Mortgage Interest Calculator
See the real number: total interest paid over your mortgage's full term, what hits in the first 5 years, and what the other common term would cost instead.
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Total Interest (Full Term)
$427,180
133% of the amount borrowed
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Last updated: August 17, 2026 · Reviewed by the DoCalc team
Why Total Interest Matters More Than the Monthly Payment
Most home shopping focuses on the monthly payment, but the total interest paid over a loan's full term is often the bigger number — and the one most easily reduced by choices you make before signing. On a typical 30-year mortgage, total interest frequently exceeds the original loan amount itself, meaning you pay for the home more than twice over by the time it's paid off.
This calculator isolates that number so it's not buried inside a monthly payment breakdown, and shows two things most payment calculators don't: how much of that interest lands in just the first 5 years, and what choosing the other common loan term would have cost instead.
The Formula
The monthly payment itself is fixed by the standard amortization formula, but total interest is simply everything paid over the full term minus the original loan amount — the rest is interest, distributed unevenly (front-loaded, as covered in the Amortization Calculator).
Worked Example
A $320,000 loan at 6.75% over 30 years produces a $2,076/mo payment. Over 360 payments, total payments come to about $747,180 — meaning roughly $427,180 of that is interest, about 133% of the amount actually borrowed. Switching to a 15-year term at the same rate would cut total interest to about $189,708 — still a large reduction — and an actually lower real-world 15-year rate would cut it further, in exchange for a meaningfully higher monthly payment.
15-Year vs. 30-Year: The Real Interest Cost
| Factor | 15-year | 30-year |
|---|---|---|
| Monthly payment | Higher | Lower |
| Total interest paid | Roughly 50-60% less | Significantly more |
| Interest rate | Typically lower | Typically higher |
| Best for | Minimizing lifetime cost | Maximizing monthly flexibility |
Pros and Cons of Optimizing for Lowest Total Interest
Pros: a shorter term or extra payments can save tens or hundreds of thousands of dollars in interest, and build equity dramatically faster.
Cons: the higher monthly payment that comes with minimizing interest reduces monthly cash flow that could otherwise go toward retirement investing, which historically outpaces mortgage rates over long horizons — minimizing interest isn't always the same as maximizing net worth.
Who Should Use This Calculator
Use it if you're comparing loan terms or deciding whether a rate difference of even 0.25% is worth negotiating for — on a 30-year loan, small rate differences compound into large total-interest differences. Skip it if you already have a signed Loan Estimate; that document's amortization table is the authoritative number.
Frequently Asked Questions
How much interest will I actually pay on a 30-year mortgage?
It depends heavily on rate, but it's common for total interest on a 30-year loan to exceed the original loan amount — meaning you effectively pay for the home more than twice by payoff, before accounting for property appreciation.
Does a lower interest rate always mean less total interest?
Usually, but not always — a lower rate over a longer term can still produce more total interest than a higher rate over a shorter term, since total interest depends on both the rate and how many payments accrue it.
How much does refinancing to a lower rate save on total interest?
It depends on how much term remains and the new rate, but refinancing resets the amortization clock, so it's worth modeling separately — see the Refinance Calculator for a break-even analysis specific to your situation.
Why is interest so front-loaded on a 30-year loan?
Interest is charged on the outstanding balance each month, which is largest early in the loan. See the Mortgage Amortization Calculator for the full year-by-year breakdown.
Does making extra payments reduce total interest?
Yes, significantly — any extra principal payment reduces the balance interest is calculated on for every remaining month of the loan, which compounds into real total-interest savings.
Is total interest paid the same as the true cost of the loan?
It's the biggest piece, but not the whole picture — closing costs, PMI, and points also add to the loan's true cost. See the Closing Costs Calculator and Mortgage APR Calculator for those pieces.