A cash-flow yield is the discount rate that makes the price of a mortgage-backed or asset-backed security equal to the present value of its cash flows. It is equivalent to the yield to maturity measure. In mortgage-backed and asset-backed securities, because of prepayments, the cash flow is unknown; so some assumption about the prepayment rate must be made to calculate the cash-flow yield.
Different from a normal bond, which typically pays a coupon semi-annually, a mortgage-backed or asset-backed security makes monthly payments. Therefore, the investor has the opportunity to generate greater reinvestment income by reinvesting the monthly cash flows. In a treasury coupon security, bond-equivalent yield can be found out by doubling the semi-annual yield, but because of more frequent payments the same concept cannot be used in calculating the bond-equivalent yield for mortgage-backed and asset-backed securities. The market convention is to calculate a yield so as to make it comparable to the yield to maturity on a bond-equivalent basis. The formula for annualizing the monthly cash flow yield for a monthly pay product is as follows:
Bond-equivalent yield = 2 [(1 + iM)6 - 1]
Where, iM is the monthly interest rate that will equate the present value of the projected monthly cash flow equal to the market price (plus accrued interest) of the security.
Example: If the monthly cash flow yield is 0.8%, then the bond equivalent yield is
2 x [(1.008)6 - 1] = 0.0979 = 9.79%.