Put $10,000 into two hypothetical three-month choices and the annual percentages alone will not tell you which leaves more cash. A savings APY and a bill’s quoted yield can use different conventions; tax, costs and the date money becomes spendable can reverse a small gross-interest difference. Compare the dollars over the same 91 days.
A useful comparison starts with the same amount and the same period, then calculates what remains after the relevant costs and taxes. This article uses U.S. products and rules. Every price, rate and tax rate in the worked example is hypothetical; none is a current offer or recommendation.
Put both choices on one calendar
Write down the date money leaves your spendable account, the date the comparison ends and the date you actually need the cash. If a purchase or transfer introduces a gap, record it instead of assuming that every dollar earns interest continuously.
For an individual bill, distinguish its remaining term from the term it originally had when issued. The price you pay and the remaining days matter to your return. For savings, check when deposited funds begin earning interest and when interest is credited.
The CFPB’s account-disclosure rules cover information such as rate changes, compounding, fees, minimum balances and transaction limitations. Read the actual account disclosures. “High yield” does not supply those details.
Convert an APY into a period estimate
The CFPB defines APY as a rate reflecting interest and compounding over a 365-day period. An annual number is not the percentage earned in ninety-one days.
For a simplified comparison, assume $10,000 remains invested for 91 days at an unchanged 4% APY, with no fees or withdrawals. An effective-rate estimate is:
Interest = $10,000 × ((1.04)^(91/365) − 1) ≈ $98.26.
This is a mathematical illustration, not an account-statement prediction. Actual results depend on the institution’s interest calculation, crediting rules and rate path. In a variable-rate account, today’s APY does not promise the same rate throughout the period.
Simply multiplying $10,000 by 4% would calculate a full year’s amount under a simplified assumption. Dividing that result by four is a rough shortcut, but it ignores the exact days and compounding convention. Use the approximation only if you label it as such.
Calculate the bill from price and face value
TreasuryDirect’s bill guide says bills are sold at a discount or face value, pay face value at maturity and have interest equal to the difference between purchase price and that maturity payment.
Suppose a fictional bill with 91 days remaining costs $10,000 and pays $10,100 at maturity. Its gross interest is $100. The holding-period return is $100 divided by $10,000, or 1%. These invented prices keep the initial investment equal to the savings example. They are not an auction result or an available trade.
At this stage, the bill produces about $1.74 more gross interest than the savings estimate. That difference is much more informative than comparing two annual numbers without reading their definitions.
If a real quotation instead gives a $10,000 face value at a lower price, use the actual cost as the denominator. Do not compare a $10,000 savings deposit with a bill requiring less initial cash without accounting for the leftover money.
Add tax as a separate assumption
IRS Topic 403 identifies bank-account interest as taxable interest and states that Treasury bill interest is federally taxable but exempt from state and local income taxes. That distinction can change the comparison; it does not calculate an individual reader’s liability.
For illustration only, apply a fictional 22% federal rate and 5% state rate to the savings interest, and 22% federal tax to the bill interest. Assume the rates simply add, ignore deductions and other interactions, and assume no fees:
| Fictional 91-day holding | Gross interest | Assumed tax | Interest remaining |
|---|---|---|---|
| $10,000 savings at unchanged 4% APY | $98.26 | 27% | $71.73 |
| Bill costing $10,000 and paying $10,100 | $100.00 | 22% | $78.00 |
The difference is approximately $6.27. In this constructed example, tax assumptions matter more than the $1.74 gross difference. Change the assumptions and the answer changes. Do not reuse these percentages as your own tax calculation or apply the Treasury exemption automatically to a fund with “Treasury” in its name.
Test costs and early access
A hypothetical $10 extra cost attached only to the bill route would exceed the $6.27 advantage in the table. That does not mean such a cost always exists. It means actual charges belong in the comparison before deciding that a higher return is worthwhile.
Timing can invalidate the example altogether. The bill calculation assumes holding to maturity. If you must sell early, replace the face-value payment with an executable sale price, costs and the date proceeds become available. A maturity payment is not an immediate withdrawal quote.
Savings access also depends on the account’s transfer rules and the receiving account. Ask when the money would be spendable for the specific payment you need to make. Neither an annual rate nor the word “liquid” establishes that date.
Protection is another separate question. The FDIC’s estimator guidance states the standard insurance amount as $250,000 per depositor, per insured bank, for each ownership category. Coverage depends on records and applicable rules; a Treasury bill is a security rather than an FDIC-insured bank deposit.
Keep the comparison small and explicit
Save the actual quoted rate or price with its date, the amount invested, the assumed holding period and the costs you checked. Mark the savings-rate path as unknown if it is variable. Mark an early-sale price as unknown until you have a real quotation.
Then compare the dollar difference with the access and administration involved. A calculation can identify a tradeoff without deciding it for you. For the broader distinction between deposits, individual bills and funds, see matching cash to its job.
