PSA Method Calculator
Analyze Mortgage-Backed Securities (MBS) Prepayment Speeds
2.40%
0.202%
2.40%
Standard Ramp
Formula: CPR = min(Age × 0.2%, 6%) × (PSA Multiplier / 100)
Prepayment Ramp Visualization
Annual CPR % over 60 months based on your PSA input
Blue line: Your PSA Speed | Grey line: 100% PSA Benchmark
PSA to CPR Conversion Table
| Month | 100% PSA (Benchmark) | Your PSA CPR (%) | Your PSA SMM (%) |
|---|
What is the PSA Method Calculator?
The PSA Method Calculator is a specialized financial tool used by investors and analysts to estimate the prepayment speeds of mortgage-backed securities (MBS). Developed by the Public Securities Association (now SIFMA), the PSA model provides a standardized benchmark for comparing how quickly homeowners are likely to pay off their mortgages ahead of schedule.
The psa method calculator allows market participants to move beyond simple flat rates and account for the “seasoning” of a mortgage pool. New mortgages rarely prepay immediately; instead, they follow a “ramp” where prepayment probability increases over the first 30 months before stabilizing. Understanding this behavior is critical for accurately pricing bonds and managing interest rate risk.
Who should use it? Portfolio managers, individual bond investors, and students of finance use the psa method calculator to determine the mortgage-backed securities cash flows and potential yield-to-maturity of various mortgage products.
PSA Method Formula and Mathematical Explanation
The PSA model is expressed as a percentage of the standard “100% PSA” benchmark. To calculate the prepayment speed for any given month, the formula follows a piecewise linear logic during the first 30 months, followed by a constant rate.
The 100% PSA Benchmark Formula:
- Months 1 to 30: CPR = 0.2% × Month Number
- Month 30 and beyond: CPR = 6.0%
The General PSA Method Calculation:
To find the CPR for a specific psa method calculator multiplier (e.g., 150 PSA):
CPR = [min(Month × 0.2%, 6%)] × (PSA Multiplier / 100)
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| PSA Multiplier | Relative speed vs benchmark | Percentage (%) | 0% – 1000% |
| Month (Age) | Time since loan origination | Months | 1 – 360 |
| CPR | Conditional Prepayment Rate (Annualized) | Percentage (%) | 0% – 100% |
| SMM | Single Monthly Mortality (Monthly) | Percentage (%) | 0% – 10% |
Practical Examples (Real-World Use Cases)
Example 1: Analyzing a New Mortgage Pool (150 PSA)
Imagine an investor looking at a new pool of mortgages that are 10 months old. The market suggests these loans are prepaying at 150 PSA. Using the psa method calculator:
- Step 1 (Benchmark): 10 months × 0.2% = 2.0% CPR.
- Step 2 (Multiplier): 2.0% × (150 / 100) = 3.0% CPR.
- Result: The annual prepayment rate is 3.0%.
Example 2: Mature Pool (200 PSA)
Consider a mortgage pool that is 40 months old. Since it is older than 30 months, the benchmark is capped at 6.0% CPR. If the speed is 200 PSA:
- Step 1 (Benchmark): 6.0% CPR (Max reached at month 30).
- Step 2 (Multiplier): 6.0% × (200 / 100) = 12.0% CPR.
- Result: The annual prepayment rate is 12.0%.
How to Use This PSA Method Calculator
- Enter PSA Multiplier: Input the expected speed (e.g., 100 for the standard ramp, or 250 for high speeds during a refinancing wave).
- Input Loan Age: Enter the number of months since the mortgages in the pool were originated.
- Review CPR and SMM: The psa method calculator will instantly show the annual prepayment rate and the monthly mortality rate.
- Analyze the Chart: View the visual ramp to see how prepayments evolve over the next 5 years.
- Consult the Table: Check the monthly breakdown to understand the specific month-by-month cash flow expectations.
Key Factors That Affect PSA Method Results
- Interest Rate Environment: When market rates drop, CPR vs SMM values usually spike as homeowners refinance.
- Housing Market Health: Strong housing markets lead to more sales, which triggers loan payoffs and higher PSA speeds.
- Seasonality: Prepayments typically rise in summer (moving season) and fall in winter.
- Burnout Effect: If interest rates stay low for a long time, the pool “burns out” because everyone who could refinance already has, lowering the PSA.
- Credit Quality: Borrowers with lower credit scores may not be able to refinance, leading to slower PSA speeds even in low-rate environments.
- Loan Size: Larger loans have more financial incentive to refinance, often resulting in higher prepayment risk management concerns.
Frequently Asked Questions (FAQ)
The 30-month limit was chosen empirically by the Public Securities Association. It represents the historical average time it takes for a mortgage pool to reach its peak “seasoned” prepayment speed.
CPR is an annualized rate, while SMM (Single Monthly Mortality) is the percentage of the remaining principal that prepays in a single month. The psa method calculator converts between them automatically.
A 0 PSA speed means that no prepayments are occurring; homeowners are only making their scheduled interest and principal payments.
Mathematically, yes. During massive refinancing booms, some “hot” mortgage pools can briefly experience speeds that exceed 1000 PSA, though this is rare for long periods.
Higher PSA speeds shorten the life of a bond. If the bond was bought at a premium, faster speeds hurt yield; if bought at a discount, faster speeds help yield.
No, there are other models like the CPR model (constant speed) or proprietary econometric models used by major banks, but the psa method calculator remains the industry standard benchmark.
The PSA model was originally designed for fixed-rate mortgages. Adjustable-rate mortgages (ARMs) often exhibit different prepayment behaviors that might not follow the standard PSA ramp perfectly.
It is the risk that principal is returned to the investor earlier than expected, usually when interest rates are low, forcing the investor to reinvest at lower yields.
Related Tools and Internal Resources
- Mortgage-Backed Securities Analysis: Deep dive into MBS structures.
- CPR to SMM Converter: Specialized tool for rate conversions.
- MBS Yield Analysis: Calculate yields based on prepayment assumptions.
- Prepayment Risk Guide: Managing risks in a falling rate environment.
- Weighted Average Life Calculator: Determine the expected life of your MBS investment.
- Bond Valuation Tools: Comprehensive suite for fixed-income investors.