T-Bill Calculator

What is a T-bill calculator?

A T-bill calculator is a tool that computes the yield of a U.S. Treasury bill, a short-term government debt security sold at a discount to its face value that pays no coupon. The metric behind it is the T-bill yield, the annualized return you earn from buying the bill below its face value (par) and being repaid the full face value at maturity. Because a Treasury bill is a zero-coupon instrument, there are no interest payments along the way: your entire return is the gap between the purchase price and the face value. Buy a $1,000 bill for $985 and, 91 days later, you are repaid $1,000, so your $15 gain is the interest.

This is a U.S.-specific tool, because Treasury bills are issued by the U.S. Treasury and the day-count conventions it uses follow U.S. market practice. A T-bill is a different security from a Treasury note or Treasury bond, which run longer and pay coupons, and from a Series I savings bond, which is a non-tradable, inflation-linked savings product. The calculator exists mainly because the same bill is quoted in three different yield conventions, and it shows all three side by side.

Why is the T-bill calculator important for investors?

The T-bill calculator is important for investors because it quantifies the real return on cash held in a government-backed security before that cash is committed elsewhere. A Treasury bill is the classic capital preservation instrument: it is backed by the U.S. government, it matures in a year or less, and it is highly liquid, which makes its yield the benchmark that a savings account, a certificate of deposit (CD) or a money-market fund is measured against. Knowing the exact yield tells you whether parking money in a bill beats the alternatives for a known, short horizon.

Investors reach for the calculator at the planning stage, before the cash is deployed rather than after. You use it whenever you are deciding where to hold money you will need soon, comparing a bill's return against a CD, or checking what a dealer's quoted discount rate actually works out to in real terms. Sizing that decision correctly is part of the wider discipline of investing for a goal, where matching the right instrument to the right time horizon matters as much as the headline rate, and running the number before you commit the cash is what turns a rough assumption into a measured choice.

How do you use the T-bill calculator?

To use the T-bill calculator, enter the face value, the purchase price you paid and the days to maturity, and the tool returns your dollar return alongside all three yields.

The steps to use the T-bill calculator are listed below:

1. Enter the face value (par). This is the amount the U.S. Treasury repays you at maturity, typically $1,000 per bill; it is the base the discount rate is measured against.
2. Enter the purchase price you paid. This is what you actually paid for the bill, which is below face value; the difference between the two is your total return.
3. Set the days to maturity. This is the number of days from settlement until the bill is repaid, capped at 364 because T-bills never run longer than 52 weeks, and it is what annualizes the return.

The tool also works in reverse: switch to the From discount rate mode and enter the discount rate a dealer quotes instead of the price, and the calculator solves for the purchase price before showing the same yields. In Advanced options, a term quick-pick sets the days for you (4, 8, 13, 17, 26 or 52 weeks), a marginal tax rate field reveals the after-tax angle, and a currency selector labels the output. Press Calculate to update the result, and the preset chips load a 13-week, 26-week or 52-week bill in one click.

What formula does the T-bill calculator use?

The T-bill calculator uses three standard formulas that annualize the same dollar return on different bases: the bank discount rate on a 360-day year against face value, the investment (coupon-equivalent) yield on a 365-day year against the price paid, and the effective annual yield (APY) that compounds the investment yield.

In these formulas, face is the face value the Treasury repays at maturity, price is what you paid for the bill, and days is the number of days to maturity. The two day-count bases are the whole point: the discount rate divides the return by the larger face value on a 360-day year, while the investment yield divides it by the smaller price on a true 365-day year.

Plugging in the default values, the investment yield is ($1,000 − $985) ÷ $985 × 365 ÷ 91 = 6.11%.

The investment yield here uses the simple actual/365 form, which is exact for bills of 182 days or less; for longer bills the U.S. Treasury applies a small quadratic adjustment, so on 52-week bills this figure can differ by a sub-decimal amount.

What is an example of a T-bill calculation?

An example of a T-bill calculation is a $1,000 bill bought at $985 with 91 days to maturity, which earns a $15 return and works out to a 5.93% discount rate, a 6.11% investment yield and a 6.25% APY, worked out as follows:

1. Total return = $1,000 face − $985 price = $15.00.
2. Bank discount rate = $15 ÷ $1,000 × 360 ÷ 91 = 5.93%.
3. Investment yield = $15 ÷ $985 × 365 ÷ 91 = 6.11%.
4. Effective annual yield (APY) = (1 + $15 ÷ $985) raised to the power of 365 ÷ 91, minus 1 = 6.25%.

