.50% APY, or a Treasury Bill quoted at 4.80%. The T-Bill wins, right? Not necessarily — and definitely not by the margin that comparison suggests once taxes enter the picture. These two instruments aren't taxed the same way, and for a lot of people, that difference completely flips which one actually puts more money in their pocket.

This isn't a small technicality that only matters to accountants. It's the difference between correctly evaluating where to park tens of thousands of dollars in cash and simply guessing based on whichever bank's marketing email had the bigger number in the subject line. The good news is the actual math takes about thirty seconds once you know which two numbers to compare — and the interactive calculator below does exactly that with your own real rates and tax situation, not a generic example.

The Tax Rule Almost Nobody Accounts For

Interest from a high-yield savings account is taxed as ordinary income at both the federal and state level. Interest from a Treasury Bill is taxed at the federal level only — it's fully exempt from state and local income tax. That single rule is the entire reason this comparison isn't as simple as reading two percentages off a screen. If you live in a state with no income tax, this exemption is worthless to you. If you live in a high-tax state, it can be worth more than a full percentage point of real, after-tax return.

🧮 See Your Own After-Tax Numbers

The Surprising Part: A Lower Rate Can Still Win

Run the calculator above at its default numbers — a 4.80% T-Bill against a 5.00% HYSA, both at a 24% federal bracket. At 0% state tax, the HYSA edges ahead, 3.80% to 3.648%, exactly as the headline rates suggest it should. But move the state tax slider up to just 3%, and the T-Bill takes the lead despite its lower advertised rate. By the time you reach California's 9.3% top marginal rate, the T-Bill is ahead by nearly a third of a percentage point — a real, meaningful gap that the two headline numbers never hinted at.

State-by-State: Where the Crossover Happens

Using the same example rates (4.80% T-Bill, 5.00% HYSA, 24% federal bracket), here's exactly where the winner flips as state tax rises:

The pattern is consistent: the higher your state's income tax, the more the balance tips toward T-Bills — regardless of which one has the higher sticker rate. Anyone in a no-tax state (Texas, Florida, Washington, Nevada, and a handful of others) never sees this advantage, since there's no state tax being exempted from in the first place.

What This Actually Means in Real Dollars

Percentages can feel abstract, so here's the same comparison in cash. Say a California resident (9.3% top state rate, 24% federal bracket) has $50,000 to park for a year. At the same example rates — 4.80% T-Bill, 5.00% HYSA — the T-Bill earns $1,824 after tax, while the HYSA earns $1,667.50 after tax. That's a $156.50 gap in favor of the option with the lower advertised rate, purely because of how the two are taxed. Scale that up to $200,000 in savings, and the same gap becomes over $600 a year — real money left on the table by anyone comparing headline rates alone.

It's Not Just About Tax — Three Other Real Differences

• Liquidity. A HYSA can be accessed anytime with no penalty — transfer the money out today if you need it. A T-Bill locks your cash until maturity (anywhere from 4 to 52 weeks), though it can technically be sold early on the secondary market if you need the cash sooner, potentially at a small gain or loss depending on where interest rates have moved since you bought it.
• Rate stability. A T-Bill's rate is locked in the moment you buy it and won't change for the life of the bill. A HYSA's APY is variable and can drop at any time — often within days of a Federal Reserve rate cut — meaning a 5.00% HYSA today could easily be paying 4.25% or lower six months from now, while a T-Bill purchased today keeps its rate regardless of what the Fed does next.
• How they're backed. HYSAs are FDIC-insured up to $250,000 per depositor, per bank — a meaningful protection, but one with a dollar ceiling. T-Bills aren't FDIC-insured because they don't need separate insurance at all: they're backed directly by the full faith and credit of the US government, the same guarantee behind every other US Treasury security.

None of these differences make one option universally "better" than the other — they simply mean the after-tax rate isn't the only factor worth weighing, even once you've done the tax math correctly. Someone who might need the cash within a few weeks has a real reason to prefer a HYSA's flexibility even if the after-tax T-Bill rate is slightly higher.

