Prepayment risk is uncertainty about principal returning earlier than expected and changing an investor's yield, duration, or reinvestment income.
Prepayment risk is the risk that borrowers return principal earlier than an investor or lender expected, changing the timing and value of future cash flows. The returned principal may have to be reinvested at lower yields, and the effective life and duration of a loan or security may shorten.
Mortgage-backed securities (MBS) are the best-known example because homeowners can refinance, sell, or otherwise repay mortgages before scheduled maturity. Prepayment risk also affects mortgage servicing rights, some asset-backed securities, whole-loan portfolios, and any credit asset with meaningful early-payoff rights.
Suppose an investor expects a pool of amortizing loans to return principal gradually over several years. If borrowers repay faster than modeled:
Early principal return is not necessarily a credit loss. The economic cost comes from losing above-market cash flows or receiving them at an inconvenient time.
Assume an investor expected $2,000,000 of principal to remain invested at 5% for the next year. Borrowers unexpectedly prepay that principal, and comparable reinvestment opportunities now yield 3%.
Expected annual interest before prepayment:
Illustrative annual interest after reinvestment:
The simplified one-year income difference is $40,000. The investor still receives the $2,000,000 principal, but sooner than expected and at a time when it earns less.
Actual security returns also depend on purchase price, amortization, servicing and guaranty fees, default cash flows, timing within the year, and future prepayment speeds. The example isolates reinvestment risk rather than calculating total return.
Prepayment behavior can hurt investors in opposite rate environments:
| Rate environment | Borrower tendency | Investor effect |
|---|---|---|
| Rates fall | Refinancing incentive generally increases | Faster prepayment, shorter average life, and reinvestment at lower yields |
| Rates rise | Refinancing incentive generally decreases | Slower prepayment, longer average life, and continued exposure to below-market coupons |
The first outcome is often called contraction risk. The second is extension risk. Together they help explain why mortgage-backed securities can display negative convexity: their expected cash-flow timing changes in ways that limit price appreciation when rates fall and deepen duration exposure when rates rise.
Rates are important, but they are not the only driver. Borrowers also prepay after a property sale, relocation, cash windfall, loan modification, casualty event, or scheduled curtailment.
Analysts commonly consider:
A large rate incentive does not guarantee refinancing. A borrower may lack sufficient equity or credit, face high transaction costs, plan to move, or simply choose not to act.
Two common measures are the single monthly mortality rate (SMM) and conditional prepayment rate (CPR).
SMM is the share of principal that prepays during a month after accounting for scheduled principal. CPR annualizes an assumed monthly prepayment rate:
The inverse conversion is:
For example, a 6% CPR corresponds to an SMM of approximately 0.514%. This does not mean 6% of the original pool prepays every year. The rate is conditional on the balance that remains after scheduled principal and prior prepayments.
The Public Securities Association, now SIFMA, developed a benchmark convention for mortgage prepayments. At 100% PSA, CPR starts at 0.2% in month one, rises by 0.2 percentage points each month until reaching 6% in month 30, and remains at 6% thereafter.
PSA is a scenario convention, not a forecast that every pool will follow. Analysts use it to compare cash-flow sensitivity under standardized speed assumptions.
In a basic pass-through MBS, scheduled and prepaid principal from the mortgage pool flows through to investors. Because borrowers control much of the timing, the investor does not know the security’s exact average life in advance.
A collateralized mortgage obligation (CMO) redirects principal among tranches under a payment waterfall. One tranche may receive principal first while another receives it later. This can redistribute contraction and extension exposure, but it does not eliminate the underlying pool’s prepayment uncertainty.
Investors should examine:
| Risk | Adverse event | Primary uncertainty |
|---|---|---|
| Prepayment risk | Principal returns earlier than expected | Cash-flow timing and reinvestment yield |
| Extension risk | Principal returns later than expected | Longer duration and below-market exposure |
| Reinvestment risk | Interim cash flow is reinvested at a lower rate | Future income |
| Default risk | Borrower fails to perform | Principal and interest recovery |
| Interest-rate risk | Market yields change | Price and duration |
| Liquidity risk | The position cannot be sold efficiently | Execution price and timing |
| Call risk | Issuer exercises a contractual redemption option | Early redemption initiated by issuer rather than underlying borrowers |
These risks can interact. For example, a falling-rate environment can increase MBS prices while also increasing prepayment, shortening the period over which the investor receives the higher coupon.
Mortgage servicing rights are valued partly from expected future servicing income. If mortgages prepay sooner, servicing on those loans ends sooner. The Federal Reserve notes that faster prepayments can therefore reduce mortgage-servicing-right values.
The effect differs from holding the mortgage principal itself, but both exposures depend on uncertain borrower behavior. Models should use realistic prepayment, default, cost, and discount-rate assumptions rather than a single static speed.
Financial institutions may manage prepayment exposure through security selection, tranche structure, scenario analysis, duration targets, hedging, loan pricing, and contractual prepayment protection. Each approach has limitations:
No method guarantees a particular return or cash-flow path.
This page provides general financial education, not a recommendation to buy, sell, or hedge a particular security.