How are Treasury bill interest rates determined?

U.S. Treasury bills (T-bills) are typically sold at a discount from their par value. The level of discount is determined during Treasury auctions.

Unlike other U.S. Treasury securities such as Treasury notes (T-notes) and Treasury bonds (T-bonds), T-bills do not pay periodic interest at six-month intervals. The interest rate for Treasuries is therefore determined through a combination of the total discounted value and the maturity length.

The Treasury holds auctions for different maturities at separate, reoccurring intervals. Auctions for the 13-week and 26-week T-bills happen every Monday, as long as the financial markets are open during the day. Fifty-two-week T-bills are auctioned every fourth Tuesday. Each Thursday, announcements are made about how many new T-bills will be issued and their face values. This allows potential buyers to plan their purchases.

There are two types of bidders for Treasury bills: competitive and noncompetitive. Competitive bidders are the only ones who actually influence the discount rate. Each competitive bidder declares the price they are willing to pay, which the Treasury accepts in descending order of price until the total face value of any particular maturity is sold out. Noncompetitive bidders agree to buy at the average price of all accepted competitive bids.

Buyers who hold T-bills until maturity always receive face value for their investments. The interest rate comes from the spread between the discounted purchase price and the face value redemption price.

For example, suppose an investor purchases a 52-week T-bill with a face value of $1,000. The investor paid $975 up front. The discount spread is $25. After the investor receives the $1,000 at the end of the 52 weeks, the interest rate earned is 2.56%, or 25 / 975 = 0.0256.

The interest rate earned on a T-bill is not necessarily equal to its discount yield, which is the annualized rate of return the investor realizes on an investment. Discount yields also change over the course of the life of the security. The discount yield is sometimes referred to as the discount rate, which should not be confused with the interest rate.

Several external factors can influence the discount price paid on T-bills, such as changes in the federal funds rate, which impacts T-bills more than other types of government securities. This is because T-bills directly compete with the federal funds rate in the market for low-risk, short-term debt instruments. Institutional investors are particularly interested in the difference between the federal funds rate and T-bill yields.

The prices for Treasury bills (T-bills) can have a significant impact on the risk premium charged by investors across the entire market. T-bills, are priced like bonds; when prices rise, yields drop and the opposite is also true. In the world of debt securities T-bills represent the greatest liquidity and the lowest risk of principal. They act as the closest thing to a risk-free return in the market; all other investments must offer a risk premium in the form of higher returns to entice money away from Treasuries.

There are other drivers of T-bill prices. During times of high economic growth, investors are less risk-averse and the demand for bills tends to drop. As T-bill yields rise, other interest rates rise as well. Other bond rates climb, the required rate of return on equities tends to rise, mortgage rates tend to rise and the demand for other "safe" commodities tends to drop.

Similarly, when the economy is sluggish and investors are leaving riskier investments, T-bill prices tend to rise and yields drop. The lower T-bill interest rates and yields drop, the more investors are encouraged to look for riskier returns elsewhere in the market. This is particularly true during times when inflation rates are higher than the returns on T-bills, essentially making the real rate of return on T-bills negative.

Inflation also affects T-bill rates. This is because investors are reluctant to purchase Treasuries when the yield on their investments does not keep up with inflation, making the investment a net loss in terms of real purchasing power. High inflation can lead to lower Treasury prices and higher yields. Conversely, prices tend to be high when inflation is low. The second reason inflation affects T-bill rates is because of how the Federal Reserve targets the money supply.

T-bill prices have a large influence on the relative levels of risk investors are comfortable taking. High T-bill prices drive investors into longer-term bonds, lower-grade bonds, equities or derivatives. This is true of institutional investors and individual investors. In effect, the price and yield of T-bills and other Treasuries helps inform the fundamentals of nearly every other investment class on the market.