The choice between a 15-year and a 30-year mortgage is fundamentally a trade-off between monthly cash flow and total borrowing cost. When the numbers are laid out side by side at the same loan amount and market rate differential, the total cost gap is substantial — but so is the payment difference that determines affordability.
Informational calculation reference only.
All equations, tools, and outputs on this page are intended strictly for educational modeling and mathematical illustration. They do not constitute certified financial, legal, or tax advice. For specific scenarios, consult a certified public accountant (CPA) or a fiduciary financial advisor.
Why this metric dictates profitability
Total interest paid over the life of a mortgage can exceed the original loan amount on a 30-year term. The 15-year mortgage eliminates roughly half those interest payments but requires a monthly payment approximately 40%–50% higher. Whether that trade-off is advantageous depends on the borrower's cash flow, alternative investment opportunities, and tax situation.
Equation and data inputs
Monthly payment for both scenarios:
M = P \cdot \frac{r(1+r)^n}{(1+r)^n - 1}
Total interest paid:
\text{Total Interest} = (M \times n) - P
Effective annual cost difference:
\Delta\text{Cost} = \text{TotalInterest}_{30yr} - \text{TotalInterest}_{15yr}
Benchmark ranges
Modeling a $400,000 loan at representative 2026 rates (15-year: 6.25%, 30-year: 6.75%):
| Feature | 15-year mortgage | 30-year mortgage |
|---|---|---|
| Loan amount | $400,000 | $400,000 |
| Interest rate | 6.25% | 6.75% |
| Monthly payment | ~$3,429 | ~$2,594 |
| Monthly difference | +$835 more | Baseline |
| Total interest paid | ~$217,220 | ~$534,040 |
| Interest savings | $316,820 less | — |
| Equity at year 10 | ~$238,000 | ~$94,000 |
| Breakeven on rate premium | N/A | Immediate |
Common variable mistakes
Using the same interest rate for both terms. Lenders typically offer lower rates on 15-year mortgages (historically 0.50%–0.75% lower). Modeling both options at the same rate understates the 15-year advantage.
Ignoring the opportunity cost of the higher payment. The $835 per month in extra principal payments on the 15-year loan cannot be invested elsewhere. If that capital would earn more than the mortgage rate after tax in alternative investments, the 30-year with deliberate investing of the difference may produce greater net wealth.
Overlooking refinancing optionality. A 30-year mortgage can be converted into a shorter effective term through voluntary extra payments, preserving cash flow flexibility. A 15-year payment is contractually fixed.
Use the mortgage calculator to model your specific loan amount and current rate differential between terms.
Disclaimer: While we strive for absolute mathematical precision, actual real-world financial outcomes may vary based on institutional fees, localized tax brackets, changes in federal legislation, or fluctuating market indexes.