The trade-off in one sentence
A 15-year mortgage charges a lower interest rate and slashes total interest, but the monthly payment is much higher. A 30-year mortgage has an affordable payment and frees up cash — but you pay far more interest over time and stay in debt twice as long. This calculator shows both numbers, then answers the question that actually matters: which leaves you wealthier after 30 years if you commit the same monthly cash either way?
The raw numbers
On a $300,000 loan at 5.75% (15-year) vs 6.5% (30-year), the 15-year payment is roughly $2,490/month versus about $1,896 — nearly $600 more. But the 15-year borrower pays around $148,000 in total interest versus about $383,000 on the 30-year. That's roughly $235,000 of interest avoided, plus being mortgage-free 15 years sooner. Purely on interest, the 15-year is a landslide.
The honest counterargument: invest the difference
The 30-year's defenders make a fair point: its lower payment frees up ~$600/month that could be invested. If that money earns more than the mortgage rate, the 30-year borrower could come out ahead in total wealth despite paying more interest. This calculator tests that directly — it assumes you spend the same monthly amount either way (the higher 15-year payment), and compares two paths over 30 years:
- 15-year: pay it off in 15 years, then invest the (now freed-up) full payment for the remaining 15 years.
- 30-year: invest the payment difference every month for the full 30 years.
The winner depends heavily on your assumed investment return versus the rate gap. At high assumed returns (say 8–10%), investing the difference in the early years — when compounding has longest to work — often edges ahead. At modest returns (5–6%), the 15-year's guaranteed interest savings usually win. Because the 15-year's benefit is certain and the investment's is expected, many people rationally prefer the guaranteed path even when the projected numbers are close.
Which should you choose?
- Choose the 15-year if: you can comfortably afford the higher payment, value guaranteed interest savings and being debt-free sooner, and worry you wouldn't actually invest the difference (most people don't).
- Choose the 30-year if: you want payment flexibility, will genuinely invest the difference (ideally automatically), or need the lower payment to keep housing costs within a safe share of income.
- A middle path: take the 30-year for flexibility but make extra principal payments toward a 15-year pace — see our extra payment calculator. You keep the option to fall back to the lower required payment in a tight month.
Whatever the term, make sure the payment fits your budget first with the home affordability calculator, and see the full monthly picture with the mortgage calculator.