31 C.F.R. § 344.7
Demand Deposit securities are one-day certificates of indebtedness that are automatically rolled over each day until you request redemption.
(a) How is the rate for Demand Deposit securities determined? Each security shall bear a rate of interest based on an adjustment of the average yield for 13-week Treasury bills at the most recent auction. A new annualized effective Demand Deposit rate and daily factor for the Demand Deposit rate are effective on the first business day following the regular auction of 13-week Treasury bills and are shown in the SLGS rate table. Interest is accrued and added to the principal daily. Interest is computed on the balance of the principal, plus interest accrued through the preceding day.
(1) How is the interest rate calculated?
(i) First, you calculate the annualized effective Demand Deposit rate in decimals, designated “I” in Equation 1, as follows:

(Equation 1)
Where: I = Annualized effective Demand Deposit rate in decimals. If the rate is determined to be negative, such rate will be reset to zero. P = Average auction price for the most recently auctioned 13-week Treasury bill, per hundred, to six decimals. Y = 365 (if the year following issue date of the 13-week Treasury bill does not contain a leap year day) or 366 (if the year following issue date of the 13-week Treasury bill does contain a leap year day). DTM = The number of days from date of issue to maturity for the most recently auctioned 13-week Treasury bill. MTR = Estimated marginal tax rate, in decimals, of purchasers of tax-exempt bonds. TAC = Treasury administrative costs, in decimals.
(ii) Then, you calculate the daily factor for the Demand Deposit rate as follows:
DDR = (1 + I)1/Y −1
(Equation 2)
(b) What happens to Demand Deposit securities during a debt limit contingency? At any time the Secretary determines that issuance of obligations sufficient to conduct the orderly financing operations of the United States cannot be made without exceeding the statutory debt limit, we may invest any unredeemed Demand Deposit securities in special 90-day certificates of indebtedness.
[70 FR 37911, June 30, 2005, as amended at 77 FR 33635, June 7, 2012; 89 FR 15449, Mar. 4, 2024]