What does a Treasury bill calculator estimate?
This calculator estimates price and two annualized rate measures for a bill from terms you provide. Treasury bills are sold at a discount or at par, and Treasury pays face value at maturity. The difference between purchase price and face value is the bill’s interest. TreasuryDirect explains this discount structure; the calculator does not check current offerings.
When you enter a discount rate, the calculator uses Treasury’s formula: price per $100 = 100 × (1 − discount rate × days remaining ÷ 360). The discount rate is a decimal in that formula, so 4.5% is entered as 4.5 and converted to 0.045. Treasury’s regulation says the calculated price per $100 is rounded to six decimal places. Estimated purchase price is that rounded price multiplied by face value divided by 100; the displayed dollar amount is rounded to cents. If you enter a price, it is first rounded to six decimals per $100 and used to derive the discount rate.
How is the investment rate calculated?
The investment rate, also called coupon-equivalent yield in the regulation, compares the discount with the purchase price and annualizes using the days remaining and the number of days in the year following issue. For a bill with no more than half a year remaining, the formula is investment rate = ((100 − price per $100) ÷ price per $100) × (year days ÷ days remaining). For a bill over half a year, Treasury specifies the compound equation P × [1 + (r − y/2)(i/y)] × (1 + i/2) = 100. Here P is price per $100, r is days remaining, y is the selected 365 or 366, and i is investment rate as a decimal. The calculator solves this quadratic and uses its positive root.
The year-day input follows the eCFR definition: normally 365, or 366 if the year following issue contains February 29. Since the exact issue date is not an input, select this value from the security’s dates. Long-bill results depend on the day count and six-decimal price rounding.
What are the limitations?
This is an input-driven estimate, not a current auction result, official quote, or realized return. It assumes the entered price and days remaining are applicable to a bill and does not model fees, taxes, secondary-market settlement, purchase eligibility, or reinvestment. TreasuryDirect describes Treasury bill purchase increments and redemption at face value, but actual available securities and transaction terms should be checked with Treasury or the broker handling a purchase.
Frequently asked questions
What is the difference between discount rate and investment rate?
The discount rate measures the discount against face value on a 360-day basis. The investment rate uses purchase price and the annual day count, so it is a coupon-equivalent yield measure.
Why is there a separate formula for bills over half a year?
Treasury’s published method uses a compound equation for bills with more than one half-year to maturity. This tool solves that equation as a quadratic.
Does this use current auction results?
No. You supply all inputs. The calculator does not retrieve auction data or quote a security.
How do I choose 365 or 366?
Use Treasury’s rule for the year following the issue date: 366 if that one-year period contains February 29, otherwise 365. Check the issue date and maturity terms for the bill you are reviewing.
Primary sources checked 2026-11-03: TreasuryDirect’s Treasury Bills and Understanding Pricing and Interest Rates; 31 CFR Part 356, Appendix B, section VI (eCFR current through 2026-10-30). The formulas and rounding description above follow the cited Treasury/eCFR methods.