91 Day T-Bill Calculator
Estimate the purchase price, maturity value, dollar discount, and annualized investment yield for a 91 day U.S. Treasury bill using a fast, premium calculator built for investors, savers, and cash managers.
Calculator Inputs
Enter the bill details below. This calculator uses the standard bank discount pricing convention commonly used for Treasury bills.
Your Results
See the estimated purchase amount and what the bill pays at maturity.
Expert Guide to Using a 91 Day T-Bill Calculator
A 91 day T-bill calculator helps you estimate how much you pay today for a short-term U.S. Treasury bill and how much you receive when the bill matures. A 91 day Treasury bill, often called a 13 week T-bill, is one of the most widely used cash-management instruments in the world. It is backed by the full faith and credit of the U.S. government, matures quickly, and is commonly used by individual savers, institutions, and businesses looking for relatively low-risk places to hold money over a short horizon.
Unlike a traditional savings product that pays periodic interest, a Treasury bill is usually sold at a discount to face value. That means you pay less than the amount you will receive at maturity. The difference between your purchase price and the amount paid back at maturity is your earnings. A 91 day T-bill calculator automates the pricing math and helps you compare Treasury bills with other short-term options such as money market funds, high-yield savings accounts, bank certificates of deposit, and commercial paper.
Key idea: If you buy a $10,000 91 day T-bill at a discount, you do not receive coupon payments along the way. Instead, you buy below par and receive the full $10,000 at maturity. Your return is the difference.
What exactly is a 91 day T-bill?
A 91 day Treasury bill is a short-term marketable U.S. government security that matures in approximately 13 weeks. Treasury bills are auctioned regularly and can also be purchased in the secondary market. They are usually quoted on a bank discount basis, which differs from the yield methods consumers are used to seeing for bank deposits or bond funds.
The U.S. Treasury issues bills in several standard maturities. The 91 day bill has become especially important because it serves as a benchmark for short-term interest rates, liquidity pricing, and cash alternatives. Professionals in banking, corporate treasury, and portfolio management frequently reference the 13 week bill when discussing the risk-free short end of the yield curve.
| Treasury Bill Term | Approximate Weeks | Day Count | Common Use Case |
|---|---|---|---|
| 4 week bill | 4 weeks | 28 days | Ultra-short cash parking and rolling liquidity |
| 8 week bill | 8 weeks | 56 days | Short reserve management |
| 13 week bill | 13 weeks | 91 days | Benchmark cash equivalent for many investors |
| 26 week bill | 26 weeks | 182 days | Intermediate cash allocation |
| 52 week bill | 52 weeks | 364 days | Longer short-term Treasury exposure |
How a 91 day T-bill calculator works
The calculator above uses the standard discount pricing method. The core pricing formula for a Treasury bill quoted on a discount basis is:
Price = Face Value × (1 – Discount Rate × Days to Maturity / 360)
In this formula, the discount rate is expressed as a decimal, not a percentage. If the quoted discount rate is 5.20%, the decimal form is 0.052. The day-count basis is 360, which is standard for bank discount calculations. Once the price is known, your dollar earnings are simply the maturity value minus the purchase price.
Suppose you want to buy one $10,000 91 day bill at a 5.20% discount rate. The estimated price would be:
- Convert 5.20% to decimal form: 0.052
- Multiply 0.052 by 91/360
- Subtract the result from 1
- Multiply by $10,000
The result is an estimated purchase price of about $9,868.56, which means the bill would mature at $10,000 and produce about $131.44 in earnings over the 91 day term.
Why the investment yield is different from the discount rate
Many investors are surprised that the quoted discount rate and the actual return on dollars invested are not the same thing. The discount rate is calculated as a percentage of face value, while your actual return is earned on the lower purchase price you paid. Because of that, the investment rate or bond-equivalent yield will generally be slightly higher than the discount quote for the same bill.
A strong 91 day T-bill calculator should therefore show more than one output. At a minimum, it should display the purchase price, maturity value, total discount earned, and an annualized investment yield estimate. That makes the output much easier to compare against bank APYs, money market yields, or short-term bond fund returns.
| Discount Rate | Estimated Price on $10,000 Face | Dollar Earnings | Approx. Investment Yield Annualized |
|---|---|---|---|
| 4.00% | $9,898.89 | $101.11 | 4.11% |
| 5.00% | $9,873.61 | $126.39 | 5.14% |
| 5.25% | $9,867.29 | $132.71 | 5.40% |
| 5.50% | $9,860.97 | $139.03 | 5.66% |
| 6.00% | $9,848.33 | $151.67 | 6.17% |
These figures are mathematical illustrations for a 91 day bill using the standard discount formula and annualized investment yield approximation. Actual auction and secondary market pricing can vary slightly because of settlement conventions and market movements.
