# How to calculate the pre-tax cost of debt for a mix of bonds allotted to a company?

I need to calculate the effective interest rate a company X is paying on the total debt it has been loaned (to arrive at the Cost of Debt) for the FY 2011-12. Its long term borrowings are a mix of bonds that will be redeemed on different dates carrying different coupon rates as such:

1. 10,700 Bonds of face value of 1,000,000/- each, allotted on 10th September 2008, are redeemable at par on 10th September 2018. The bonds carry a coupon rate of 11.00 % p.a. payable annually on 15th September.
2. 14,150 Bonds of face value of 1,000,000/- each, allotted on 21st December 2011, are redeemable at par on 21st December 2016 with put/call option after 18 months from the date of allotment. The bonds carry a coupon rate of 9.28 % p.a. annually on 21st June each year.
3. 16,000 Bonds of face value of 1,000,000/- each, allotted on 11th December 2008, are redeemable at par on 11th December 2016. The bonds carry a coupon rate of 10.70 % p.a. payable annually on 30th June each year.
4. 158 Bonds of face value of 26,000,000/- each allotted on 18th July, 2001 are redeemable in 13 equal installments from the end of the 3rd year upto the end of 15th year from the date of allotment. Accordingly, 8th installment was paid in July 2011. The Bonds carry a coupon rate of 10.25% p.a. payable annually on 30th September.

My question is, how do I arrive at the pre-tax Cost of Debt for this company, if it is to be calculated as the interest rate on its borrowings?