To calculate the discount you need the following information:

• date of purchase
• date of maturity
• discount rate
• amount you wish to purchase

For example, if you wanted to purchase $1000 of Treasury bill issue number 1293 on November 08, 2010. This issue matures on January 19, 2011, and the discount rate is 0.35%. The number of days between the purchase date and the maturity date is 72.
The discount is calculated as follows:

Discount = Principal x Rate x Time / 100
= (1000 x 0.35 x 72) / (100 x 365)
= $0.69

The price is Face Value – Discount, or in this example:    $1000.00 – $0.69 = $999.31