What it means
When thousands of home loans are bundled into a mortgage-backed security (a bond backed by a pool of mortgages), the pool's principal falls each month as homeowners make payments. The paydown factor measures how much of the original amount was repaid in that month.
A factor of 0.0075 means 0.75% of the starting principal was returned. The calculation is simple: divide the principal repaid in the period by the original principal.
This is separate from the pool factor, which divides the principal still outstanding by the original principal. The two are linked, because the pool factor falls by the paydown factor each period, but they answer different questions.
Investors use the paydown factor to judge how quickly they are being repaid. A pool with a high paydown factor is returning cash fast, which shortens the expected life of the bond and may force the investor to reinvest at lower rates.
A low factor means capital is tied up for longer, but interest keeps flowing on a bigger balance. What drives the factor?
Two things: scheduled principal, which comes from the regular instalments, and prepayments, which come from borrowers refinancing, selling homes or paying extra. When interest rates fall, many homeowners refinance, prepayments jump, and the paydown factor rises sharply.
The same idea can be applied to a single loan. A lender can calculate what portion of a $350,000 mortgage was repaid in a month to see how slowly a loan amortises (pays down) in its early years.
Because early payments are mostly interest, the factor for a new loan is small and grows over time. A practical point is that the factor is reported for a single period, so it should be tracked as a trend.
Analysts often look at the average over three or six months to smooth out one-off prepayments before drawing conclusions about the pool.
In practice
Real-world examples.
Example
An asset manager holds a $10,000,000 slice of a mortgage bond. The monthly report shows a paydown factor of 0.0050, so $50,000 of the slice (0.0050 x 10,000,000) came back in cash this month.
Example
A credit analyst compares two car-loan pools of equal size. Pool A has a paydown factor of 0.9% a month and pool B has 2.1%, which tells her that pool B's borrowers are repaying or prepaying more than twice as quickly.
Example
A bank treasurer models its own $350,000 home loan. In month one, $1,300 of the payment goes to principal, so the paydown factor is 1,300 / 350,000 = 0.37%, and he expects the factor to creep up each month.
Formula
Calculation
Paydown factor = Principal repaid in the period / Original principal
Suppose a mortgage-backed security was created with an original principal of $200,000,000. In March, homeowners repaid $1,500,000 of principal through scheduled payments and prepayments. Paydown factor = 1,500,000 / 200,000,000 = 0.0075, or 0.75%. If the pool factor at the start of March was 0.80, it falls to 0.80 - 0.0075 = 0.7925 at the end, so $158,500,000 of the original $200,000,000 remains outstanding (0.7925 x 200,000,000). As a sense check, a factor of about 0.75% a month means roughly 9% of the original principal is returned each year (0.75% x 12 = 9%). Data providers differ on whether they quote monthly or annualised factors, so always confirm the definition before comparing pools.Case study
Seen in the real world.
Meridian Pension Partners is an illustrative, fictional fund that bought $50,000,000 of a mortgage-backed security expecting a ten-year life. Over the first year, mortgage rates fell and the monthly paydown factor climbed from 0.6% to 1.8%.
The portfolio manager saw that capital was coming back much faster than planned and that she would have to reinvest at lower yields. She used the factor trend to update the cash-flow forecast, shortening the expected life to about six years.
The illustrative lesson is that a rising paydown factor is a warning of reinvestment risk as well as good news about borrowers repaying.
Watch out
Common mistakes.
- Confusing the paydown factor with the pool factor, when one measures what was repaid and the other what remains.
- Reading one month's factor as the long-term pattern, when a single large prepayment can distort it.
- Assuming a high paydown factor is always good news, when it can force reinvestment at lower yields.
Questions
People also ask.
How is the paydown factor different from a paydown?
A paydown is the dollar amount of principal reduced, while the factor expresses that amount as a share of the original principal.
Why is the factor usually small at first?
Early loan payments are mostly interest, so only a small part reduces principal.
Who uses this measure?
Bond investors, credit analysts, and lenders who track how fast principal is being returned on mortgage and other asset-backed pools.
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