To determine whether a bond is subject to this tax, calculate the amount of full years between the discounted bond's purchase date and the maturity date and multiply this by 0.25. Subtract the calculated amount from the bond's par value. If this amount is above the purchase price of the discount bond, the purchased bond is subject to capital gains tax.
For example, if you are looking at a 10-year bond with a par value of 100 and five years left until maturity, simply multiply five years by 0.25 to get 1.25. You then subtract the 1.25 from the par value to get the de minis cut off amount, which in this example is 98.75 (100-1.25). If the price of the discount bond you purchased is below 98.78 per 100 of par value you will be subject to capital gains tax under the de minimis tax rule.