15 vs 30 Year Mortgage Calculator
The classic mortgage argument, settled with numbers: both payments, the interest the 15-year saves, and — fairly — what investing the payment difference would build instead.
The argument, in full, with the other side allowed to speak
The 15-year mortgage charges a lower rate and clears the debt in half the time; its interest bill is not slightly smaller but dramatically so. The honest counter-argument is not the higher payment — it is the alternative: take the 30-year, invest the monthly difference, and let markets outrun the mortgage rate. This calculator presents both cases on your numbers.
Interest saved = total interest (30y) − total interest (15y) · Investing case = the payment difference, compounded monthly
Example: $300,000 at 6.5% for 30 years costs $382,632 of interest. At 5.5% for 15 years it costs $141,039. Interest saved by the 15: $241,593 — against a payment that is about $552 higher each month. Investing that $552 for 30 years at 7% builds about $672,000 instead, which is the real form of the argument.
The facts both camps skip
The 15-year's win is guaranteed; the investing win is probable but not. The 30-year also keeps the required payment low if income falls — flexibility has value no spreadsheet cell holds. And most 30-year borrowers who swear they will invest the difference do not. Choose the plan you will actually execute.
How this is calculated
Both loans are amortized in full at their own rates (15-year rates are normally lower — enter the real pair you are quoted). Interest saved is the direct difference of total interest. If you enter an investment return, two terminal values are computed: investing the payment difference monthly for 30 years, versus the 15-year path investing the entire 15-year payment monthly during years 16–30; contributions at month end, return divided by 12.
Frequently asked questions
Why is the 15-year rate lower?
The lender's money is at risk for half as long and the borrower's equity builds faster, so the loan is safer — and priced like it. A gap of 0.5–1.0 percentage points is typical.
Can I get the 15-year result on a 30-year loan?
Mostly: pay the 30-year loan as if it were a 15 (the amortization calculator shows how). You keep the flexibility of the low required payment and pay a little more interest for the option, because the 30-year rate is higher.
Does the investing comparison account for tax?
No — it is pre-tax and ignores the mortgage-interest deduction as well. It is a fair first approximation for comparing shapes of plans, not a tax plan.
Which do most people regret?
Surveys find regret concentrated among 30-year borrowers who spent (rather than invested) the difference. The 15-year's pain is monthly and visible; the 30-year's is deferred and silent.
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