CDs vs. Treasury Bills: Which Is Better for Safe Cash?

The tax exemption is the whole comparison

Interest from Treasury bills is exempt from state and local income tax, though still subject to federal tax. CD interest gets no such exemption — it is taxed as ordinary income at both the federal and state level, wherever you live. In a state with no income tax, that difference disappears and the higher headline rate simply wins. In a state that taxes income, the exemption can make a T-bill paying a lower stated rate the better deal after tax. Local income tax, where a city or county levies one on top of state tax, follows the same exemption — T-bill interest is free of both, while CD interest remains subject to whichever local taxes apply to ordinary income where you live.

The tool for making this comparison correctly is the tax-equivalent yield: divide the T-bill's yield by (1 minus your state marginal tax rate). That converts the T-bill's exemption into a CD rate you can compare directly, rather than comparing two headline numbers that are not measuring the same thing.

A worked example: comparing after tax, not by headline

Suppose a bank quotes 4.50% APY on a 12-month CD, and a 12-month T-bill is priced to yield 4.30%. On $10,000, the CD's interest is $450, using A = P(1 + r/n)^(nt) with daily compounding. The T-bill's interest is roughly $430 on the same deposit.

Now suppose your state taxes interest income at a 7% marginal rate. The CD's $450 becomes $418.50 after state tax. The T-bill's $430 is exempt from state tax entirely, so you keep the full $430 — $11.50 more than the CD, despite the T-bill's lower headline rate. Checking this with the tax-equivalent-yield formula confirms it: 4.30% divided by (1 − 0.07) is 4.62%, which is above the CD's 4.50% APY, meaning the T-bill is the better deal once the exemption is priced in. In a state with no income tax, none of this applies and the 4.50% CD is simply the higher yield.

The formula only runs in one direction: it translates a tax-exempt T-bill yield into the taxable rate it is equivalent to. Do not apply it to the CD side — a CD's APY is already the number to compare, once state tax has been subtracted from its dollar return.

Safety and the insurance ceiling

Both instruments are about as safe as cash gets, but the mechanism differs. T-bills carry the full faith and credit of the US government with no dollar limit on any single holding. CDs are protected by FDIC insurance up to $250,000 per depositor, per bank, per ownership category — ample for most savers, but a real constraint on a balance that exceeds it at one institution.

For a balance comfortably under the insured limit, this difference is close to irrelevant. For a balance well above it, T-bills avoid the need to split money across multiple banks or ownership categories just to stay insured, since there is no ceiling to work around in the first place.

Credit union share certificates carry the equivalent NCUA coverage at the same $250,000 threshold, so this comparison against T-bills applies identically whether the CD sits at a bank or a credit union.

Buying mechanics and account types

T-bills are bought either directly through TreasuryDirect, the Treasury's own platform, where individual investors typically submit a non-competitive bid and accept whatever yield the auction sets, or through a brokerage account, which may offer more flexibility for reinvestment. CDs are opened directly with a bank or credit union and require no separate brokerage relationship at all. For a saver who does not already have a brokerage or TreasuryDirect account, setting one up purely to buy T-bills is a real, if modest, extra step compared with using an existing bank relationship for a CD.

TreasuryDirect accounts are free to open and carry no minimum balance requirement beyond the smallest T-bill increment, which removes cost as a reason to prefer a brokerage over the direct government platform.

Reinvestment and what happens at maturity

A T-bill matures at face value with no compounding decision to make — the full amount becomes available in cash, and TreasuryDirect offers an optional automatic reinvestment into a new bill of the same term if it is set up in advance. A CD's maturity works through a grace period, commonly seven to ten days, after which an un-instructed CD typically auto-renews into a new CD at the bank's current rate. Both processes require action from you to actually change term or issuer; passivity defaults differently in each case, which is worth knowing for money you are not actively managing.

Missing the reinvestment window on either side has a real cost. An un-rolled T-bill simply sits as idle cash earning nothing until you act, while an auto-renewed CD locks into whatever rate the bank currently offers, which is frequently below the rate that first attracted you to it.

Liquidity and how each is bought and sold

• T-bills can be sold on any day the market is open, at whatever the current market price is — a market outcome, not a fixed fee.
• CDs charge an early withdrawal penalty for exiting before maturity — a known, contractual cost rather than one set by the market.
• T-bills are bought at auction through TreasuryDirect or through a brokerage account.
• CDs are opened directly at a bank or credit union, typically in minutes, with no brokerage account required.
• T-bills are sold at a discount to face value and mature at par, rather than paying a stated periodic interest rate the way a CD does.

Where each one wins

In a high-tax state, with a balance large enough that the insurance ceiling matters, and a comfort level with using a brokerage or TreasuryDirect account, T-bills usually come out ahead once the comparison is run after tax. In a state with no income tax, for a saver who wants the simplicity of walking into a bank and opening an account in minutes, and with a balance under the insured limit, a competitive CD is frequently the higher after-tax yield for less effort.

Many savers do not need to pick one exclusively. Holding both lets a portion benefit from a CD's simplicity and guaranteed rate while another portion benefits from a T-bill's tax treatment and its complete absence of an insurance ceiling.

What people get wrong

The most common mistake is comparing the two headline rates directly, as if a 4.50% CD simply beats a 4.30% T-bill. That comparison ignores the tax treatment entirely, and in a taxing state it can point to the wrong answer. The second most common mistake is assuming T-bills are illiquid because they carry a maturity date; unlike a CD, a T-bill can be sold on any trading day, so the maturity date is a ceiling on the term, not a lock on your access to the money.

• Comparing a CD's raw nominal rate, rather than its APY, against a T-bill's quoted yield, when only the APY reflects what the CD actually pays over the term.
• Assuming a T-bill requires a brokerage account; TreasuryDirect lets you buy directly from the government with no broker involved at all.
• Forgetting that CD interest is taxed in the year it is credited even on a CD you cannot yet access, the same accrual-basis principle that applies to most interest income generally.