Direct answer: The rate lock-in effect means many homeowners keep mortgages with note rates far below today’s market, so selling and rebuying would raise their monthly payment sharply. FHFA’s National Mortgage Database (NMDB) is the government research source for how common those low coupons still are in the outstanding stock.
Why it matters for calculators
- Refinance Calculator — break-even when your rate is already low.
- Mortgage Calculator — model the payment you would face on a new purchase.
- Weekly PMMS payment index — track how market rates translate to dollars.
Methodology note
This page explains NMDB concepts and points to FHFA publications. State-level share tables will ship as a downloadable CSV after the next scheduled NMDB aggregate pull (see _docs/data-refresh/gov-data-refresh-calendar.md).
Last reviewed
Last reviewed: · Source: FHFA National Mortgage Database (public aggregates)
Frequently Asked Questions
What is the mortgage rate lock-in effect?
Homeowners who financed when rates were much lower (often under 3-4%) face a large payment jump if they sell and buy again at today's rates. Many delay moving, which reduces listed inventory. FHFA’s National Mortgage Database (NMDB) publishes aggregate statistics on outstanding mortgage coupons that researchers use to measure how widespread lock-in is.
Where does the NMDB data come from?
The FHFA National Mortgage Database is a representative sample of U.S. mortgages assembled for research and policy. Public NMDB aggregate tables (not loan-level) show distributions of note rates, credit metrics, and performance. Daily Calcs cites the published aggregates; we do not redistribute restricted microdata.
How should buyers use lock-in context?
Treat lock-in as market context, not a personal prediction. Compare your current rate and remaining term in the Refinance Calculator, and stress-test a new purchase payment with local taxes on our mortgage pages. Lock-in helps explain inventory; it does not set your quote.
Does lock-in affect refinancing?
Yes. Borrowers already below market rates have little incentive to refinance for rate alone. Cash-out or term changes may still make sense. Run break-even months in the Refinance Calculator before paying closing costs.
Will you publish a state CSV?
When we next pull FHFA NMDB public aggregates for the share of outstanding balances below 3% and 4% by state, we will add a CSV under /data/reports/ and update this page’s verified date. Until then, use FHFA’s official NMDB publications as the primary source.
Try the calculator
Refinance Calculator
Try the calculator
Mortgage Calculator
Try the calculator
Mortgage Payoff Calculator
Related guides
- Conforming Loan Limits by County 2026 - FHFA CSV Free 2026 FHFA conforming loan limits by county with high-cost flags and FHA floor/ceiling context. Download the CSV and see how jumbo thresholds work.
- Historical Mortgage Rates Chart - 1971-2026 Interactive Freddie Mac PMMS chart of U.S. mortgage rates since 1971, plus decade payment impact on a $300k loan. Free to cite.
- Jumbo Mortgage Rates Guide (2026) 2026 jumbo loan guide: the conforming limit, rate premiums, down payment rules, and payment examples. Free jumbo mortgage calculator.
- Metro Housing Affordability Scorecard - ACS PITI (2026) See how many of 20 metros clear a 28% PITI test on ACS median homes at the latest PMMS rate. Free CSV download, methodology, and cite block.
- Mortgage Rate History Payment Impact (2026) See what every PMMS rate era cost on a $300k 30-year loan - monthly P&I and lifetime interest from 1971 to 2026. Free analysis.