Core Extra Payment Rules of Thumb
- At 6.5–7.5% mortgage rate: every $10,000 of extra principal payment saves approximately $8,000–$12,000 in total future interest (depending on remaining term)
- One extra mortgage payment per year shortens a 30-year loan by approximately 3 years (biweekly payment method)
- To save 1 year of payments on a 25-year remaining mortgage: add approximately 4% of your monthly payment as a monthly extra (example: $80 extra on $2,000 payment)
- The earlier in the mortgage, the higher the interest savings multiplier — a dollar paid in year 1 saves 5–7x more in interest than a dollar paid in year 25
- Never make extra mortgage payments while carrying credit card debt above 10% APR — the spread is too large
- Always capture employer 401k match before making extra mortgage payments — free money beats guaranteed interest savings
- At rates below 4.5%, investing in a diversified index fund likely outperforms extra mortgage payments over 20+ year horizons
- At rates above 6.5%, extra mortgage payments are competitive with investing after retirement accounts are funded
- A $100/month extra payment on a typical $250K–$300K mortgage saves $20,000–$30,000 in total interest — compelling for a modest commitment
- Target mortgage payoff by age 60–65 to reduce required retirement income and simplify retirement cash flow
Extra mortgage payment rules of thumb and their mathematical basis
| Rule of Thumb | Math Behind It | When It Breaks Down |
|---|---|---|
| $10K extra = $8–12K interest saved | $10K reduces interest at mortgage APR × remaining years | Less savings with few remaining years |
| One extra payment = 3 years shorter | 26 biweekly payments vs. 24 = 2 extra half-payments = 1 full payment extra annually | Varies with rate and remaining term |
| Early dollar saves 5–7x more | Longer compounding window for reduced balance | True only for early-term payments |
| Rate below 4.5%: invest instead | Expected stock returns (7–10%) clearly exceed guaranteed 4.5% | Short investment horizons change this |
🔑The Most Useful Single Rule
At current rates (6.5–7.5%): after capturing all employer retirement match and having a 3-month emergency fund, extra mortgage payments are among the best guaranteed-return uses of surplus monthly cash flow. The returns are larger and more certain than bonds, competitive with the stock market on a risk-adjusted basis.
Verify Your Rule of Thumb Estimate
Get the exact interest savings and payoff timeline for your specific mortgage details — beyond these useful approximations.