how to calculate the after tax interest cost?

Wilbur Marksberry: Interest Cost

Jannie Ariola: Example. Suppose your earnings put you in the 20% tax bracket and suppose you own a home and pay 5% interest on the mortgaged amount. Because the home mortgage interest is tax deductible, you can deduct the interest paid and therefore not have to pay the 20% tax on that amount. Hence, after tax, the actual amount the mortgage is costing you is 5% ( 0.8) = 4.0%.

Raymon Fiene: the after tax cost is equal to: before tax cost x (1 - tax rate)