The three yields describe the same $15 gain, but they climb from 5.93% to 6.25% because each one measures it more favourably: the discount rate uses the big face value and a short 360-day year, the investment yield switches to the smaller price and a full 365-day year, and the APY then compounds that yield across the year. In the From discount rate mode the calculation runs backwards: enter a 5% discount rate on a 182-day bill and the tool solves the purchase price as $1,000 × (1 − 0.05 × 182 ÷ 360) = $974.72, then reports the matching yields from that price.

How do you read the T-bill calculator's yield?

You read the T-bill calculator's yield by using the investment yield, not the headline bank discount rate, as the number that reflects your real return on the price you actually paid. The calculator shows three figures for a reason. The bank discount rate (5.93% in the worked example) is the old dealer quote, measured against face value on a 360-day year, and it always understates your return. The investment yield, also called the coupon-equivalent yield (6.11%), measures the same $15 against the $985 you really invested on a 365-day year, so it is the figure to compare against a savings account or a CD. The effective annual yield (APY) (6.25%) goes one step further and compounds that yield, showing what you would earn holding and rolling the bill for a full year.

The gap between the three is not an error, it is convention, and reading the result correctly means always comparing like for like. A bank or broker will usually quote the discount rate, so when you weigh a bill against another product you should convert to the investment yield first, which is exactly the yield figure that lets you put a Treasury bill, a CD and a money-market fund on the same scale before you decide where the cash goes.

What are the limits of the T-bill calculator?

The T-bill calculator returns an estimate that is only as accurate as the inputs you enter, and it deliberately leaves out several real-world factors. It does not pull live auction rates: the price and discount rate are figures you type in, so the yields reflect your bill, not today's Treasury market. It does not model reinvestment risk, the possibility that when the bill matures you have to reinvest the proceeds at a lower rate, and it does not adjust for inflation, which can quietly erode the real value of a nominal yield over the holding period.

Two further limits are worth stating plainly. The investment yield uses the simple actual/365 formula, which is exact for bills of 182 days or less but differs slightly from the U.S. Treasury's official coupon-equivalent yield on longer bills, as noted in the formula section. And the calculator sizes one bill in isolation: it can tell you the yield of a Treasury bill, not whether a bill is the right home for your money. It is an educational tool, not investment advice.

How does the T-bill calculator handle taxes on Treasury bills?

The T-bill calculator handles taxes through an optional marginal tax rate input that reflects how Treasury bill interest is taxed in the United States: subject to federal income tax, but exempt from state and local income tax. That state and local exemption is set in federal law (31 U.S.C. §3124) and confirmed in IRS guidance on interest income (Topic no. 403), and it is what makes a Treasury bill's after-tax return higher than a fully taxable product showing the same headline yield, especially for investors in high-tax states such as California, New York or New Jersey.

When you enter your marginal tax rate, the tool shows the taxable-equivalent yield, the rate a fully taxable alternative would need to offer to match the bill once state tax is accounted for. This is educational context, not tax advice, and your own result depends on your state and bracket. For reporting, the interest a bill pays is recorded as original issue discount and reported to you on Form 1099-INT from the U.S. Treasury in the year the bill matures.

What is the difference between a T-bill discount rate and an investment yield calculation?

A T-bill discount rate calculation and an investment yield calculation measure the same dollar return on different bases: the discount rate divides the return by the face value on a 360-day year, while the investment yield divides it by the price you paid on a 365-day year. Both describe one bill, but they never produce the same number, and the discount rate is always the lower of the two.

Because the discount rate uses the larger denominator and the shorter year, it understates the true return: the same bill quoted at a 5.93% discount rate actually earns a 6.11% investment yield. This is also why the two conventions use 360 versus 365 days, a money-market habit that predates the true-calendar yield. Whenever you compare a Treasury bill to another investment, use the investment yield, not the headline discount rate.

Which calculators are related to the T-bill calculator?

The calculators related to the T-bill calculator cover the rest of the fixed-income and real-return toolkit, from coupon-paying bonds to the inflation that eats into a nominal yield.

The calculators related to the T-bill calculator are listed below:

• Bond calculator: prices a coupon-paying Treasury note or bond, the longer-dated sibling of the zero-coupon T-bill.
• Yield to maturity calculator: works out the total return of a coupon bond held to maturity, the fuller yield concept the T-bill simplifies.
• I bond calculator: values a Series I savings bond, the other U.S. government retail security, whose rate is linked to inflation.
• Expense ratio calculator: shows the cost drag of a bond ETF or fund, the packaged alternative to buying a T-bill directly.
• Inflation calculator: converts a nominal T-bill yield into a real, after-inflation return, the figure that shows how much of the yield you keep.

FAQ

These results are educational estimates based on the inputs you enter and current U.S. conventions. Yields and tax treatment can change, and this is not tax or investment advice.