Given the annual interest rate and days to maturity, the price of a US Treasury Bill can be calculated. Remember, T-bills are discount bonds. This means they sell below par value and mature at par. The lower the price, the higher the effective annual interest rate will be, and vice versa. The result is too small. Given the purchase price and days to maturity, the rate of return of a US Treasury Bill can be calculated. Remember, T-bills are discount bonds. This means they sell below par value and mature at par. The lower the price of the T-bill, the higher the annual rate of return will be, and vice versa. Find the purchase price of a T-bill by calculating the discount over the term of the T-bill. T-bills are quoted according to their discount rates, so you pay less than their $1,000 face value. When they mature, you receive $1,000. The difference between what you paid and $1,000 is your interest earned.