Mortgage interest calculator Calculate the total interest paid over the life of a mortgage, see your monthly payment, and view the amortization breakdown year by year.
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Enter valid values Provide a positive loan amount, rate, and term to calculate mortgage interest.
Mortgage interest calculator: find how much total interest you will pay over the life of
Enter your loan amount, interest rate, and term to see the total interest cost, monthly payment, and a year-by-year amortization breakdown showing how principal and interest shift over time.
What a mortgage interest calculator does
A mortgage interest calculator takes your loan amount, annual interest rate, and term length, then computes your fixed monthly payment, the total amount you will pay over the life of the loan, and — most importantly — how much of that total is interest rather than principal repayment.
The headline number is total interest: the cost of borrowing. On a typical 30-year mortgage, total interest often exceeds the original loan amount. Seeing this figure can motivate strategies like making extra payments, choosing a shorter term, or refinancing to a lower rate.
This tool also shows the interest-to-loan ratio — a quick way to gauge how expensive the loan is relative to what you borrowed.
How mortgage interest is calculated
Fixed-rate mortgages use the standard amortization formula to compute a constant monthly payment. Each month, a portion of that payment covers the interest accrued on the remaining balance, and the rest reduces the principal.
In the early years, most of your payment goes to interest because the balance is large. As you pay down principal, the interest portion shrinks and more of each payment goes toward principal — this is the amortization effect visible in the chart.
The monthly interest charge is calculated as the annual rate divided by 12, multiplied by the current remaining balance. Over a full term, the sum of all monthly interest charges equals the total interest cost.
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
Where M is the monthly payment, P is the loan principal, r is the monthly interest rate (annual rate ÷ 12 ÷ 100), and n is the total number of monthly payments (years × 12).
How loan term affects total interest
A 30-year mortgage has lower monthly payments than a 15-year mortgage, but the total interest paid is dramatically higher. For example, a $300,000 loan at 6% costs about $347,515 in interest over 30 years versus about $156,108 over 15 years — a difference of nearly $200,000.
The trade-off is monthly cash flow: the 30-year payment is roughly $1,799/mo while the 15-year payment is about $2,532/mo. Choosing the right term depends on your budget, other financial goals, and how long you plan to stay in the home.
Some borrowers take a 30-year mortgage for the lower required payment but make extra payments when cash flow allows, capturing some of the interest savings of a shorter term without the commitment.
Strategies to reduce mortgage interest
Making extra principal payments — even small ones — can significantly reduce total interest. An extra $200/month on a $300,000 loan at 6% saves over $100,000 in interest and shortens the loan by about 7 years.
Refinancing to a lower rate reduces the interest component of every payment. A common guideline is that refinancing makes sense when you can reduce your rate by at least 0.5–1.0 percentage points and you plan to stay long enough to recoup closing costs.
Biweekly payments — making half your monthly payment every two weeks — result in 26 half-payments per year (13 full payments instead of 12), effectively adding one extra payment per year toward principal.
Understanding the amortization schedule
An amortization schedule breaks your mortgage into individual payments, showing how much of each goes to interest versus principal. The chart on this page displays the annual summary.
The crossover point — where principal exceeds interest in each payment — typically occurs about two-thirds of the way through a 30-year term. For a 6% rate, this happens around year 19 or 20.
Reviewing the schedule helps you understand why paying down a mortgage early has outsized benefits: each extra dollar of principal paid in the early years avoids interest that would have compounded for decades.
Limitations of this calculator
This calculator models fixed-rate mortgages only. Adjustable-rate mortgages (ARMs) have variable rates that change after an initial fixed period, making total interest unpredictable without knowing future rate adjustments.
It does not include property taxes, homeowners insurance, PMI, or HOA fees — these are part of your total monthly housing cost but are not interest charges on the loan itself.
The calculator assumes you make exactly the scheduled payment each month for the full term. Extra payments, lump-sum prepayments, or missed payments will change the actual total interest paid.
Frequently asked questions
How much interest will I pay over the life of my mortgage?
The total interest depends on your loan amount, rate, and term. For a $300,000 loan at 6% over 30 years, you would pay about $347,515 in interest — more than the loan itself. Use this calculator to see the exact figure for your situation.
How is mortgage interest calculated each month?
Each month, interest is calculated as the remaining loan balance multiplied by the monthly rate (annual rate ÷ 12). The rest of your fixed payment goes toward reducing the principal balance.
What is the difference between interest rate and APR?
The interest rate is the cost of borrowing the principal. APR (Annual Percentage Rate) includes the interest rate plus other loan costs like origination fees and points, expressed as a yearly rate. APR is typically higher than the interest rate and gives a more complete picture of loan cost.
Should I choose a 15-year or 30-year mortgage?
A 15-year mortgage has higher monthly payments but saves a significant amount in total interest. A 30-year mortgage offers lower monthly payments and more flexibility. The right choice depends on your monthly budget, other financial goals, and how long you plan to stay.
