Treasury Bills: The Safest Place to Park Cash You'll Need Soon
Cash sitting in a low-interest checking account quietly loses purchasing power every year to inflation, yet many investors leave meaningful sums there simply because a Treasury bill sounds more complicated than it is. It isn't: it pays no coupon, is sold below its face value, and repays that face value at maturity, and understanding exactly how that discount produces a yield is the only real prerequisite to using it well.
The core principle
A Treasury bill, or T-bill, is a short-term debt security issued by the U.S. Treasury with a maturity of one year or less, commonly issued in 4, 8, 13, 26, and 52-week terms. Unlike a standard bond, a T-bill pays no periodic coupon. Instead, it is sold at a discount to its face value, and the holder receives the full face value at maturity; the gap between the discounted purchase price and the face value repaid is the entire return. Because the U.S. government has never defaulted on Treasury debt, T-bills are widely treated as the closest practical approximation to a risk-free asset denominated in dollars, and their short maturities mean they carry very little interest rate risk even if sold before maturity, unlike longer Treasury securities.
The quirk that trips up many first-time buyers is that T-bills are typically quoted two different ways, and the two numbers are not the same. The discount yield is calculated as (face value - price) / face value x (360 / days to maturity), a convention dating back to an era before financial calculators, using a 360-day year and the face value, not the actual amount invested, as the denominator. The investment yield, sometimes called the bond-equivalent yield, is calculated as (face value - price) / price x (365 / days to maturity), using the actual purchase price as the denominator and a real 365-day year, which is the more accurate and more comparable figure when weighing a T-bill against a bank CD or savings account quoted as an annual percentage yield.
T-bill interest is exempt from state and local income tax, though still subject to federal income tax, a genuine, often overlooked edge over an equivalent-yielding bank CD or savings account for anyone living in a state with meaningful income tax.
T-bills are auctioned on a regular weekly or monthly schedule depending on maturity, and can be purchased directly at auction through TreasuryDirect.gov with no transaction fee, or through most brokerage accounts, which typically also charge no commission for Treasury purchases. A secondary market also exists, allowing an investor to buy a bill originally auctioned to someone else, or sell a bill before its own maturity, though a sale before maturity means accepting whatever price the market is currently offering rather than the guaranteed face value that comes from holding to maturity. Because T-bills carry virtually no credit risk and only minimal price sensitivity given their short maturities, they function as one of the closest available substitutes to holding literal cash while still earning a competitive, market-based yield, which is why they anchor so much of how short-term interest rates are discussed and quoted throughout the rest of the financial system.
How the math works
Example 1: discount pricing and the gap between the two yield conventions. Suppose you buy a 26-week (182-day) T-bill with a $10,000 face value for $9,800, a $200 discount. The discount yield is ($200 / $10,000) x (360 / 182) = 2.0% x 1.978 ≈ 3.96%. The investment, or bond-equivalent, yield is ($200 / $9,800) x (365 / 182) = 2.041% x 2.0055 ≈ 4.09%. The two figures describe the identical $200 gain on the identical $9,800 investment over the identical 182 days, yet they differ by about 13 basis points purely from the calculation convention, which is why the investment yield, not the discount yield, is the number to compare against a bank's advertised APY.
Example 2: the state tax exemption, quantified. An investor living in a state with a 9.3% marginal state income tax rate is choosing between a $50,000 T-bill and an equally safe $50,000 bank CD, both yielding 5.0% and both fully taxable at the federal level. Interest earned on either: $50,000 x 5.0% = $2,500. The CD's interest is taxed at both the federal and state level; the T-bill's interest is exempt from state tax entirely. The state tax the CD holder owes that the T-bill holder avoids: $2,500 x 9.3% = $232.50. Even with identical headline yields, the T-bill holder in this example keeps $232.50 more after taxes, purely from the state exemption, an advantage that scales directly with both the account size and the investor's state tax bracket.
How it shows up in real portfolios
An emergency fund is a natural home for T-bills purchased through a brokerage account, particularly a short ladder of 4, 8, and 13-week bills that rolls over continuously, offering a government-backed yield typically competitive with, and sometimes better than, a bank's advertised high-yield savings rate, in exchange for slightly less instant liquidity than a savings account provides.
A high-earning professional in a high-tax state like California or New York gets a meaningfully larger after-tax edge from the state tax exemption than an investor in a state with no income tax, exactly as in Example 2, which is one of the more reliable, low-effort tax optimizations available to anyone holding meaningful cash reserves in a taxable account.
A saver with a known near-term goal, such as a house down payment due in eight months, can buy a T-bill maturing on or just before the exact date the funds are needed, locking in a known yield with no risk of a bank promotional rate expiring or a CD's early withdrawal penalty being triggered by an unexpectedly earlier need for the cash.
Small business owners and self-employed professionals setting aside cash for a known future tax payment, such as quarterly estimated taxes, sometimes use short-dated T-bills timed to mature just before the payment is due, capturing a competitive yield on money that would otherwise sit idle in a checking account for months at a stretch, while accepting the minor tradeoff that the funds are not instantly accessible the way a checking or savings balance would be if an emergency required the money sooner than planned.
Retirees drawing from a portfolio in fixed, predictable installments sometimes hold a rolling ladder of T-bills specifically to cover the next one to two years of planned withdrawals, a structure that lets the rest of the portfolio remain invested for growth without forcing a sale of stocks or longer bonds during a downturn simply to generate near-term cash, since the near-term spending need is already funded separately by maturities timed to arrive when the money is actually needed.
Actionable breakdown
- How to buy T-bills:
- Directly through TreasuryDirect.gov, with no fees.
- Through a brokerage account, often also fee-free.
- On the secondary market for a specific maturity date.
- Before comparing to a CD or savings rate:
- Convert the discount yield to the investment yield.
- Add back the value of the state tax exemption.
- Check the exact maturity date against your actual need.
- Consider laddering several maturities for rolling liquidity.
Common pitfalls
- Comparing a T-bill's discount yield directly to a bank's advertised APY without converting to the investment yield first, which understates the T-bill's true return by roughly 10 to 15 basis points in typical conditions.
- Forgetting the state and local tax exemption when comparing T-bills to an equally safe bank CD, understating the T-bill's real after-tax advantage, especially for residents of high-tax states.
- Assuming a T-bill is completely risk-free in every sense, when selling before maturity on the secondary market still exposes the holder to some price fluctuation if rates have moved since purchase.
- Locking cash into a T-bill maturity that does not match an actual anticipated need, creating friction, and possibly a small loss, if the money is needed unexpectedly early.
Related concepts
T-bills anchor the risk-free rate used to evaluate every other investment, and they belong alongside cash equivalents like a money market fund or a certificate of deposit. For longer-dated government debt, see Treasury bond, and for an inflation-protected alternative, see TIPS and I bond. For a broader framework, see the guide on cash and emergency funds.
The bottom line
For safe, short-term cash, a T-bill offers a government-backed yield that, once you convert to the right yield figure and account for the state tax exemption, often beats a comparable bank CD.