What Is Single Monthly Mortality?

By Kristi Waterworth – Updated Jun 3, 2025 at 9:54 AM

Key Points

Single monthly mortality helps gauge MBS investment health by tracking premature loan payoffs.
Initial fast prepayment phase of an MBS is now shorter than in the 1980s, impacting profits.
Calculate single monthly mortality by comparing expected vs. actual payments, expressed as a percentage.

If you're considering becoming a mortgage-backed securities investor, there's a lot about mortgage lending that you might want to learn. Along with interest rates and how notes are engineered, it's important to be able to do calculations like the single monthly mortality rate. This figure can help you better understand what's happening with your investment.

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A hand holding a pen and writing in the loan amount on a mortgage application. — Image source: Getty Images.

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About the Author

Kristi Waterworth has been a contributing real estate and financial expert at The Motley Fool, covering real estate, investing, and personal finance topics, since 2020. Kristi has been interviewed by various TV media outlets and podcasts for her real estate investing expertise – from residential real estate to digital real estate and everything in between. Kristi’s journalist career spans over 25 years, and she has written about real estate investing, homeownership, and home construction specifically for the last decade. She was also previously a licensed Realtor and general building contractor before the Great Recession and housing market collapse. She holds a bachelor’s degree in American history from Columbia College in Missouri.

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