15-Year vs 30-Year Mortgage: Total Cost Compared

Lenders typically price 15-year mortgages about 0.5–0.75 percentage points lower than 30-year mortgages because the shorter payoff window reduces their risk. Using representative rates — 6.5% for a 30-year and 5.9% for a 15-year — here is what a $300,000 loan actually costs over its full life:

The 15-year borrower pays $620 more per month but saves $229,680 in interest over the life of the loan. That is not a rounding error — it is nearly the price of a second home.

Two forces compound against the 30-year borrower simultaneously:

1. Higher rate. Even a 0.6-point rate difference costs thousands on a $300k balance.
2. Longer amortization. In early years, almost all of your payment is interest. With a 30-year loan, you carry a large principal balance for 15 extra years — each of those months accrues interest on money you would have already paid off with a 15-year loan.

By month 60 (year 5) on the 30-year loan at 6.5%, you still owe roughly $279,000 — you have paid down only $21,000 of principal despite sending in about $114,000 in payments. On the 15-year loan, you would already owe roughly $237,000 after five years. The principal erosion is dramatically faster.

The 30-year is not automatically the wrong choice. The $620/month you save versus the 15-year payment can be invested. If you consistently invest that difference in a diversified index fund at a long-run average return of 7% annually, after 30 years that side fund grows to approximately $750,000 — well above the $229,680 in extra interest you paid.

This math only works, however, if you actually invest the difference every month without exception. Most people do not. The 15-year mortgage functions as forced savings with a guaranteed return equal to your mortgage rate. For borrowers who struggle with discretionary investing, the 15-year is the better behavioral choice regardless of the spreadsheet.

The invest-the-difference strategy also requires that your alternative investments outperform your mortgage rate after tax — not guaranteed in all rate environments.

The $620/month payment gap matters enormously depending on your income stability:

• Dual high-income households with stable salaries often absorb the 15-year payment comfortably and build equity at an accelerated pace.
• Single-income households or variable earners (freelancers, commission-based workers, small business owners) face real risk: missing a mortgage payment damages your credit and can trigger default proceedings. The lower 30-year payment provides margin during slow months.
• Buyers stretching to afford a home should be cautious. If the only way you can afford the house is by choosing a 30-year term, consider whether the house is priced correctly for your budget at all.

One middle path: take the 30-year mortgage but make extra principal payments whenever cash flow allows. You get the safety of a lower required payment while reducing total interest if your income permits accelerated paydown. Most mortgages allow this with no prepayment penalty — verify yours explicitly.

• You have stable, predictable income and the higher payment is under 28–30% of gross monthly income.
• You are buying a forever home — you plan to stay long enough to benefit from the full interest savings.
• You are within 20 years of retirement and want the mortgage paid off before you stop working.
• You already max out retirement accounts and have an emergency fund; the extra equity build-up is the highest-return use of your cash.
• You want the lower rate with certainty, rather than relying on the discipline to invest the difference.

• Your income is variable or your job security is genuinely uncertain.
• You carry high-interest debt (credit cards, personal loans) — eliminating 20% APR debt first beats paying down a 6.5% mortgage.
• You have not yet fully funded an emergency reserve (3–6 months of expenses) or employer-matched retirement accounts.
• You plan to sell or refinance within 7–10 years — you will not hold the loan long enough to justify locking into the higher payment.
• You are a disciplined investor who will reliably redirect the monthly savings into a taxable or tax-advantaged brokerage account.

Run these checks before committing to either term:

1. Payment-to-income ratio: PITI (principal, interest, taxes, insurance) should stay below 28% of gross monthly income. If the 15-year payment exceeds that, the 30-year is safer.
2. Opportunity cost rate: If your mortgage rate is under 5%, the math almost always favors investing the difference rather than accelerating payoff. Above 6–7%, the guaranteed return from paydown becomes more attractive.
3. Time horizon: Plan to sell in under 8 years? The extra interest you would avoid by choosing a 15-year may not materialize — run the numbers for your actual expected hold period.
4. Rate differential: Check current lender quotes for both terms. If the rate gap is only 0.25 points instead of the typical 0.5–0.75, the 15-year becomes even more compelling since you lose less of the rate advantage.

Use the mortgage calculator on this page to enter your exact loan amount, both rate quotes from your lender, and your local tax/insurance estimates — the side-by-side comparison will make the right call obvious for your specific numbers.