On a $300,000 mortgage at 6%, a 30-year term costs $347,515 in total interest (Fixture A, engine-verified). A 15-year term at the same 6% costs approximately $155,700 in total interest — a difference of roughly $192,000. The 15-year payment is approximately $2,531/month vs $1,798.65 for the 30-year — about $732/month more.
Those numbers are the starting point, not the whole story. The real question is what you would do with the $732 monthly difference — and whether the 30-year's flexibility or the 15-year's discipline is worth more to you. The calculator shows both schedules side by side for your actual loan amount and rate.
The $192,000 interest gap — and why it compounds
The 15-year mortgage's massive interest advantage comes from two sources: a shorter period (180 months vs 360) and — in practice — a lower interest rate (lenders typically charge 0.5–0.75% less for 15-year loans). In the calculator above, both terms use the same 6% rate for an apples-to-apples comparison; in the real market, the 15-year's advantage is even larger.
The compounding effect matters too. By paying off in 15 years, you stop generating interest on your outstanding balance 15 years earlier. Every extra month of a 30-year mortgage adds interest on a balance that would already be zero under the 15-year.
For context, $192,000 at the same 6% rate invested for 30 years grows to roughly $1.1 million. The interest you save on the mortgage is not just money kept — it is a principal that could itself compound if deployed elsewhere.
The case for the 30-year mortgage
The 30-year's biggest advantage is not financial — it is optionality. A 30-year borrower who invests the $732 monthly payment difference into an index fund at historical market returns may come out ahead of the 15-year borrower, purely on numbers. The 15-year is a forced savings mechanism; the 30-year requires the discipline to actually invest the difference.
The 30-year also provides a lower required minimum payment. In months when income drops — job loss, illness, family changes — the lower contractual obligation is a genuine safety valve. A 15-year borrower who cannot make the higher payment is in default; a 30-year borrower facing the same shortfall has more margin.
The honest answer: if you will reliably invest the payment difference, the 30-year can be mathematically competitive at low rates. If the extra $732 would be spent rather than invested, the 15-year wins both the math and the behavioral game.
Using the compare mode for other term pairs
The calculator is not limited to 15 vs 30. Change the "Alternative term" field to compare any two terms: 20-year vs 30-year, 10-year vs 15-year, or any custom combination. The side-by-side balance chart shows both loan payoff curves simultaneously, making the equity-building difference visual.
A 20-year term at 6% on $300,000 produces a payment of approximately $2,149/month and total interest of roughly $215,800 — a middle ground between the 15- and 30-year options. Use the compare mode to find the term that fits your budget and goals.
Frequently asked questions
Is a 15-year mortgage always better than a 30-year?
Not always. The 15-year wins on total interest paid and speed of equity building. The 30-year wins on monthly payment flexibility and — if you reliably invest the payment difference — potentially on total wealth. The right choice depends on your income stability, investment discipline, and how long you plan to stay in the home.
What is the payment difference between a 15-year and 30-year mortgage on $300,000?
At 6%, the 30-year payment is $1,798.65/month (engine-verified, Fixture A). The 15-year payment at 6% is approximately $2,531/month — about $732/month more. In practice, 15-year mortgage rates are typically 0.5–0.75% lower, which narrows the payment gap: a 15-year at 5.5% on $300,000 costs approximately $2,451/month — about $652 more than the 30-year at 6%.
How much interest do I save with a 15-year vs 30-year mortgage?
On $300,000 at 6% for both terms: 30-year costs $347,515 in total interest (Fixture A); 15-year costs approximately $155,700 in total interest — a difference of roughly $192,000. With the typical rate advantage (15-year at 5.5% vs 30-year at 6%), the interest saved rises further.
Can I get the benefits of a 15-year mortgage without the higher payment?
Yes — by taking a 30-year mortgage and making extra principal payments equal to the difference. On $300,000 at 6%, paying an extra $732/month on the 30-year accelerates payoff to roughly 15 years and produces similar total interest. The advantage of this approach is flexibility: in a tight month, you can skip the extra payment. The disadvantage is that the discipline to consistently pay the extra is not guaranteed.