The formulas
- Bank discount yield = (face - price) / face x 360 / days
- Money market yield = (face - price) / price x 360 / days
- Bond-equivalent yield, half a year or less: (100 - P) / P x y / r
- Over half a year: solve P[1 + (r - y/2)(i/y)](1 + i/2) = 100 for i
- P = price per 100 of face, r = days to maturity, y = 365 or 366
- Effective annual yield = (face / price)^(365 / days) - 1
How it works
Treasury bills pay no coupon. TreasuryDirect states that bills are sold at a discount or at par, and you are paid face value at maturity, so the discount is the return. Treasury auctions bills on a discount rate basis, which divides the discount by face value and uses a 360-day year. That understates the return on the money invested.
The bond-equivalent yield, which Treasury calls the investment rate or coupon-equivalent yield, divides by the price paid and uses the days in the year after the issue date: 365, or 366 when that year contains February 29. For bills over half a year, the federal formula in 31 CFR Part 356 adds the coupon a note would pay at six months and is solved as a quadratic. The money market yield divides by price on a 360-day year, the convention for other money market instruments.
Illustrative figures only. Check the result against your firm's methodology before relying on it.
Common questions
Why is the discount yield the lowest figure?
It divides by face value, which is larger than the price, and uses a 360-day year.
Which yield compares with a savings account APY?
The effective annual yield, which assumes you roll the bill at the same rate all year.
Is T-bill interest taxed?
Yes, federally. TreasuryDirect states that federal tax is due on the interest and no state or local tax applies. IRS Publication 550 says the same for Treasury bills, notes and bonds.
Sources
- TreasuryDirect, Treasury Bills
- 31 CFR Part 356, Appendix B: Formulas and Tables (Treasury bill investment rate)
- IRS Publication 550, Investment Income and Expenses
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