A 30-year mortgage at 7% is a commitment to pay 2.4 times the purchase price. On a $320,000 loan, you'll pay $751,000 total — $431,000 in principal and interest is actually $320,000 principal + $431,000 interest over three decades.
Cutting that timeline down saves an enormous amount of money. I calculated exactly what it takes.
Disclaimer: Calculations assume a fixed-rate mortgage. Individual results vary based on rate, loan balance, and timing of extra payments.
The Baseline: $320,000 Mortgage at 7%, 30 Years
| Metric | Value | |---|---| | Monthly P&I payment | $2,129 | | Total paid over 30 years | $766,478 | | Total interest paid | $446,478 | | Interest as % of loan | 139.5% |
You borrow $320,000 and pay $446,478 in interest — you pay more in interest than you borrowed. This is not a scandal; it's math. You're borrowing money for 30 years. The question is whether you can afford to accelerate it.
The Payoff Target Table: Extra Monthly Payment Required
Starting from year 1 on a $320,000 loan at 7%:
| Payoff Target | Extra Monthly Payment | Total Interest Paid | Interest Saved | |---|---|---|---| | 30 years (baseline) | $0 | $446,478 | — | | 25 years | $198 | $366,000 | $80,478 | | 22 years | $313 | $320,000 | $126,478 | | 20 years | $413 | $291,000 | $155,478 | | 18 years | $569 | $258,000 | $188,478 | | 15 years | $871 | $199,000 | $247,478 | | 12 years | $1,318 | $153,000 | $293,478 | | 10 years | $1,779 | $125,000 | $321,478 |
The most powerful insight: An extra $871/month (for 15-year payoff) saves $247,000 in interest. Every dollar of extra payment directly reduces principal — that same dollar no longer accrues 30 years of interest.
Why Extra Payments Are So Powerful Early
The earlier you make extra payments, the more interest you avoid. This is because interest accrues on the remaining balance.
Month 1 of a $320,000 loan at 7%:
- Interest portion: $320,000 × (7%/12) = $1,867
- Principal portion: $2,129 – $1,867 = $262
An extra $262 in month 1 doubles the principal paid that month, eliminating 1 month from the end of the loan — roughly $2,129 in future payments avoided.
Month 300 of the same loan:
- Remaining balance: ~$83,000
- Interest portion: $83,000 × (7%/12) = $484
- Principal portion: $2,129 – $484 = $1,645
An extra $262 in month 300 eliminates far less future interest (only months with small balances remain).
Early extra payments have 10–15× the impact of late extra payments. Refinancing to a 15-year mortgage at loan origination captures this advantage from day one.
The Biweekly Payment Strategy
Instead of 12 monthly payments, make 26 half-payments (every two weeks). Since there are 52 weeks in a year, biweekly = 26 half-payments = 13 full payments/year.
Effect on $320,000 at 7%:
- Extra payments per year: 1 full payment = $2,129
- Payoff: ~26 years (4 years early)
- Interest saved: ~$72,000
The biweekly strategy requires no willpower after setup — it's automatic acceleration. Most lenders allow it; some charge a setup fee ($200–$500), which is trivially worth it given the $72,000 savings. Alternatively, simply add 1/12 of your monthly payment to each payment yourself.
Lump Sum Payments: The Impact at Different Points
If you receive a bonus, tax refund, or inheritance, a lump sum payment can significantly alter the trajectory.
$10,000 lump sum on a $320,000 mortgage at 7%:
| When Applied | Years Saved | Interest Saved | |---|---|---| | Year 1 | 2.8 years | $62,000 | | Year 5 | 2.1 years | $46,000 | | Year 10 | 1.5 years | $31,000 | | Year 20 | 0.7 years | $14,000 |
A $10,000 payment in year 1 saves $62,000 in total interest. The same payment in year 20 saves $14,000. Apply windfalls early.
Should You Pay Off the Mortgage Early? The Investment Alternative
The payoff strategy has a clear cost: opportunity cost of the extra payments invested.
Extra $413/month (20-year payoff) vs. invested at 7%:
Investing $413/month at 7% for 20 years = $219,000
Paying off mortgage in 20 years vs. 30 years saves $155,478 in interest.
The investment wins by $63,522 in this scenario — purely on expected returns. But this assumes:
- You actually invest the money (not spend it)
- The market returns 7% (not guaranteed)
- You're comfortable with market volatility on funds you could have used to eliminate debt
At 7% mortgage rate vs. 7% expected return: The math is approximately equal before taxes. After taxes (mortgage interest may be deductible; investment gains are taxable), the after-tax calculation tips slightly toward investing in taxable accounts but toward mortgage payoff in retirement accounts.
At 3–4% mortgage rate: Investing wins clearly. At 7%+ mortgage rate: paying off the mortgage is a competitive alternative to investing.
The Practical Strategy: Both
The optimal approach for most people isn't either/or:
- Contribute to 401k up to employer match (guaranteed 50–100% return)
- Max HSA (triple tax advantage)
- Max Roth IRA ($7,000/year)
- Then: split extra cash between extra mortgage principal and taxable investing
This approach builds net worth on multiple fronts while reducing the psychological weight of the mortgage balance. The "pay off the house" motivation is real and worth some financial sacrifice in expected returns.
The $247,000 in interest savings from 15-year payoff is real money. Whether it's better than investing the same funds depends on your mortgage rate, risk tolerance, and tax situation. But knowing the exact numbers — $871/month extra, $247,000 saved — lets you make the decision consciously rather than simply paying the minimum because you've always paid the minimum.