Inputs that matter most
- Face value: The amount paid at maturity. This is the par amount of the bill.
- Quantity: Number of bills you want to purchase.
- Discount rate: The annualized discount quote used to estimate the bill price.
- Days to maturity: For a 13 week bill, this is usually 91 days, but actual remaining days can differ in the secondary market.
- Settlement date: Helps estimate the maturity date and your expected holding period.
Who should use a 91 day T-bill calculator?
This tool is useful for a surprisingly wide range of users. Individual investors may want to compare a Treasury bill against a certificate of deposit or a savings account. Retirees might use it to keep part of their cash reserve in short-duration government securities. Businesses can use it to manage near-term working capital. Financial advisors may use it to illustrate risk-managed income alternatives for clients who do not want to lock up capital for long periods.
It is also especially valuable during periods when short-term rates change quickly. In rising-rate environments, investors often rotate cash into Treasury bills to seek higher yields while keeping duration low. In uncertain markets, bills can become attractive because of their government backing and short maturity schedule.
How 91 day T-bills compare with other cash options
A 91 day T-bill is not always the best option, but it is often one of the strongest benchmark choices. Compared with a savings account, a T-bill usually gives you a fixed known maturity value if held to maturity. Compared with a CD, it may offer more flexibility in the secondary market, though market prices can fluctuate before maturity. Compared with a money market fund, a bill can provide a direct holding in a specific U.S. government security rather than a pooled vehicle.
- Versus savings accounts: Bills can offer competitive yields, but savings accounts provide ongoing liquidity.
- Versus CDs: Bills often avoid early-withdrawal penalties, but selling before maturity can still result in a gain or loss.
- Versus money market funds: Bills are direct securities; money market funds offer diversification and daily convenience.
Important tax considerations
Interest income from U.S. Treasury bills is generally subject to federal income tax, but it is typically exempt from state and local income taxes. That tax treatment can make Treasury bills more attractive than taxable bank interest for investors in high-tax states. Even so, tax outcomes depend on your personal situation, account type, and where you hold the investment. A calculator can estimate gross returns, but it cannot replace tax advice from a qualified professional.
Primary market vs secondary market pricing
When you buy through a Treasury auction, you usually receive the auction-determined rate. In the secondary market, however, price and yield move continuously with market conditions. If rates rise after you buy a bill, the resale price may fall. If rates decline, the bill may trade at a premium to your original purchase price. That is why a 91 day T-bill calculator is most accurate as a held-to-maturity planning tool unless it also includes live market pricing data.
Common mistakes investors make
- Confusing discount rate with actual return: The quoted discount rate is not the same as the annualized return on invested cash.
- Ignoring the day count: Treasury bill pricing uses a 360-day convention for discount calculations.
- Comparing quoted yields incorrectly: APY, SEC yield, money market yield, and discount rate are not interchangeable.
- Forgetting taxes: Treasury tax treatment can improve after-tax returns versus some bank products.
- Overlooking reinvestment risk: After a 91 day bill matures, your next bill may carry a very different rate.
How to use this calculator effectively
Start with the current discount rate from a reliable market source or recent auction result. Enter the face value you want, the number of bills, and keep the term at 91 days if you are evaluating a standard 13 week T-bill. Review the purchase price and earnings. Then compare the annualized investment yield to the return available on other short-term cash alternatives. If you are laddering bills, repeat the process across several maturity dates so you can understand both yield and cash-flow timing.
If you are comparing multiple options, focus on net usable cash, liquidity needs, and tax-adjusted return instead of headline rate alone. That is where a good calculator becomes much more than a convenience. It becomes a decision tool.
Authoritative sources for Treasury bill research
If you want official details on Treasury auctions, purchase rules, and security terms, review the following authoritative resources:
- TreasuryDirect.gov: Treasury Bills overview
- U.S. Department of the Treasury: Interest rate statistics
- Investor.gov: U.S. Treasury securities basics
Bottom line
A 91 day T-bill calculator gives you a fast and reliable way to estimate the economics of one of the world’s most important short-term securities. By translating a quoted discount rate into a purchase price, maturity value, and annualized investment return, the calculator helps you make smarter cash decisions. Whether you are preserving liquidity, building a Treasury ladder, comparing yield alternatives, or simply trying to understand how Treasury bills work, a dedicated 91 day T-bill calculator is one of the most practical tools you can use.