How to Actually Buy Each One

A HYSA is about as simple as personal finance gets — open an account online with a bank offering a competitive rate, transfer in cash, and start earning interest immediately with no minimum holding period. A T-Bill takes one extra step: you can buy directly from the source at TreasuryDirect.gov with no broker fees and a minimum purchase of just $100, choosing from the standard 4, 8, 13, 17, 26, or 52-week maturities at regular auctions. Many brokerages also let you buy T-Bills directly inside a regular investment account, sometimes with a small markup compared to buying straight from TreasuryDirect.

The Mistake This Whole Comparison Corrects

The single most common error is comparing the two headline rates directly, as if a dollar of HYSA interest and a dollar of T-Bill interest cost the same in taxes. They don't, and the gap isn't trivial for anyone in a state with meaningful income tax. Before assuming the higher advertised rate wins, it's worth running the actual purchase price and yield through a T-Bill calculator and comparing that after-tax figure against your HYSA's after-tax rate directly, using your own real tax brackets rather than a generic example.

If you're specifically deciding between a fixed T-Bill term and rolling it forward, it's also worth checking how the same rate compares across every standard maturity — sometimes a shorter or longer tenor changes the picture more than people expect. And if you want to see the exact dollar amount a specific T-Bill purchase would earn you before committing any cash, the full T-Bill price and yield calculator handles that instantly.

Why This Gap Matters More When Rates Are High

The after-tax advantage of a T-Bill scales directly with the nominal rate itself. When both T-Bills and HYSAs were paying under 1% during the near-zero rate years, the dollar difference between the two after tax was too small to bother calculating — a few dollars a year on a typical balance. But in a higher-rate environment, where both are paying 4-5%+, that same percentage-point gap in after-tax terms turns into hundreds of real dollars on a meaningful cash balance, exactly as the $50,000 example above shows. The higher the rate environment, the more this comparison is actually worth doing carefully instead of just picking whichever number looks bigger.

A Quick Decision Checklist

Before choosing between the two, it helps to run through a short list rather than just comparing two percentages:

• Do you live in a state with income tax? If not, skip straight to comparing the two headline rates — the T-Bill's main advantage doesn't apply to you.
• Will you need this cash before the T-Bill matures? If there's a real chance you'll need it in the next few weeks, the HYSA's flexibility may be worth more than a small after-tax rate advantage.
• Is the nominal rate gap large or small? A HYSA paying noticeably more than a T-Bill (over a point or two) can still win even after the tax adjustment, especially in lower-tax states — run your actual numbers rather than assuming.
• Do you want a locked-in rate, or are you comfortable with a variable one? If you expect rates to fall, locking in a T-Bill rate now protects you from a HYSA's APY dropping later.

Frequently Asked Questions

Are T-Bills really tax-free?
No — T-Bill interest is subject to federal income tax. It's only exempt from state and local income tax, which is the specific advantage that makes T-Bills more attractive than a comparable HYSA for residents of high-tax states.

Is a high-yield savings account taxed differently than a T-Bill?
Yes. HYSA interest is taxed as ordinary income at both the federal and state level, while T-Bill interest is only taxed federally — that gap is the entire source of the after-tax difference between the two.

Do I pay state tax on T-Bills if I live in a state with no income tax?
If you live in a state with no income tax, such as Texas, Florida, or Washington, the state tax exemption on T-Bills provides no extra benefit over a HYSA, since neither is subject to state tax in the first place.

Which is more liquid, a T-Bill or a HYSA?
A HYSA is more liquid — you can withdraw at any time with no penalty. A T-Bill locks your money until maturity, though it can be sold early on the secondary market at a potential gain or loss.

Are T-Bills FDIC insured like a savings account?
T-Bills aren't FDIC insured because they don't need to be — they're backed directly by the full faith and credit of the US government, generally considered at least as safe as FDIC insurance up to its coverage limit.

The Bottom Line

Whichever headline rate looks bigger isn't necessarily the one that pays more. For residents of no-tax states, the comparison really is close to a straight read of the two rates. For anyone in a high-tax state, the state tax exemption on T-Bills can turn a "lower" rate into the actually higher after-tax return — sometimes by a wide enough margin to change the decision entirely. The only way to know for sure is to run your own numbers, not someone else's example.