Research question
How much does the interest rate change the monthly payment and the total lifetime cost of a fixed-rate mortgage, and how do state rate anchors sit around the national benchmark?
Dataset
- A fixed-rate, fully-amortized $400,000 loan over 30 years (kept constant to isolate rate).
- National benchmark 6.71% — Freddie Mac PMMS 30-year average (2026-09-03).
- State rate anchors from the TruePITI market dataset (directional model around the national anchor).
Methodology
- Compute P&I with the standard amortization formula P·r(1+r)^n / [(1+r)^n − 1] at each rate.
- Lifetime interest = monthly P&I × 360 − principal.
- Rank the 50 state anchors and plot the four lowest and four highest.
Calculations
| Rate | Monthly P&I | P&I per year | Lifetime interest |
|---|---|---|---|
| 5.00% | $2,147 | $25,767 | $373,023 |
| 5.50% | $2,271 | $27,254 | $417,616 |
| 6.00% | $2,398 | $28,778 | $463,353 |
| 6.50% | $2,528 | $30,339 | $510,178 |
| 7.00% | $2,661 | $31,935 | $558,036 |
| 7.50% | $2,797 | $33,562 | $606,869 |
| 8.00% | $2,935 | $35,221 | $656,621 |
$400,000 loan, 30-year term, principal and interest only (excludes tax, insurance, MI).
State rate anchors — four lowest vs four highest
Directional state estimates around the 6.71% national benchmark (PMMS, 2026-09-03).
Hawaii6.62%
Washington6.64%
California6.65%
Massachusetts6.66%
Ohio6.75%
Louisiana6.76%
Mississippi6.76%
West Virginia6.76%
Findings
- A half-point rate move (6.5% → 7.0%) changes the monthly P&I by about $133, and lifetime interest by roughly $47,858 on the same loan.
- Rate sensitivity compounds: the same loan at 5.0% vs 8.0% differs by hundreds of dollars per month for the full 30 years.
- The spread between the lowest and highest state anchors is narrower than the spread across a 2-point rate range — rate shopping within a state matters more than the state label.
Limitations
- Holds the loan amount and term fixed; real borrowers change both.
- State anchors are direction estimates, not lender quotes — your rate depends on credit, LTV, points and lender.
- Excludes tax, insurance and mortgage insurance, which vary independently of rate.
Sources
Last updated & change history
Last updated: 2026-09-21
- 2026-09-21 — Initial study.