Quick answer: The highest advertised rate is not automatically the best place for short-term cash. A high-yield savings account offers bank-deposit liquidity but a variable APY. A CD can lock a rate but may charge an early-withdrawal penalty. A Treasury bill has different pricing and tax mechanics. A money market mutual fund is an investment fund whose yield generally follows short-term rates; it is not an FDIC-insured bank deposit. Compare after-tax dollars, access, exit friction, and the type of protection rather than ranking the four by one headline percentage.
| Choice | Rate / yield behavior | Tax note | Early-access trade-off | Protection / backing |
|---|---|---|---|---|
| High-yield savings | Variable APY | Interest generally subject to federal and state income tax | Usually liquid, subject to account terms and transfer timing | Eligible deposits can be FDIC-insured within limits |
| Bank CD | Usually fixed APY for the term | Interest generally subject to federal and state income tax | Early-withdrawal penalty may apply | Eligible bank CDs can be FDIC-insured within limits |
| U.S. Treasury bill | Yield depends on purchase price and maturity | Federal tax applies; state and local income tax generally do not | Selling before maturity can introduce market-price and transaction friction | Backed by the full faith and credit of the U.S. government; not FDIC insurance |
| Money market mutual fund | 7-day yield changes with portfolio income and short-term rates | Federal tax generally applies; state treatment depends on the fund and its holdings | Usually redeemable, but it is a security and can lose value | Not FDIC-insured; qualifying fund shares at a SIPC-member broker can be protected as securities if the broker fails, not against market loss |
Why the tax line can change the winner
Suppose a Treasury and a bank deposit advertise similar annualized yields. The Treasury may produce a higher after-tax result for a saver who owes state income tax because Treasury interest is exempt from state and local income taxes. The advantage disappears in a state with no individual income tax, and federal tax still applies.
Do not treat a quoted Treasury yield as identical to a bank APY. They are calculated differently. For a real decision, compare the expected dollars you would receive over the same holding period, after taxes and any exit costs.
CD vs. Treasury vs. money-market fund vs. high-yield savings calculator
Compare the same dollars over the same holding period
Use rates and yields actually available to you. APY, Treasury yield and money-market-fund 7-day yield are different measures; the calculator converts them into simplified dollar estimates rather than pretending they are identical quotations.
The calculator’s assumptions
- High-yield savings and CD inputs are treated as APYs for the simplified holding-period estimate.
- The Treasury input is treated as an annualized yield estimate, not a quoted-price or sale-before-maturity model.
- The money-market-fund input is a 7-day yield. The SEC requires money market funds to report 7-day yield information, but it is not the same measure as a bank APY.
- Bank-deposit interest is modeled using your entered federal plus state/local marginal rates.
- Treasury interest is modeled using your entered federal rate only because Treasury interest is generally exempt from state and local income taxes.
- Money-market-fund income is modeled using federal tax plus the state/local rate multiplied by your entered taxable-share assumption. Actual state treatment depends on fund holdings and state rules.
- The CD penalty is subtracted from modeled gross CD interest to show an early-exit earnings estimate before tax adjustments. Exact penalties can reduce principal and tax treatment can vary.
Liquidity matters more than a few extra basis points when the money has a job
Money for next month’s rent, an insurance deductible, or a near-term home closing has a different job from cash you are confident you will not need for a year. A slightly higher yield can be a bad trade if accessing the money creates a penalty, a forced sale, or operational delay at the wrong time.
For emergency reserves, start by deciding how much must be available immediately. Dollar Beacon’s emergency fund stress-test calculator helps estimate the reserve target. Then decide which portion, if any, can tolerate a maturity date or transfer process.
What happens if you need the money early?
| Product | Possible early-access issue | What to check before opening/buying |
|---|---|---|
| High-yield savings | Rate can change; transfer timing or account restrictions may matter | Withdrawal/transfer process, minimum balance, fees, rate conditions |
| CD | Early-withdrawal penalty can reduce interest and may reduce principal depending on terms | Exact penalty formula, maturity date, grace period, renewal terms |
| Treasury | Selling before maturity exposes you to market price and transaction/transfer mechanics | Where the security is held, ability to sell, settlement timing, maturity match |
| Money market mutual fund | Yield can change quickly; fund shares are securities and may lose value | 7-day yield, expense ratio, fund type/holdings, redemption terms, broker/SIPC status |
Safety and account protection are not the same across the four choices
Eligible savings accounts and bank CDs can be covered by FDIC insurance within the applicable ownership limits. U.S. Treasury securities are not FDIC-insured; they are obligations of the U.S. government. Money market mutual funds are also not FDIC-insured. If fund shares are held at a SIPC-member brokerage, qualifying shares are treated as securities for SIPC purposes if the brokerage fails, but SIPC does not protect you from a decline in the fund’s market value. A money market deposit account at a bank is a different product from a money market mutual fund.
If you are comparing more than $250,000 at one institution, do not assume every dollar is insured. Ownership categories and account titling affect coverage.
A better decision sequence
- Set the date: When is the earliest realistic time you may need the money?
- Protect the liquid layer: Keep near-term emergency cash where access is simple.
- Enter real rates: Use rates available to you today, not a national headline.
- Adjust for taxes: Include your state rate when comparing bank interest with Treasury interest.
- Price the exit: Read the CD penalty and understand how a Treasury would be sold before maturity.
- Then compare yield: Choose the structure that fits the cash’s job, not merely the highest advertised percentage.
FAQ
Are Treasury bills exempt from federal income tax? No. Treasury interest is subject to federal income tax. It is generally exempt from state and local income taxes.
Are CDs safer than Treasury bills? They use different protection structures. Eligible bank CDs can have FDIC insurance within coverage limits; Treasury securities are direct U.S. government obligations.
Can I lose money on a Treasury bill? If you hold a bill to maturity, the payment structure is known from the purchase terms. If you sell a marketable Treasury before maturity, the market price can differ from what you paid.
Is the highest APY usually best for an emergency fund? Not necessarily. Access, penalties, transfer timing, insurance coverage, tax treatment, and the stability of the rate can matter more than a small yield difference.
Is a money market fund the same as a money market account? No. A money market mutual fund is a security and is not FDIC-insured. A bank money market deposit account is a deposit product and can be FDIC-insured if eligibility requirements are met.
Primary sources and methodology
- TreasuryDirect: Tax Forms and Tax Withholding
- TreasuryDirect: Treasury Bills
- IRS Publication 550
- FDIC: Insured Deposit Products
- FDIC: Financial Products That Are Not Insured
- Investor.gov: Money Market Funds
- SIPC: What SIPC Protects
- CFPB: What Is a Certificate of Deposit?
Dollar Beacon provides general financial education. The calculator uses simplified tax and yield assumptions and does not model every institution’s compounding method, Treasury purchase price, brokerage spread, or individual tax rule. Verify actual product terms before moving cash.
Next reading
For long-term rate context, read 10-Year Treasury Near 5%: What Changes for Long-Term Investors?. For cash-reserve sizing, use the Emergency Fund Stress-Test Calculator.