Conditional Prepayment Rate (CPR)

A mortgage pool sends scheduled principal and interest to investors, but borrowers may also pay more principal than required, perhaps after refinancing or selling their homes. Those extra payments shorten the pool's life and change future interest cash flows.

CPR expresses an annualised pace conditional on the principal still outstanding, so an 8% CPR is a modelling assumption about the remaining balance, not a guarantee that exactly 8% of all original loans will close during the next year. Market participants also use single monthly mortality, or SMM, to describe the unscheduled share for one month, and a standard relation is CPR = 1 - (1 - SMM)^12.

The compounding conversion assumes the selected monthly rate persists for comparison, and a Federal Housing Finance Agency working paper uses the same relation to annualise a monthly prepayment measure. Specific pool reporting conventions can vary by deal, so inspect the transaction's method and cash-flow definitions.

For example, a monthly SMM of 1% converts to about 11.36% CPR, not 12% exactly, because compounding reflects that each month's unscheduled payment is measured against a smaller remaining balance. Falling mortgage rates can encourage borrowers to refinance, which may raise prepayments and return investors' principal earlier than planned, so the investor then may have to reinvest at a lower prevailing rate.

Rising rates can slow refinancing, extending a pool's effective life, so investors might be locked into an older lower coupon for longer while new bonds offer higher yields, which means both faster and slower prepayments can hurt in different market conditions. Borrower credit, loan age, home sales, servicing practices and contractual terms also affect prepayment, so a single rate-shock forecast cannot capture every household decision and historical CPR from one pool may not transfer cleanly to another.

Prepayment assumptions affect valuation: a mortgage security bought above face value may suffer when high-coupon loans pay back quickly because the investor recovers principal sooner and loses expected premium-related yield, while discount-priced securities can respond differently. CPR should be paired with a time profile, since a mortgage pool might start with slow repayments and speed up after borrowers settle in or refinancing becomes practical, and a flat annual CPR is a simplification that can miss these patterns.

Deal reports sometimes quote realised CPR over a prior period, while analysts forecast a future CPR, so label observed and projected figures separately, because a realised one-month spike does not establish that the pace will continue. Portfolio managers stress multiple CPR paths rather than one point estimate, comparing expected cash flow, weighted average life and price sensitivity under faster and slower repayment.

This reveals whether a return depends heavily on one borrower-behaviour assumption. The practical reading is to ask which pool, which measurement period, whether scheduled principal was removed, and whether the number is realised or forecast.

Without those facts, an annualised percentage can be misleading.